Download measurement of thermal performance of building envelope

MEASUREMENT OF THERMAL PERFORMANCE OF BUILDING
ENVELOPE - A COMPARISON OF SOME INTERNATIONAL
LEGISLATION
Hongyu LI (1), Laverick MALCOLM (1)
(1) AECOM Ltd.
Abstract:
The purpose of the sustainable building envelope is to create a comfortable thermal
internal environment and enhance the overall building energy performance. This is
accomplished by regulating air temperature, radiant heat loss or gain, solar heat gain, relative
humidity and air movement with use of building energy standards or codes of practice.
In the current design standards worldwide there are two standards popularly used to
measure thermal performance of a building envelope: one is thermal insulation standards in
cold climates with the use of U-values, the other is methods of control of solar heat gain
through the building envelope for buildings in hot climates with the use of Overall Thermal
Transfer Value (OTTV). Comparison of these two methods adopted in various countries has
presented understanding of various thermal performance standards for building envelope.
Air-tightness was not taken into account in the measurement for thermal performance of
building envelope before the 21st-century. In reality air permeability of a building envelope
has a great influence on energy performance of a building. Air-tightness standards adopted in
various countries’ legislation has been explored and discussed.
Performance based approach to analyse the building energy consumption in building
envelope and other building service systems is the trend to achieve building energy efficiency
goals.
Keywords: Thermal performance, Building envelope, OTTV, U-values, Energy consumption,
air-tightness
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1.
INTRODUCTION
The building envelope is the interface between the interior of a building and the outdoor
environment and provides the enclosure that allows the internal environment to be controlled.
The building envelope comprises the walls, windows, doors and roof(s) which enclose the
building interior. A climate-responsive building envelope deliberately uses a combination of
shading, high performance windows, and the thoughtful placement of windows to enhance the
comfort and energy performance of the building. Additionally opaque wall and roof insulation
and reflectance options will greatly impact the energy demand and occupant comfort inside
the building.
Two different standards of measurement of the thermal performance of buildings are
considered. One is the “overall thermal transmittance value” (OTTV) method and the other is
“U-value”, and a comparison of each standard used in various countries has been explored.
The designed OTTV values and U-values adopted in practical project cases are presented and
the findings from the comparison study are discussed.
Air permeability of a building envelope has a great influence on energy performance of a
building. Air-tightness standards adopted in various countries’ legislation has been explored
and discussed.
With good standards of OTTV and U-values as well as air tightness of a building, its
thermal performance will be enhanced.
2.
MEASUREMENT OF THERMAL PERFORMANCE OF BUILDING ENVELOPE
2.1
Hot climates: the OTTV method
OTTV is an index for comparing the thermal performance of buildings. It is a measure of
the average heat gain into a building through the building envelope and consists of three
major components:
• Conduction through opaque walls,
• Conduction through window glass, and
• Solar radiation through window glass.
The usual practice is to have two sets of OTTV – one for the exterior walls and the other
for the roof. The general form of OTTV equation for an external wall is [2]:
Qwc + Qgc + Qsol
OTTV =
Ai
( Aw × Uw × TDeq) + ( Af × Uf × DT ) + ( Af × SC × SF )
Ai
=
109
= (1 − WWR ) × Uw × TDeq + WWR × Uf × DT + WWR × SC × SF
OTTVi = overall thermal transmittance value of the external wall (W/m2)
Qwc = heat conduction through opaque walls (W)
Qgc = heat conduction through window glass (W)
Qsol = solar radiation through window glass (W)
Aw = area of opaque wall (m2)
Uw = U-Value of opaque Wall (W/m2.K)
TDeq = equivalent temperature difference (K)
Af = area of fenestration (m2)
Uf = U-value of fenestration (W/m2.K)
DT = temperature difference between interior and exterior (K)
SC = shading coefficient of fenestration = SCwin × SSF
SCwin = shading coefficient of window glass (dimensionless)
SSF = solar shade factor of external shading devices (dimensionless)
SF = solar factor of fenestration (W/m2)
Ai = gross area of the walls (m2) = Aw + Af
WWR = window to wall area (gross wall area) = Af / Ai
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The OTTV of the whole exterior wall is given by the weighted average of the OTTV’s of
individual walls at different orientations as follows:
OTTVwall =
∑ (OTTV × A)
∑A
Where OTTVwall = OTTV of the whole exterior wall (W/m2)
The approach for calculating the roof OTTV is similar to that for walls. ASHRAE was the
first body to propose the OTTV method [5][6], but subsequently changed to
performance-based energy budgets generated through computer simulations [7]. In Asia,
Singapore, Hong Kong and other countries including Indonesia, Malaysia, Philippines and
Thailand adopted an OTTV standard in the 80s based on the ASHRAE early Standards with
some refinements to suit local climate and construction practices.
Comparison of OTTV standards
Table 1: Comparison of OTTV standards [2]
Singapore
Malaysia
Thailand
Jamaica
HK
13o41’ N
Bangkok
Philippine
s
14o35’ N
Manila
Latitude
(city)
1o20’ N
Singapore
Year
adopted
Status
OTTV
limits for
walls
OTTV
limits for
roof
1979
3o7’ N
Kuala
Lumpur
1989
17o56’ N
Kingston
22o18’ N
HK
1992
1993
1992
1995
Mandatory
Voluntary
Mandatory
Mandatory
Mandatory
45
45
45
48
55.1 – 67.7
Tower: 35
Podium: 80
(average for
walls and
roof)
45
(max.
