Download Monitoring ventilation in homes built before 1995: A pilot study

Monitoring ventilation in
homes built before 1995:
A pilot study
Prepared for: James Russill
Energy Saving Trust (EST)
July 2006
Client report number 229615
1
Monitoring ventilation in homes built before 1995: A pilot study
Prepared by
Name
Vina Kukadia and Martin White
Position
Associate Director and Senior Consultant, Environmental Consultancy
Signature
Approved on behalf of BRE
Name
Anthony Slater
Position
Director, Environmental Consultancy
Date
Signature
BRE
Garston
WD25 9XX
T + 44 (0) 1923 664000
F + 44 (0) 1923 664010
E [email protected]
www.bre.co.uk
This report is made on behalf of BRE. By receiving the report and acting on it, the client - or any third party relying on it - accepts that
no individual is personally liable in contract, tort or breach of statutory duty (including negligence).
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Monitoring ventilation in homes built before 1995: A pilot study
Executive Summary
Energy savings from the installation of energy efficiency measures in UK housing are usually estimated
using the BRE Domestic Energy Model (BREDEM). BREDEM has previously been tested against a large
number of measurements from field trials, for a range of typical house types. However, the calculated
energy savings depend on assumed values for important input parameters such as air permeability, which
are subject to significant uncertainty and at present information included in BREDEM may be out-of-date.
To reduce this uncertainty, the Energy Saving Trust (EST) requested BRE to carry out a pilot study to
measure the ventilation rate and air permeability in a small sample of pre-1995 homes. This was to enable
an initial comparison to be made between ventilation rates measured in this study with those currently
predicted by BREDEM. The study was also intended to determine whether or not the results from the pilot
study would be representative of the pre-1995 building stock within the five types of housing studied. If not,
then the intention was to determine the optimum numbers and types of property that would need to be
monitored in a future study, so that the results would be as representative as possible of the UK housing
stock.
The findings from this study are as follows:

Average measured whole house ventilation rates in the homes were found to be in the range
0.29-0.55 ach. The home with the highest measured ventilation rate also had the highest measured air
permeability and by far the longest time periods during which windows were opened. However, this is
contrary to the BREDEM calculation method which assumes occupants open their windows if the
infiltration due to air permeability and fan use is low.

For each of the homes investigated, the measured ventilation rates were considerably lower (usually by
a factor of around 2) than those calculated by either of the methods used within BREDEM. Therefore,
for these homes, if the survey information alone had been used, BREDEM would have over-estimated
their ventilation rates. This, in turn means that the calculated ventilation energy losses would have been
higher and the corresponding SAP rating lower.
The measured wind speed was also used in BREDEM to calculate ventilation rates. The corresponding
ventilation rates were lower than those calculated using BREDEM Method 1, ranging from 19% to 44%
smaller. However, there was still a significant difference between the ventilation rate calculated by
BREDEM method 1using measured wind speed and the measured values of ventilation rate, ranging
from 25% to 42% higher.
In each of the homes measured internal temperatures varied significantly from those calculated by
BREDEM. In most cases the internal temperature in the homes was lower than that calculated by
BREDEM. BREDEM does not use temperature in its ventilation rate calculation. However, temperature
plays an important role in driving natural ventilation in homes.



The measured wind speeds were significantly lower than those calculated by BREDEM. The wind
speed factor in BREDEM was therefore significantly overestimated in the ventilation calculation.

BREDEM overestimates the space heating energy consumption. When the measured ventilation rates
were input to BREDEM the estimated space heating energy consumption fell by between 6% to 20%.

The measured air permeabilities in the five homes were mostly less than 10 m 3 h-1 at 50 Pa pressure
difference (one of the homes measured 13.3 m 3 h-1). Although three of the homes tested were built in
the 1930’s and two in the 1980’s, they compared favourably with the 10 m 3 h-1 permeability requirement
recently specified in the Building Regulations Approved Document L (ODPM 2006). However, the
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Monitoring ventilation in homes built before 1995: A pilot study
number of homes i.e. five, investigated here is too small a sample size to be representative of the UK
housing stock for home built prior to 1995.

