Download 12 – Condensation in Buildings

16293: Environmental Engineering Science 1
12 – Condensation in Buildings
Condensation
• Condensation in buildings is a very common problem in the UK and especially
Scotland and causes unhealthy living conditions and damage to materials and
structures.
• Condensation in buildings occurs where moist air meets a cold surface and is
cooled below its dew point temperature, the vapour close to the surface becoming
saturated and excess vapour turning to liquid.
Types of Condensation in Buildings
In buildings there are two types of condensation:
A) Surface condensation. As the name suggests this occurs at the surface of the
material. Surface condensation can occur in both warm and cold conditions, the
processes being slightly different:
• Warm weather condensation. In summer the outside air is warm and can
therefore hold a lot of moisture. Inside a building, interior surfaces may still be
cool due to thermal inertia effects or due to air conditioning inside the
building. When the warm outside air contacts these cool surfaces,
condensation may occur if the surface is cooler than the dew point temperature
of the air.
• Cold weather condensation. During cold weather the outside air does not
hold much moisture. However, when brought into a building and heated, the
capacity of the air to hold moisture increases. This moisture is readily
provided by respiration, cooking, washing and drying of clothes. If internal
surfaces of the building are cold and below the dew point (e.g. due to poor
insulation) then the moisture in the air will condense.
16293: Environmental Engineering Science 1
•
Warm weather condensation
Cool massive
surface T<Tdb
Warm moist air
Condensation
Cold surface
T<Tdb
Cold dry air
Condensation
Vapour addition
from occupancy
Cold weather condensation
16293: Environmental Engineering Science 1
NOTE that the moisture generated in one part of a building may migrate
and condense in another part of the building.
• For example, air is often heated and humidified in the kitchen and may
then migrate to a cool bedroom, where the excess water vapour will
condense on a cool surface such as a window.
B) Interstitial condensation: Interstitial condensation occurs inside a material.
Vapour can pass through porous building materials. If the dew point temperature
occurs within that material then, as with surface condensation, the vapour will
condense.
condensation
Tdb
Tdp
inside
outside
water vapour
• Interstitial condensation can reduce the thermal effectiveness of insulation
material, and damage structural steel and concrete. It will be covered in detail in
the next lecture.
Factors Influencing Condensation
• The major factors influencing condensation in buildings are:
-
Moisture production from sources inside the building
-
Air and structural temperatures
-
Ventilation
16293: Environmental Engineering Science 1
Moisture Sources:
• Most of the moisture inside buildings comes from the occupants, with the
average family producing 20-30kg of moisture per day through respiration,
cooking, washing and the drying of clothes.
• Un-flued gas heaters and cookers also produce large amounts of vapour.
Temperature:
• If the temperature of a surface is at or below the dew point of the room air
then condensation will occur.
• This usually happens at windows or cold bridges (points of poor insulation in
a wall).
• Structure weight will also affect condensation. Heavy structures are slow to
heat up and can therefore be prone to both warm weather and cold weather
condensation.
Ventilation:
• Cool outside air holds less moisture than warm moist air, so if cool air in
introduced into a building its relative humidity will be low when heated,
provided no moisture is added to this air.
• Replacing moist air inside the building with cold air from outside can
therefore reduce condensation risk. This of course increases the amount of
heating energy required.
• Note that as buildings have become more airtight over the years so the
incidence of condensation has increased.
16293: Environmental Engineering Science 1
Mould Growth
• Dampness and mould growth are recognised as major problems affecting a
significant proportion of houses in the UK. Approximately 2.5 million UK
residences (250,000 in Scotland) are affected. It is estimated that the cost of
repairing the damage caused by timber decay in the UK housing stock is about
£400M per annum.
• Apart from aesthetic considerations there is now considerable epidemiological
evidence to support the view that mouldy housing has a detrimental effect on the
physical and mental health of children and adults residing in such environments.
This is a cause for concern, especially since many individuals spend up to 90% of
their day indoors
• High levels of airborne spores may occur due to growth of fungi on walls and
furnishings in addition to other internal/ external sources. Data from the 1991
Scottish housing condition survey indicated that around 12.3% of Scottish houses
are affected, with inadequate heating, insulation and ventilation cited as the
principal causal factors.
• Based on an examination of the incidence of moulds within Scottish dwellings,
and an epidemiological assessment of the inhabitants, it was concluded that there
exists a significant correlation between the incidence of mould spores and illhealth. Mycotoxins produced by moulds are implicated in a range of problems
including respiratory problems and allergic reactions.
• Requirements for mould growth in buildings are:
-
nutrients: found in the materials which make up or are deposited on indoor
surfaces
-
oxygen
-
light
-
suitable temperature (5-25oC)
-
moisture: the RH at a surface is the best indicator of moisture available for
mould growth φ =
Ps @ T surface
Pss @ T surface
16293: Environmental Engineering Science 1
• Contrary to popular belief, condensation does not need to occur at a surface for
mould growth to occur, as moisture in the form of water vapour can be absorbed
directly from the air.