Since a sample size of only five homes was considered to be too small to carry out any meaningful
statistical analysis, as part of this study a dataset from previous work was analysed. This was to
determine the optimum numbers of properties that would need to be monitored in a future study so that
the results would be representative of the UK housing stock.
Confidence limits were therefore determined for studies of different sizes. This analysis showed that the
smaller the 95% confidence intervals, the larger is the sample size required to give a greater level of
precision and hence a better representation of the UK housing stock. Thus, if, for example, an average
95% confidence interval of 0.004 is taken from the statistical analysis carried out here, then it is
recommended that the future study should include monitoring of 112 homes.
From the findings of this pilot study and experiences gained, the following is recommended for future
studies.
1. The ventilation rates in homes need to be measured over a reasonable time period to obtain
representative average values. The period of two weeks used in the pilot study proved to be sufficient
for this purpose.
2. Depending on the size of the home, at least four rooms need to be monitored to give a reasonable
representative average of the whole house ventilation rate.
3. The temperature dependent aspect of ventilation and the procedure for calculating demand
temperatures needs to be assessed. To allow this internal and external temperatures need to be
recorded throughout the monitoring period in all homes in the study.
4. Calculation of ventilation rates via Method 2 in BREDEM, is only possible if measured air permeability
values are available. Therefore, it is recommended that these measurements are also carried out in any
future study. This will help to build up a database in BREDEM for Method 2.
5. Accurate wind speed data is required. Therefore it is recommended that UK Meteorological Office
weather data should be purchased to provide a better estimate of the regional wind speed. It is also
recommended that detailed wind speed measurements be made at selected homes for comparison
with the data purchased from the UK Meteorological Office. This will allow the site exposure factor to be
assessed and if necessary corrected. It is possible to measure wind speeds on a continual basis.
However, the equipment to do this is expensive especially if each home is to have wind speed
monitored. If there are several homes in one location then one meteorological station to service all the
homes would be sufficient. It must be noted that the there are other important considerations that need
to be taken into account, such as where will the equipment be located and who will be responsible for it
during the monitoring period.
6. The occupant activity diary used for the pilot study needs to be improved and made more ‘occupant
friendly’. In particular, a record sheet for window opening and extract fan usage should be provided for
every room in the home that is monitored.
7. The home characteristics survey should be:
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Monitoring ventilation in homes built before 1995: A pilot study
a. Targeted more towards the features and characteristics of the home relevant to ventilation;
b. Simplified with respect to the heating system assessment.
8. The home characteristics survey form should always be completed by the trained investigators that are
conducting the ventilation and air permeability tests.
9. Data on typical occupant activities in the present day, together with information on home characteristics
needs to be collected as part of the future study. This then needs to be compared with such data
already used in BREDEM calculations to determine its validity for the type of home studied.
10. An incentive to the homeowners will almost certainly need to be included to persuade sufficient of them
to participate in this project, otherwise it may be difficult to obtain the required numbers and different
types of property required.
11. Regarding the sample size required for a future study, this needs to be discussed with the client to
determine the level of precision required before the actual number of homes that should be investigated
can be decided. If, for example, an average 95% confidence interval of 0.004 is taken from the
statistical analysis carried out here, then it is recommended that the future study should include
monitoring of at least 112 homes.
Remembering that homes can be categorised into potentially five groups (for example detached, semidetached, terrace, end-terrace and bungalow), discussions will be required on the type of homes that
need to be monitored. Again depending upon the 95% confidence interval selected, the number of
homes to be monitored from each category will need to be discussed.
12. BREDEM calculates the space heating energy consumption over the heating season. If it is decided to
carry out a more detailed study, BRE is well set up to carry out the monitoring work over the heating
season 2006-07, assuming that the contract begins by July 2006.
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Contents
1
Introduction
6
2
Objectives
7
3
Description of BREDEM
8
4
Work Programme
10
Selection of homes
Ventilation rate measurements
Air permeability measurements
Wind speed measurements
Site surveys
Home characteristics surveys
Occupant activity diaries
Comparison of ventilation rates measured with BREDEM results
Statistical analysis of data from current and previous studies
10
10
11
11
11
12
12
12
12
Results
14
Ventilation rates
Temperatures
Air permeability
Site surveys
Windspeeds
Home characteristics surveys
Occupant activity diary to determine window and fan operation
14
14
17
17
18
19
21
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
5
5.1
5.2
5.3
5.4
5.5
5.6
5.7
6
Comparison of measured ventilation rates with BREDEM results22
6.1
6.2
Comparison of ventilation rates
Space heating energy consumption
22
23
Statistical analysis of data from current and previous studies
25
A confidence interval approach
25
8
Summary and conclusions
27
9
Recommendations
29
10
References
31
11
Acknowledgements
32
7
7.1
Appendix A – Detailed description of BREDEM
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Monitoring ventilation in homes built before 1995: A pilot study
Introduction
Energy savings from the installation of energy efficiency measures in UK housing are usually estimated
using the BRE Domestic Energy Model (BREDEM), which has been previously subjected to testing against
a large number of measurements from field trials, for a range of typical house types (Dickson et al, 1996).
However, the calculated energy savings depend on assumed values for important input parameters such as
air permeability, which are subject to significant uncertainty.
BREDEM contains procedures for assessing ventilation rates from building characteristics and location; this
typically yields rates between 0.5 and 1 air changes per hour (ach) but with unknown accuracy. Other
researchers occasionally assume very different rates. For example, the recently published “40% House”
report (Boardman et al, 2005) assumed the average ventilation rate in 1996 to be 3.5 ach.
To reduce this uncertainty, the Energy Saving Trust (EST) requested BRE to carry out a pilot study to
measure the ventilation rate and air permeability in a small sample of pre-1995 homes. This was to enable
an initial comparison to be made between ventilation rates measured in this study with those currently
predicted by BREDEM. The study was also intended to determine whether or not the results from the pilot
study would be representative of the pre-1995 building stock within the five types of housing studied. If not,
then the intention was to determine the optimum numbers and types of property that would need to be
monitored in a future study, so that the results would be as representative as possible of the UK housing
stock.
This report describes the building monitoring work carried out during the pilot study, the comparisons made
between the measured ventilation rates and those predicted by BREDEM together with conclusions and
recommendations derived from the study.
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2
Objectives
The main objective of the project was to monitor ventilation rates and air permeability for a small number of
homes for comparison with the calculated values of ventilation rate and air permeability used in the
BREDEM software.
This was achieved by carrying out the following tasks. Full details are given in the work programme in
Section 4.
1. Selection of homes, one each from the following:

Detached;

Semi-detached;

Mid-terrace;

End-terrace;

Bungalow.
2. Ventilation rate measurements in each home
3. Air permeability measurements in each home
4. Site survey to determine the exposure of each home
5. Home characteristics survey to determine the structure of each home
6. Occupant activity diary to determine window and fan operation
7. Comparison of ventilation rates measured with BREDEM results
8. Statistical analysis of data from current and previous studies
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3
Description of BREDEM
BRE’s Domestic Energy Model (BREDEM), (Anderson et al, 2002) is the UK’s most widely used calculation
tool for estimating the energy consumption for a dwelling (details are given in Appendix A). BREDEM
requires a reasonable estimate of the ventilation rate (air change rate) to enable an overall heating
requirement to be calculated. However, it does not allow the direct input of measured ventilation rates, but
instead determines them within the software by the use of other parameters. BREDEM can calculate
ventilation rates using one of two methods depending upon the information available on other parameters.

Method 1: This method determines the ventilation rate by a series of calculation procedures within
BREDEM using the following:
1. Information on the type of home, its characteristics and exposure to the wind needs to be
known and input to the model. For the current study, this was obtained from the home and
site survey questionnaire. This information is required so that relevant data, such as
leakage effects of different building components, from the database within BREDEM, can
be obtained.
2. Once information from 1 is input to the model, the air permeability of the building is
determined from tabulated data on the air leakage effects of the building fabric, building
components and fans and vents. This is a simple additive procedure
3. The calculated air permeability has then to be corrected for site and dwelling exposure and
wind effects:

Site exposure relates to the general land use of the local area. This factor applies
to areas of a few miles across and is determined from data from the database;

Dwelling exposure is classified according to the number of sides of the building that
are sheltered by an obstacle, for example a mid-terraced house is sheltered on two
sides. This factor is determined from data in the database;

Wind effects are calculated by taking the regional wind speed and multiplying it by
the site exposure factor which gives the site wind speed. The regional wind speed
typically applies to an area fifty miles across and is normally taken from the
BREDEM database.
4. The occupant’s effect on the ventilation rate is then calculated using certain assumptions
already included in BREDEM.
5. The overall ventilation rate is then determined by adding the calculated air permeability in 3
above and the occupant’s effect on the ventilation from 4 above (see Equation 2 in
Appendix A).
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
Method 2: This method uses measured air permeability of the building fabric and components
which is obtained by carrying out a fan pressurisation test on the building. The ventilation rate is
then calculated in the following way:
1. The measured air permeability is divided by 20 (as a rule of thumb estimate of infiltration
rate) to account for the fact that the pressure test would have been carried out at a building
pressure differential of 50 Pa.
2. To this air permeability, is added the air leakage effects due to fans and vents only. This
information is taken from the BREDEM database as in Method 1.
3. This air permeability is then corrected for wind effects and site exposure again by using
data from the database as in Method 1.
4. The occupant’s effect on the ventilation rate is then calculated using certain assumptions
already included in BREDEM in the same way as for method 1.
5. The overall ventilation rate is then determined by adding the calculated air permeability in 3
above and the occupant’s effect on the ventilation from 4 above (see Equation 3 in
Appendix A).
In general, Method 1 is normally used as it does not require any measurements. However, the accuracy of
the ventilation rates and hence the heating requirements is then in question. In Method 2, a measured
value of air permeability due to the building fabric and components is used. At present, this method can
only be used if this data is available. Whichever method is used the resultant ventilation rate is then used to
calculate the ventilation heat loss.
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4
Work Programme
4.1
Selection of homes
For this study, a small sample of homes built prior to 1995 was required. To keep the fee as low as possible
and to facilitate a rapid start to the pilot study, the homes studied were selected from those belonging to
BRE Staff. Five homes, one each from the following were chosen:

Detached (H1): four-bedroom house built in the 1980s;

Semi-detached (H2): three-bedroom house built in the 1930s;

End-terrace (H3): three-bedroom house built in the 1930s;

Mid-terrace (H4): two-bedroom house built in the mid-1980s;

Bungalow (H5): one-bedroom bungalow built in the 1930s.
Each home was monitored for a period of two weeks.
4.2
Ventilation rate measurements
Ventilation rates were measured in the five homes using the Perfluorocarbon Tracer (PFT) technique
(Walker and White, 1995) developed originally by BRE. This tracer gas technique comprises two main
components:

Sources. These are small metal canisters containing the liquid PMCH perfluorocarbon that is
slowly emitted as a tracer gas. The sources are placed throughout the building being monitored so
as to give a uniform tracer emission across the whole building. The source emission rate is
temperature dependent. Therefore, during these measurements the temperature in each room
being monitored, was also recorded.

Sampling tubes. These are metal tubes that contain a quantity of granular carbonaceous
adsorbent. The sample tubes, in pairs, are placed in the space being monitored and left for the
sampling period which may be as much as four weeks. Air containing the PFT tracer either diffuses
naturally (as in this study) or is pumped through the sample tube. The loading of tracer on the
sample tube is the total concentration over the monitoring period and is used to calculate the
average concentration of tracer over the monitoring period of two weeks. The ventilation rate
calculated is the time-averaged value for the whole monitoring period, in this case two weeks. The
sampling locations are chosen as representing the areas of most interest. Usually in dwellings one
pair of sample tubes would be placed in each room.
The ventilation rate is calculated using the source strength, the average temperature, the loading of tracer
on the sampling tube and the sampling period. The ventilation rate calculated is a time averaged value over
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Monitoring ventilation in homes built before 1995: A pilot study
the monitoring period. For this study, the average ventilation rate over two weeks for the whole house was
calculated.
In all five homes four rooms most likely to be occupied during normal use were monitored, i.e. kitchen,
living room, bathroom and main bedroom. In the relatively larger homes, other rooms are also often
monitored. For example, in this case a second bedroom in the detached home (H1) and the dining room in
the semi-detached home (H2) were monitored. Selection of extra rooms is largely arbitrary. The extra
measurements are usually carried out in relatively larger homes to improve the estimate of the whole house
ventilation rate.
4.3
Air permeability measurements
As explained in Section 3, two methods are used within BREDEM to calculate ventilation rates. Method 1
uses air permeability values determined within BREDEM based on various assumptions, whereas Method 2
allows the direct input of air permeability from actual measurements. Therefore, air permeability was
measured in the five homes so that Method 2 could also be used to determine ventilation rates.
The air permeability measurements were carried out using a fan pressurisation system (Stephen, 2000).
This is a simple, widely used technique to measure the air permeability of the building envelope. For this
study, the test was carried out using an Infiltec blower door system according to the UK test procedure
(ATTMA, 2006). This is a small fan pressurisation system designed to quantify the air permeability of
dwellings and small buildings. The test measures the permeability of only the building fabric and therefore
all external doors and windows are closed and chimneys, vents and flues are sealed. Internal doors are
wedged open to allow the air from the fan to move freely through the building.
The air permeability test was carried out for each home prior to the start of the ventilation rate monitoring.
4.4
Wind speed measurements
During the monitoring period a number of spot measurements of wind speed were made during the visits to
each home. This was done to provide a general picture of the wind speed at the location of each home and
for comparison with the assumptions of wind speed used in BREDEM. Also during the monitoring period
the general wind environment in the area of the homes was observed visually and the Beaufort scale was
used to assess the wind speed. This was done to give an early warning of changes to the wind speed in
case it was necessary to visit the homes again and make more wind speed measurements.
Wind speed is used in BREDEM in the calculation of the wind speed factor. BREDEM does not allow the
input of a measured wind speed instead it uses a regional wind speed map of the UK. This map is used to
provide the regional wind speed in the general area of the home being monitored. The regional wind speed
is then corrected by the site exposure factor and the dwelling exposure factor to give the wind speed factor
at the location of the monitored home. The wind speed factor takes into account the effect of the land use of
the local area and the dwelling exposure of the homes on the regional wind speed. The wind speed factor is
then used to correct the calculated ventilation rate.
4.5
Site surveys
A site survey was carried out via a questionnaire to determine the exposure to the wind of each home. This
included a qualitative analysis to determine information on, for example, the surrounding terrain and shelter
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to the wind from other structures. This information needed to be determined, as it was required as one of
the inputs to the BREDEM model, so that appropriate data from the BREDEM database could be used to
run the model to determine ventilation rates.
4.6
Home characteristics surveys
A survey of the characteristics of each home, via a questionnaire, was carried out to determine the
following.

Construction methods and materials. This also included information on any extensions made and
also drawing a simple sketch layout of the home with dimensions.

The type, number and location of doors and windows and the type of glazing used.

A general description of the heating system. This included information on the type of heating
system and appliance used via a series of codes on the questionnaire. In addition, this included the
make and type of boiler, the approximate size of hot water cylinder and type of insulation used, the
number and size of the radiators and the form of controls used.

A simple estimate of how well the home was insulated including information on the type and
thickness of the insulation in the loft and the presence of any cavity wall insulation.
This information needed to be determined, as it was required as an input to the BREDEM model, so that
appropriate data from the BREDEM database could be used to run the model to determine ventilation rates
for the type of home investigated.
4.7
Occupant activity diaries
During the two week monitoring period, the occupants of each home were asked to complete an activity
diary on a daily basis of the following:

Time, duration and location of when windows were opened;