• Moisture can also be obtained from the surface itself.
• Moulds grow in varying conditions: xerophilic moulds grow at relatively low
moisture values, while hydrophilic moulds grow at much higher moisture levels.
• Generally moulds can appear on a surface if the local surface relative humidity
remains above 75% for considerable periods of time.
• Different moulds appear depending upon the prevailing RH at the surface.
RH
Hydrophilic
95%
75%
Xerophilic
20°C
Tdb
Remedies
• The solution to problems relating to condensation and mould growth is usually
obtained by using a combination of remedies.
• The remedies act to both increase surface temperatures and reduce the quantity of
moisture in the air, the net effect being to decrease the relative humidity RH:
• Ventilation: removing moist air from the building and replacing it with cooler,
drier air. Ventilation is most effective when used near the source of the moisture,
e.g. extracts in bathrooms and kitchens.
16293: Environmental Engineering Science 1
• Heating: increasing internal temperatures so that surface temperatures are greater
than the dew point. Heating also helps the air hold more moisture, which can then
be removed by ventilation. The pattern of heating is also important: low level
continuous heating is better than short heating bursts followed by unheated
periods, when surface temperatures can drop below dew point.
• Insulation: this reduces the loss of heat from a building and also increases the
temperature of internal surfaces. Note that insulation placed at the inside surface
of a wall will reduce the temperature of the structure behind and a vapour barrier
may be needed to eliminate the possibility of interstitial condensation.
• Actions such as treating surfaces with fungicidal or water repellent paints merely
treat the symptoms, not the causes of dampness and condensation.
Example
Assume that air inside a room has a relative humidity of 30% and is at a temperature
of 20oC, but that there is a cold surface at 5 oC. Assume the overall pressure, P, is 100
kPa.
From tables, the saturated vapour pressure at 20oC, Pss, is 2.339kPa; at 5oC, it is
0.8721kPa.
Determine whether condensation will occur and whether mould growth is likely
Answer:
Moisture content is found from:
g=
0.622 Ps
P − Ps
However, Ps is unknown, but Pss is known, so use the following equation:
g=
0.622φPss 0.622. ×0.3 ×2.339 0.44
=
=
= 0.0044 kg/kgda
P − φPss
100 − 0.3 ×2.339
99.23
16293: Environmental Engineering Science 1
Assume the air is cooled sensibly to the temperature of the surface. P remains the
same. The relative humidity at the surface can be determined from
φ=
gP
0.0044 ×100
0.44
=
=
= 0.8 = 80%
( 0.622 + g ) Pss ( 0.622 + 0.0044) 0.8721 0.55
a) Condensation will occur if RH = 100%, so no condensation will occur.
b) Mould will grow at a surface if the relative humidity is > 75%. Hence there is a
risk of mould growth at this surface if these conditions persist.
Alternatively, the analysis can be done using the psychrometric chart.
16293: Environmental Engineering Science 1
Tutorial Questions – Condensation in Buildings
1. Give a brief description of what condensation actually is.
2. Indicate why it is important to keep the average relative humidity in a room below
about 70%.
3. Indicate locations in a house where condensation is most likely to occur. How can
you recognise from looking at a wall, whether stains or marks are caused by
condensation or by rain penetration?
4. Why do we want to avoid condensation in a building?
5. On which factors does the occurrence of condensation in houses depend?
6. Which are the two primary strategies to remedy condensation problems?
7. Why should we be careful with draughtproofing for energy conservation, if a
condensation problem exists? One way to reduce condensation is by increased
heating. Why is this strategy not advisable when not accompanied by adding extra
insulation?
8. What do we mean by “warm weather condensation”?
9. What do we mean by “superficial condensation”?
10. What do we mean by “interstitial condensation”, and why do we want to avoid it?
11. The air in an air-conditioned room is maintained at a dry-bulb temperature (Tdb) of
20°C and at a relative humidity (RH) of 50%. The outside window of the room is
partly double-glazed and partly single-glazed. At night, The outside temperature
falls to -5°C. Determine:
i)
ii)
iii)
the rate of heat flow through both the single- and double-glazed parts of
the window;
whether condensation will occur on the inside of the single-glazed part of
the window;
whether condensation will occur on the inside of the double-glazed part of
the window.
DATA
U-value single-glazing
U-value double-glazing
Rsi
Tdew-point for Tdb = 20°C, RH = 50%
6.0
3.0
0.12
9.1
W/m2 K
W/m2 K
W/m2 K
°C
[150 W/m2, 75 W/m2, condensation on single-glazing, no condensation on
double-glazing].