Time, duration and location of any extract fans used.
The purpose of the diary was to obtain information on occupant-related usage of the windows and fans so
that these could be taken into consideration when determining ventilation rates in the homes.
4.8
Comparison of ventilation rates measured with BREDEM results
Ventilation rates measured in the five homes were compared with the ventilation rates calculated by the two
BREDEM methods described in Section 3.
4.9
Statistical analysis of data from current and previous studies
One of the objectives of the current study was to carry out a statistical analysis of the data collected. This
could not be done on a sample size of 5 homes as it was too small. Therefore, data from a previous study
(Dimitroulopoulou et al, 2004) was used to give some indication of the statistics of the data collected,
although in reality even a sample size of 33 plus 5 homes would not strictly be large enough.
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Monitoring ventilation in homes built before 1995: A pilot study
The previous study (Dimitroulopoulou et al, 2004) carried out by BRE on 33 homes were all built post 1995,
and therefore was only used to provide a statistical base for recommendations for a future study for pre1995 homes. Since it is not possible to monitor the ventilation rates in all UK homes built before 1995, a
representative random sample must be taken. Thus, one of the main aims of this part of the study was to
determine the optimum number of homes that would be needed to be monitored in a future study, so that
the results would be representative of the UK housing stock.
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Monitoring ventilation in homes built before 1995: A pilot study
5
Results
5.1
Ventilation rates
The ventilation rates from each monitored room in each house are presented in Table 1. The individual
room ventilation rates were used to determine the whole house ventilation rate by taking an average.
Within BREDEM, there is a general assumption that ventilation rates in homes normally fall within the range
0.5 to 1.0 ach. From Table 1, it can be seen that only in one case, the mid-terrace (H4) home, did the
measured ventilation rate of 0.55 ach fall within this range. This home also had the highest air permeability
and when the occupant diary was analysed in conjunction with the ventilation rate proved to have the
longest periods with open windows.
5.2
Temperatures
Five homes were used in the pilot study, all were of different types of dwelling. The mean temperatures
monitored in the rooms of the homes and in the homes overall are summarised in Table 2.
The measured mean temperatures were normally fairly constant within most of the homes monitored. The
exception to this was the bungalow (H5), which did not have central heating, resulting in the bathroom and
main bedroom having much lower mean temperatures than were recorded elsewhere in the survey.
Table 3 shows the average internal and external temperatures measured during the monitoring period and
the temperatures calculated by BREDEM. The measured internal temperature values show significant
variation from home to home with a range of values of internal temperature from 14.5 oC to 20 oC. This
reflects the type and use of heating systems. H1 for instance has central heating whereas H5 does not. The
internal temperature values calculated by BREDEM show greater consistency with a range of values from
17.81 oC to 18.31 oC. This is because BREDEM assumes a standard demand temperature (temperature
required by occupants during heating period) of 21 oC. This standard temperature is then corrected for the
type of heating and control system in the home. This gives rise to a much smaller range of temperature
variance than would be found in ‘real life’.
BREDEM does not use temperature data in the ventilation calculation procedure. For all the homes, the
calculated air flow rates used through similar building components was therefore the same. However, in
reality temperature plays an important role in the natural ventilation rate in a home as it is a factor in
buoyancy driven ventilation.
Internal temperature values are comparable between measured and BREDEM within a range of 10% to
25%. This is a significant variance that would have an effect on the ventilation within the homes. This in
turn contributes to the difference between the measured ventilation rates and those calculated by
BREDEM.
It is not possible to input real temperature data into BREDEM. The variation in the measured and calculated
values and the difference between the measured values of each home suggests that the BREDEM
calculation procedure is not flexible enough.
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Monitoring ventilation in homes built before 1995: A pilot study
Table 1. Measured ventilation rates
House type
Room monitored
Room
ventilation rate
Whole House
ventilation rate
(h-1)
(h-1)
Living room
0.39
Kitchen
0.34
Bathroom
0.38
Bedroom 1
0.33
Bedroom 2
0.25
Living room
0.34
Dining room
0.28
Kitchen
0.23
Bathroom
0.35
Bedroom 1
0.26
Living room
0.26
Kitchen
0.52
Bathroom
0.35
Bedroom 1
0.36
Living room
0.29
Kitchen
1.15
Bathroom
0.34
Bedroom 1
0.43
Living room
0.41
Kitchen
0.74
Bathroom
0.41
Bedroom 1
0.19
Detached (H1)
Semi-detached (H2)
End terrace (H3)
Mid-terrace (H4)
Bungalow (H5)
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0.34
0.29
0.37
0.55
0.44
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Monitoring ventilation in homes built before 1995: A pilot study
Table 2. Summary of the mean temperatures measured in the rooms of the homes
Description
Mean temperature (˚C)
House
Overall
Living
room
Kitchen
Main
bedroom
Bathroom
Bedroom
2
Dining
Room
Detached (H1)
19.4
19.9
17.9
20.4
19.2
19.5
-
Semi-detached
(H2)
16
16.5
14.5
16.5
15.3
-
17.2
End Terrace
(H3)
15.5
16
15.4
16
14.7
-
-
Mid Terrace
(H4)
17.5
20.3
14.4
16.3
18.9
-
-
Bungalow (H5)
14.5
18.7
16.4
11.3
11
-
-
Table 3. Summary of measured and calculated internal and external temperatures
Measured
External*
5.3
Calculated by BREDEM
Internal
External
18.14
4.78
House type
Detached (H1)
Internal
20
Semi-detached
(H2)
16.2
5.3
18.03
5.35
End terrace (H3)
14.5
5.3
18.22
6.48
Mid-terrace (H4)
15
4.9
18.31
4.47
Bungalow (H5)
18.5
5.3
17.81
6.21
*
Regional monthly average temperature measured by UK Meteorological office.
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17
5.3
Air permeability
The results of the air permeability tests are given in Table 4.
Table 4. Results of the air permeability testing
House type
Air permeability (m3/h per
m2 envelope area @ 50 Pa)
Detached (H1)
6.5
Semi-detached (H2)
7.2
End terrace (H3)
6.0
Mid-terrace (H4)
13.3
Bungalow (H5)
9.2
For domestic buildings, the Building Regulations Approved Document Part L: 2005 (ADL) (ODPM, 2006)
specifies the following:

For domestic buildings, a maximum air permeability of 10 m 3/h per m2 envelope area at 50 Pa, and

For best practise, a value of 5 m 3/h per m2 envelope area at 50 Pa.
It is now a requirement that new-build homes achieve an air permeability of 10 m 3 h-1 per m2 envelope area
at 50 Pa or less..
From Table 4, it can be seen that the air permeabilities measured in homes H1, H2, H3 and H5 were all
within the value 10 m3/h per m2 envelope area at 50 Pa, specified in ADL. However, one home (H4)
exceeded the air permeability specified in ADL.
Although at present there are no published studies to state otherwise, in general, it is believed that most
existing homes, especially those built prior to 1995, will not have been built to accommodate the
specification in ADL.
5.4
Site surveys
BREDEM uses two factors to correct the wind factor used in the ventilation rate calculation methods 1 and
2. These are:

The site exposure factor, which relates to the general land use of the locale;

The dwelling exposure, which is based on how many sides of the building are sheltered by another
structure.
The site survey for each of the five homes provided the information required to allow the correct factors to
be selected. The site and dwelling exposures of each of the five homes of the pilot study are shown in
Table 5.
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Monitoring ventilation in homes built before 1995: A pilot study
Table 5. Summary of site exposure
5.5
House type
Site
exposure
No of
sheltered
sides
Detached (H1)
Average
None
Semi-detached
(H2)
Average
2
End terrace
(H3)
Average
2
Mid-terrace
(H4)
Average
2
Bungalow (H5)
Average
1
Wind Speeds
BREDEM calculates the site wind speed using the regional wind speed and correcting this with the site
exposure factor from the BREDEM database. Table 6 shows the measured wind speed and the wind speed
calculated by BREDEM for each home.
The wind speed in the general area of the homes was visually observed from time to time during the
monitoring period. The Beaufort scale was used to estimate the wind speed in the area of the homes. In
general during the monitoring period for all the homes the wind speed assessed by using the Beaufort scale
was of the order Gale force 1 (direction of wind shown by smoke drift but not by wind vanes). Gale force 1
is taken to equate to wind speeds in the range 0.3 to 1.5 m/s.
The measured wind speed values in Table 6 are averages of a series of spot measurements. It must be
noted that on the day that H3 and H5 (the first two homes monitored) were visited, the wind was blustery
resulting in the comparatively high average wind speed. The wind speeds measured during the visits of the
other three homes seem to be more representative of the wind speeds experienced for the monitoring
period.
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Table 6. Measured and calculated wind speeds.
Wind speeds (m/s)
Calculated by BREDEM
3.42
House type
Detached (H1)
Measured
0.20
Semi-detached (H2)
0.30
3.24
End terrace (H3)
2.6
3.24
Mid-terrace (H4)
0.34
3.06
Bungalow (H5)
2.4
3.42
The measured wind speeds are significantly lower than those calculated by BREDEM for use in the
ventilation rate calculations. It must be noted that the wind speeds calculated in BREDEM are averages
over the whole heating season, whereas the values measured as part of this study represent wind speeds
experienced during the monitoring period of two weeks only. It must also be noted that the length of heating
season used by BREDEM is in itself a variable which is based on the geographic location of the home and
on the level of insulation of the home. However, generally the length of the heating season is about seven
months long.
Although, BREDEM does not normally allow ventilation rates to be determined by inputting actual
measured wind speeds, EST requested that this be carried out if possible. Therefore, a spreadsheet
enabled version of BREDEM was (with difficulty) used to allow the measured wind speed to be input.
The measured wind speeds were input into BREDEM and the corresponding ventilation rates thus
calculated. These were compared with the ventilation rates measured during the monitoring period. The
results are discussed in section 6.1.
5.6
Home characteristics surveys
Five homes were used in the pilot study, all were of different types of dwelling. The major characteristics of
the homes are summarised in Table 7.
Three of the properties were built in the 1930’s, with the other two having been built in the 1980’s. All were
brick built with cavity walls. Four of the properties were heated using gas fired boilers and radiators, while
the bungalow had a gas fire and wall-mounted balanced flue gas heaters. All of the properties were at least
90% double glazed, with three of them being 100% double glazed.
This information was required as an input to BREDEM, so that the correct data could be selected within
BREDEM for the ventilation rates calculation.
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Table 7. Summary of the characteristics of the homes used in the pilot study
House type
Built
Type of
Construction
Number
of main
rooms
Floor
area
(m2)
Main
heating
type
Double
Glazing (%)
Detached
(H1)
1980’s
Brick, cavity
wall
9
195
Gas boiler
100
Semidetached
(H2)
1930’s
Brick, cavity
wall
7
74
Gas boiler
95
End Terrace
(H3)
1930’s
Brick, cavity
wall
6
80
Gas boiler
90
Mid Terrace
(H4)
1980’s
Brick, cavity
wall
5
56
Gas boiler
100
Bungalow
(H5)
1930’s
Brick, cavity
wall
5
50
Gas fires
100
Table 8 shows the type and level of insulation used in each home studied.
Table 8. Type and level of insulation in each home.
House type
Detached (H1)
Loft insulation
Type
Thickness (mm)
Fibreglass
100
Cavity Insulation
Installed y/n
N
Semi-detached (H2)
Rockwool
100
N
End terrace (H3)
Fibreglass
100
N
Mid-terrace (H4)
Rockwool
50
N
Bungalow (H5)
Fibeglass
50
N
H1 and H4 are the most modern of the homes (1980’s) and it is likely that loft insulation would have been
installed during construction. Loft insulation is likely to have been required by Building Regulations and a
minimum thickness of insulation is likely to have been specified. H2, H3 and H5 were built in the 1930’s
and, it is assumed that loft insulation would not originally have been installed. Loft insulation is likely to have
been retro-fitted to these homes. None of the homes appeared to have cavity wall insulation.
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5.7
Occupant activity diary to determine window and fan operation
The occupant diaries to determine the window and fan operation during the two week monitoring period,
were completed by the occupants of each home on a daily basis. This was then qualitatively analysed with
the results from the ventilation rates measured during the monitoring period (see Section 5.1).
H1 was a four-bedroom detached family home where the adults and children were at work and in full-time
education respectively, during weekdays. Homes H2-4 were single occupancy homes with the homeowners
also at work during weekdays. Home H5 was occupied by a retired couple who were at home for most of
the time during the study.
Table 9 gives a summary of the ventilation and fan operating patterns recorded for each of the homes
during the pilot study. Typically extract fans were used, or windows were opened, for periods of 30 minutes
to 1 or 2 hours per day. The exception was in home H4, where an upstairs bedroom window was open for
the majority of nights during the study.
In general, there were few problems encountered in completing the diaries. However, the occupants of
home H1, the only family home, reported that it was difficult to ensure that all window and fan uses were
recorded effectively because it was not always convenient to fill in the relevant part of the diary. The prime
reason for this was that sometimes a window was opened in a different part of the house from where the
diary was kept.
Expecting home occupants to maintain a log of activities is always a potential problem area with this type of
study. One means of ensuring occupant enthusiasm in a larger survey is for an incentive, such as a cash
payment to be available on completion of the monitoring period.
Table 9. Typical extract fan usage and window opening times per day in the homes
Extracting
cooker
hood
Kitchen
extract
Bathroom
extract
Upstairs
windows
Bathroom
window
D’stairs
window
Kitchen
window
H1
0.5–2 hrs
-
0.5 hr
1 hr
2 hrs
-
-
Semidetached
H2
-
-
0.5–2 hrs
Occasional
-
Occasional
0.5–1 hr
End Terrace
H3
-
0.5–1 hr
0.5–1 hr
-
-
-
-
Mid Terrace
H4
0.5 hr
-
0.5–1 hr
2–9 hrs
-
-
-
Bungalow
H5
-
-
-
-
0.5–2 hrs
0.5 hr
2–6 hrs
Description
Code
Detached
0.5 hr
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Monitoring ventilation in homes built before 1995: A pilot study
6
Comparison of measured ventilation rates with BREDEM results
6.1
Comparison of ventilation rates
Table 10 shows the ventilation rates for each house for the three conditions:

BREDEM calculation method 1 using all the assumptions;

BREDEM calculation method 2 with air permeability input;

Measured ventilation rate from this pilot study.
Table 10. Ventilation rates from BREDEM and measured values
Ventilation rates, ach
House type
BREDEM calculations
Method 1
Method 2
Measured
Detached (H1)
0.69
0.59
0.34
Semi-detached (H2)
0.89
0.66
0.29
End terrace (H3)
0.72
0.63
0.37
Mid-terrace (H4)
0.83
0.87
0.55
Bungalow (H5)
0.86
0.89
0.44
As already stated, within BREDEM, there is a general assumption that ventilation rates in homes normally
fall within the range 0.5 to 1.0 ach. From Table 10, it can be seen that the calculated ventilation rates for the
five homes from both Methods 1 and 2 fall within this assumed range. However, the average measured
ventilation rates are generally lower than this range. Thus the ventilation rates seem to be considerably
overestimated by the BREDEM calculation methods.
Only in one case, the mid-terrace (H4) home, did the measured ventilation rate of 0.55 ach fall within the
assumed and calculated range. This home also had the highest air permeability, and when the occupant
diary was analysed in conjunction with the ventilation rate, showed to have the longest periods with open
windows. This is as expected, since the leakiest house with regular use of windows, would be expected to
have the highest ventilation rate. However, this is contrary to the BREDEM calculation methods which
assume that occupants normally tend to open their windows if the infiltration due to air permeability and fan
use is low.
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Monitoring ventilation in homes built before 1995: A pilot study
The measured wind speeds were input into BREDEM and the corresponding ventilation rates calculated.
Table 11 shows the ventilation rates for each house for the three conditions:
 BREDEM calculation method 1 using all the assumptions;
 BREDEM calculation method 1 using the measured wind speed;
 Measured ventilation rate from this pilot study.
Table 11. Ventilation rates calculated by BREDEM using Method 1 and Method 1 using measured
wind speeds, and those measured during the monitoring period.
Ventilation rates, ach
House Type
BREDEM calculations
Method 1
Measured
Detached (H1)
0.69
Method 1 using
measured wind speed
0.5
Semi-detached (H2)
0.89
0.5
0.29
End terrace (H3)
0.72
0.58
0.37
Mid-terrace (H4)
0.83
0.5
0.55
Bungalow (H5)
0.86
0.59
0.44
0.34
The ventilation rates calculated by method 1 using measured wind speed are lower, ranging from 19% to
44% smaller, than those calculated using method 1 using all the assumptions. However, they are still
significantly higher, ranging from 25% to 42% higher, than the ventilation rates measured during monitoring.
The exception being H4 where the ventilation rate calculated by method 1 using measured wind speed is
slightly lower than the measured ventilation rate, of the order 10%. It is also apparent that the two highest
ventilation rates, calculated using method 1 using measured wind speed, are those for H3 and H5 both of
which recorded the highest wind speeds.
6.2
Space heating energy consumption
BREDEM is primarily a method for calculating energy consumption in dwellings. Table 12 compares the
space heating energy consumption using the ventilation rates calculated from the two methods used in
BREDEM and the space heating energy consumption calculated using the average measured values of
ventilation rate.
The results show that for both methods 1 and 2, the space heating energy consumption is overestimated
significantly when compared with that calculated using the measured ventilation rate. It also shows that the
overestimate is least for H4 and H5 which are the two homes with the highest measured ventilation rates.
The bungalow (H5) has higher space heating energy consumption than any of the other homes. This is due
to several reasons:
 The bungalow has no thermostatic temperature control. Therefore, BREDEM assumes that there is
a higher internal demand temperature due to overheating. This would increase the space heating
energy use by about 10%;
 The bungalow has a lower heating efficiency, 50% cf 72%;
 The bungalow has a less efficient shape. This is an inherent problem in bungalows.
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Monitoring ventilation in homes built before 1995: A pilot study
Table 12. Comparison of space heating energy consumptions
Space heating energy consumption (Gj/yr)
Calculated
from
characteristics
Calculated
from
measured
air
permeability
Calculated
from
measured
ventilation
rate
% difference between
calculated value and
measured value
%
%
difference
difference
House type
Method 1
Method 2
Measured
Method 1
Method 2
Detached
(H1)
88.6
83.7
71.8
19.0
14.2
Semidetached
(H2)
70.2
64.7
56.1
20.1
13.3
End terrace
(H3)
99.6
97.9
93.0
6.6
5.0
Mid-terrace
(H4)
23.6
24.1
19.6
17.1
18.7
Bungalow
(H5)
107.3
107.8
100.8
6.0
6.5
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Monitoring ventilation in homes built before 1995: A pilot study
Statistical analysis of data from current and previous studies
Since a sample size of only five homes was considered to be too small to carry out any meaningful
statistical analysis, as part of this study a dataset from previous work was analysed. This was to determine
the optimum numbers of properties that would need to be monitored in a future study so that the results
would be representative of the UK housing stock. It is not possible to check the ventilation rates in all UK
homes therefore a representative random sample must be taken.
As a rule, the larger the sample the better the estimate of the population statistic and ideally the entire
population would be surveyed. However, this is clearly not possible, because the population to be
examined would be extremely large resulting in excessive project costs.
The size of the sample should be big enough to give a sufficient level of precision and accuracy and yet
small enough that project costs are not excessive. Sample size analysis has therefore been used to
calculate the confidence intervals for samples of different sizes.
7.1
A confidence interval approach
The type of sample size analysis conducted depends on how the sample data is intended to be analysed.
In this instance data has been used to estimate a population statistic i.e. the mean ventilation rate in UK
homes. Therefore, a confidence interval approach has been used.
Using ventilation rate data gathered from an earlier study conducted by BRE (Dimitroulopoulou et al, 2004),
sample size analysis was conducted. The sample was of 33 homes across the UK. The mean winter
ventilation rate for the 33 homes was 0.437, with a standard deviation of 0.114.
From these statistics a 95% confidence interval has been constructed. This is given by 0.437 ± 0.040. The
calculations were based on a small sample of 33 homes. In statistical terms this was not large enough to
give an accurate indication of the mean ventilation rates in homes.
The main way to increase the precision of an estimate is to reduce the size of the confidence interval. This
is shown in Figure 1. The dotted vertical line is assumed to be the mean ventilation rate based on winter
ventilation data, with the topmost horizontal line being the current confidence interval. By reducing the size
of the confidence interval it can be seen that the
The smaller the interval the larger the sample size and the precision of the estimate is increasing.
Increasing the precision of the estimate, means increasing the sample size. Table 12 shows the total
sample sizes that would be required to achieve smaller confidence intervals based on winter ventilation
data. Table 12 gives an indication of the level of precision that can be obtained with different sample sizes.
The final decision on which sample size to use is determined by the level of precision that is required in the
estimate. A sample of over 100 would provide a high level of precision, while one of over 500 would give a
very high level of precision.
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It must be noted that these calculations were based on the fact that an estimate of the population statistic
was being sought. If a comparative analysis was required to be carried out on the data, e.g. a t-test, power
analysis should then be carried out. It must be noted that a sample size for a power analysis could be larger
than that for a confidence interval calculation.
0.39
0.40
0.41
0.42
0.43
0.44
0.45
0.46
0.47
0.48
Figure 1. Confidence intervals
Table 12. Sample sizes required for 95% confidence intervals based on winter ventilation data
Interval size
Sample size
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0.040
33
0.010
45
0.005
90
0.004
112
0.003
149
0.002
224
0.001
448
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Monitoring ventilation in homes built before 1995: A pilot study
Summary and conclusions
A small scale pilot study to measure the ventilation rates and air permeability in a small sample of five
homes has been carried out. The ventilation rates measured in the homes have been compared with those
calculated by the two methods used in BREDEM.
Average measured whole house ventilation rates in the homes were found to be between 0.29 and
0.55 ach. The home with the highest measured ventilation rate also had the highest measured air
permeability and by far the longest time periods during which windows were opened. However, this is
contrary to the BREDEM calculation method which assumes occupants open their windows if the infiltration
due to air permeability and fan use is low.
For each of the homes investigated, the measured ventilation rates were considerably lower (usually by a
factor of around 2) than those calculated by either of the methods used within BREDEM. Therefore, for
these homes, if the survey information alone had been used, BREDEM would have over-estimated their
ventilation rates. This, in turn means that the calculated ventilation energy losses would have been higher
and the corresponding SAP rating lower.
The measured wind speed was also used in BREDEM to calculate ventilation rates. The corresponding
ventilation rates were lower than those calculated using BREDEM Method 1, ranging from 19% to 44%
smaller. However, there was still a significant difference between the measured values of ventilation rate
and those calculated by BREDEM using the measured wind speed, ranging from 25% to 42% higher.
In each of the homes the measured internal temperature varied significantly from that calculated by
BREDEM. In most cases the internal temperature in the homes was lower than that calculated by
BREDEM. BREDEM does not use temperature in its ventilation rate calculation. However, temperature
plays an important role in driving natural ventilation in homes.
The measured air permeabilities in the five homes were mostly less than 10 m 3 h-1 at 50 Pa pressure
difference (one of the homes measured 13.3 m 3 h-1). Although three of the homes tested were built in the
1930’s and two in the 1980’s, they compared favourably with the 10 m 3 h-1 permeability requirement
recently specified in the Building Regulations Approved Document L (ODPM 2006). However, the number
of homes i.e. five, investigated here is too small a sample size to be representative of the UK housing stock
for home built prior to 1995.
The measured wind speeds were significantly lower than those calculated by BREDEM. The wind speed
factor in BREDEM is therefore significantly over-estimated in the ventilation calculation. As well as overestimating the ventilation rate BREDEM also over-estimates the space heating energy consumption. When
the measured ventilation rates were input to BREDEM the space heating energy consumption fell by
between 6% to 20% when comparing Methods 1 and 2 to measurements. It was also noted that bungalows
have several reasons for having higher space heating energy consumptions than other types of home.
Since a sample size of only five homes was considered to be too small to carry out any meaningful
statistical analysis, as part of this study a dataset from previous work was analysed. This was to determine
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Monitoring ventilation in homes built before 1995: A pilot study
the optimum numbers of properties that would need to be monitored in a future study so that the results
would be representative of the UK housing stock.
Confidence limits have been determined for studies of different sizes. This was to enable decisions to be
made on the size of a future study, so that results obtained would be representative of the UK housing
stock. This analysis showed that the smaller the 95% confidence intervals, the larger is the sample size
required to give a greater level of precision and hence a better representation of the UK housing stock.
However, if, for example, an average 95% confidence interval of 0.004 is taken from the statistical analysis
carried out here, then it is recommended that the future study should include monitoring of 112 homes.
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Monitoring ventilation in homes built before 1995: A pilot study
Recommendations
The following is recommended for any future study of monitoring ventilation rates in homes.
1. The ventilation rates in homes need to be measured over a reasonable time period to obtain
representative average values. The period of two weeks used in the pilot study proved to be sufficient
for this purpose.
2. Depending on the size of the home, at least four rooms need to be monitored.
3. The temperature dependent aspect of ventilation and the procedure for calculating demand
temperatures needs to be assessed. To allow this internal and external temperatures need to be
recorded throughout the monitoring period in all homes in the study.
4. Calculation of ventilation rates via Method 2 in BREDEM, is only possible if measured air permeability
values are available. Therefore, it is recommended that these measurements are also carried out in any
future study. This will help to build up a database for Method 2.
5. Accurate wind speed data is required. Therefore it is recommended that UK Meteorological Office
weather data should be purchased to provide a better estimate of the regional wind speed. It is also
recommended that detailed wind speed measurements be made at selected homes for comparison
with the data purchased from the UK Meteorological Office. This will allow the site exposure factor to be
assessed and if necessary corrected. It is possible to measure wind speeds on a continual basis.
However, the equipment to do this is expensive especially if each home is to have wind speed
monitored. If there are several homes in one location then one meteorological station to service all the
homes would be sufficient. It must be noted that the there are other important considerations that need
to be taken into account, such as where will the equipment be located and who will be responsible for it
during the monitoring period.
6. The occupant activity diary used for the pilot study needs to be improved and made more ‘occupant
friendly’. In particular, a record sheet for window opening and extract fan usage should be provided for
every room in the home that is monitored.
7. The home characteristics survey should be:
a. Targeted more towards the features and characteristics of the home relevant to ventilation;
b. Simplified with respect to the heating system assessment.
8. The home characteristics survey form should always be completed by the trained investigators that are
conducting the ventilation and air permeability tests.
9. Data on typical occupant activities in the present day, together with information on home characteristics
needs to be collected as part of the future study and compared with such data already used in
BREDEM calculations to determine its validity for the type of home studied.
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Monitoring ventilation in homes built before 1995: A pilot study
10. An incentive to the homeowners will almost certainly need to be included to persuade sufficient of them
to participate in this project, otherwise it may be difficult to obtain the required numbers and different
types of property required.
11. Regarding the sample size required for a future study, this needs to be discussed with the client to
determine the level of precision required before the actual number of homes that should be investigated
can be decided. If, for example, an average 95% confidence interval of 0.004 is taken from the
statistical analysis carried out here, then it is recommended that the future study should include
monitoring of at least 112 homes.
12. BREDEM calculates the space heating energy consumption over the heating season. If it is decided to
carry out a more detailed study, BRE is well set up to carry out the monitoring work over the heating
season 2006-07, assuming that the contract begins by July 2006.
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Monitoring ventilation in homes built before 1995: A pilot study
References
ATTMA, Airtightness Testing and Measurement Association. Technical Standard 1 – Measuring air
permeability of building envelopes, Issue 1, ATTMA, 2006.
Anderson BR, Chapman PF, Cutland NG, Dickson CM, Henderson G, Henderson JH, Iles PJ, Kosmina L
and Shorrock LD (2002). BREDEM-12 Model description 2001 update. BRE, Garston. 2002.
Boardman B, Killip G, Darby S and Sinden G (2005). 40% House. Oxford University Press. 2005.
Dickson CM, Dunster JE, Lafferty SZ and Shorrock LD (1996). BREDEM: Testing monthly and seasonal
versions against measurements and against detailed simulation models. Building Services Engineering
Research and Technology. Vol 17 No.3. 1996.
Defra (2005). The Governments Standard Assessment Procedure for Energy Rating of Dwellings 2005
edition. Published on behalf of Defra by BRE, Garston. 2005.
Dimitroulopoulou C, Crump D, Coward SKD, Brown V, Squire R, Mann H, White M, Pierce B and Ross D
(2004). Ventilation, air tightness and indoor air quality in homes in England built after 1995: Part 1 - Final
report and BRE report publication draft. BRE Report number 202503, BRE Garston, 2004.
ODPM (2005). Approved Document L1, The Building Regulations. The Stationery Office, 2006.
Stephen RK. (2000). Airtightness in UK dwellings. BRE, Garston. 2000. IP 1/00
Walker RR and White MK. (1995). The passive gas tracer method for monitoring ventilation rates in
buildings. BRE, Garston, 1995. IP 13/95
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Monitoring ventilation in homes built before 1995: A pilot study
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11
Acknowledgements
Funding for this study from the Energy Saving Trust (EST) is gratefully acknowledged. Input as appropriate
from the following BRE staff is gratefully acknowledged.

John Henderson and Gavin Hodgson for carrying out the BREDEM calculations;

Lorna Hamilton for the statistical analysis of data carried out for this study;

Stuart Upton for relevant input to this report.
BRE Client report number 229615
Commercial in confidence
© Building Research Establishment Ltd 2006
Monitoring ventilation in homes built before 1995: A pilot study
Appendix A – Detailed description of BREDEM
BRE’s Domestic Energy Model (BREDEM), (Anderson et al, 2002) is the UK’s most widely used calculation
tool for estimating the energy consumption for a dwelling, based on its physical characteristics and the way
in which it is heated. BREDEM aims to provide an energy calculation that is substantially better than simple
procedures such as design heat loss, but is considerably simpler to use than detailed simulation models. It
was developed in the 1980s and is widely used. The software can be used to calculate:

Estimated energy requirements in different dwelling types;

Estimating possible running costs;

Ensuring the most appropriate selection of measure when upgrading existing dwellings;

Estimating the savings from implementing energy efficiency measures;

Calculating the energy rating for a dwelling;

Estimating the internal temperatures for a given energy input.
BREDEM underpins the Standard Assessment Procedure (SAP) (Defra, 2005) for energy rating dwellings
which is used for checking building regulation compliance and calculating a dwellings SAP rating.
BREDEM requires a reasonable estimate of the ventilation rate (air change rate) as an input parameter (to
calculate the overall heating requirement). This ventilation rate cannot be assessed from building plans or a
site survey. At present BREDEM estimates an infiltration rate (ach) and then adds an estimate for occupant
ventilation. When the infiltration rate is low BREDEM assumes that the occupants will open windows to
provide additional ventilation if required, conversely when the infiltration rate is high BREDEM assumes the
windows are kept shut. The infiltration rate also depends on the local wind speed and building exposure.
The total ventilation rate, nτ (ach), is the sum of the occupancy ventilation, n o, and the infiltration, ni, which
has been adjusted to take into account the building exposure:
nτ = ni + no
Equation 1
Two methods are available to calculate the ventilation rate used by BREDEM:

Method 1: This estimates the leakage effects of the various building components, such as
doors, windows and walls. This is based on the information obtained from the site and home
survey questionnaire and on data built- in to BREDEM.

Method 2: uses the result of an air permeability, the building infiltration can be estimated as
Q50/20, where Q50 is the air leakage test result.
BRE Client report number 229615
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© Building Research Establishment Ltd 2006
Monitoring ventilation in homes built before 1995: A pilot study
Method 1
The infiltration rate can be evaluated by adding together estimates of the leakage effects of the various
characteristics of the dwelling. The characteristics are obtained from the home survey questionnaire
completed by the investigator. There is air leakage through the building fabric components (such as, walls,
doors and windows) and through forced ventilations from fans, flues and chimneys. Additionally an extra
0.1 ach is added to the ventilation rate for each storey above ground level, this is to allow for the stack
effect.
The values for infiltration rate through the building fabric and the flow rates for fans, flues and chimneys are
tabulated in the BREDEM model description (Anderson et al, 2002). The total flow rate is calculated by
assessing the number of components of each type and multiplying by the corresponding airflow rate. The
total is then converted in to an equivalent air change rate by dividing by the volume of the dwelling. The
infiltration rate, ni (ach), is given by:
ni = Σ Li + Σ W i. ρi + ΣNi. Fi/VT
Equation 2
where:
o
Li is a building fabric leakage component (ach)
o
W i is a window leakage component (ach)
o
ρi is the proportion (by area) of windows of a given type
o
Ni is the number of items (fans, vents etc) of a given type
o
Fi is the flow rate for items (fans, vents etc) of a given type (m 3/h)
o
VT is the total house volume.
Method 2
If the building has been pressure tested and the air permeability result is available the dwelling infiltration is
estimated as Q50/20, where Q50 is the air permeability result in m 3/h per m2 dwelling envelope area at 50
Pa. During the test fans, chimneys, flues and vents are closed or sealed so their contribution needs to be
included. Therefore, the infiltration rate is given by:
ni = Q50/20 + ΣNi. Fi/VT
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Equation 3
© Building Research Establishment Ltd 2006
Monitoring ventilation in homes built before 1995: A pilot study
Wind effects

The local wind speed will affect the ventilation rate for a dwelling. BREDEM accounts for this by
employing a series of factors to adjust ni. These factors are:
o
vr. the regional windspeed.
o
ShE. the site exposure factor.
o
vsite = vr. ShE. vsite the site windspeed.
o
fwind = vsite/4 the wind factor.
o
ShDW . the dwelling exposure factor.
The overall infiltration rate adjusted for exposure is given by:
ni = fwind. ShDW . ni (ach)
Equation 4
Occupancy effects
Finally there is an adjustment to be made for occupant ventilation. This is based on occupants deliberately
ventilating their dwelling. It is assumed that this takes place if the infiltration rate is less than 1 air change
an hour. The occupant ventilation (when there is no mechanical ventilation present), no, is given by:
o
no = 0.5 – ni +0.5. ni2
for ni < 1.0
o
no = 0
for ni ≥ 1.0
The above two sets of calculations and assumptions gave two values of calculated ventilation rate for
comparison with the measured value of ventilation rate for each home.
BRE Client report number 229615
Commercial in confidence
© Building Research Establishment Ltd 2006