U-value if
no
skylights)
TDeq
for 10-15
walls (K)
25
(max.
U-value if
no
skylights)
19.1α
25
(max.
U-value if
no
skylights)
9 - 18
TDeq
for 16-24
roof (K)
DT
for 5
walls (K)
16-24
12-32
25 (max.
U-value if
no
skylights)
12.65
α
(office)
5.4
α
(hotel)
---
Neglected
5
3.35office
1.10-hotel
111
20
Varies with 1.4 - 7.5
α
Varies with 7.9 -18.6
α
Varies by Neglected
location
Table 1 continued
DT for roof
(K)
Average
SF for
walls
Average
SF for roof
Consider
exterior
shading?
Daylightin
g credits?
5
Neglected
5
---
130
194
160
320
488
370
161-office
142 -hotel
151 -store
---
Yes
Yes
Yes
No
Yes (10% N/A
or 20%)
Varies by Neglected
location
372
160
435
264
No
Yes
Yes
Yes (10%)
Yes (7.5% No
or 30%)
Notes: 1. α = solar absorptivity
2. Average SF for walls is calculated for the four principal directions
(N,E,S,W)
It can be found from the above comparison that there is similarity of OTTV limits as set
out by different countries even with use of different values and parameters. The relative
importance of the components in OTTV may be studied from the heat gain values calculated
using the coefficients TDeq, DT and SF, respectively. The solar component is the most
significant in the OTTV and may account for a portion from 44% (in the Singapore’s 1979
method) to 87% (in the Hong Kong 1995 method).
Hong Kong introduced a Building (Energy Efficiency) Regulation in 1995 [1] which has
set out the maximum OTTV criteria for commercial buildings as follows:
a) In the case of a building tower; the OTTV should not exceed 35W/m2 ; and
b) In the case of a podium; the OTTV should not exceed 80 W/m2.
In 2000, the building envelope standard was revised, with the OTTV criteria reduced from
35 to 30 W/m2 for the tower, and from 80 to 70 W/m2 for the podium [3].
Performance-based building energy code (PB-BEC) [4][10] has been subsequently
developed and implemented in Hong Kong in April 2003. The performance approach focuses
on the total energy consumption of a building design, which is termed as the DESIGN
ENERGY in comparison with the prescriptive energy consumption required for a
corresponding reference building, which is called the ENERGY BUDGET. The PB-BEC is
deemed to be met when DESIGN ENERGY ≤ ENERGY BUDGET with use of the following
key building envelope data:
Ÿ Glass wall area (m2)
Ÿ Window-to-wall ratio
Ÿ Shading coefficient of windows (shall be not less than 0.25)
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Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Gross roof area (m2)
Skylight-to-roof ratio
Shading coefficient of skylights
OTTV of exterior walls (W/m2)
OTTV of roof (W/m2)
External shading device provided
In Singapore, since 1979, the Building Control Regulations [8] had prescribed that all
air-conditioned buildings must be designed to have an Overall Thermal Transfer Value (OTTV)
of not more than 45 W/m2. A major review of the OTTV standard was carried out by the
Building and Construction Authority (BCA) in the early 2000 to provide a more accurate
measure of the thermal performance of building envelope: the ‘Envelope Thermal Transfer
Value’ (ETTV) [9] and ‘Roof Thermal Transfer Value’ (RTTV). The maximum permissible
ETTV as well as RTTV has been set at 50 W/m2 [9][11]
The Code on Envelope Thermal Performance for Buildings in Singapore has been
implemented in April 2008 [9]. It has largely adopted the BCA Green Mark’s criteria as the
compliance method in assessing the environmental performance of a building development,
covering the following Envelope Thermal Performance Standards:
i.
ETTV for air-conditioned non-residential buildings
ii.
RTTV for air-conditioned non-residential buildings (with skylight)
iii.
Residential Envelope Transmittance Value (RETV) for residential buildings
iv.
Roof insulation for air-conditioned non-residential buildings (without skylight) and
residential buildings
It can be seen that although the OTTV method for building envelope has its limitation, i.e.
it does not account for the interactions between the envelope, internal gains, and equipment
efficiency, however it forms a prescriptive standard for measuring the energy performance of
a building envelope and is easier for developer and building owners and professionals to
comply with. It can be co-used with other energy performance criteria and standards for
building services systems designed for a building development. The trend nowadays is to
adopt a performance based approach to analyse the building energy consumption in building
envelope and other building service systems including lighting, air-conditioning, electrical
installations and in turn to optimize the energy efficiency of a building as a whole.
2.2
Cold climates: U-Values
Conservation of Fuel and Power in New Buildings Other Than Dwellings, L2A, UK,
[12] which took effect in April 2006 and deals with the energy efficiency requirements in the
Building Regulations 2000. In this document, the design limit for envelope standards is
measured by U-values as shown in the following table:
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Table 2: Limiting U-value standards (W/m2K), L2A, UK
Element
(a) Area-weighted
average
(b) For any individual
element
Wall
0.35
0.70
Floor
0.25
0.70
Roof
0.25
0.35
Windows, roof
windows, rooflights and
curtain walling
2.2
3.3
Pedestrian doors
2.2
3.0
Vehicle across and
similar large doors
1.5
4.0
High usage
doors
entrance
6.0
6.0
Roof ventilators (inc.
smoke vents)
6.0
6.0
Notes:
1. Excluding display windows and similar glazing. There is no limit on design
flexibility for these exclusions but their impact on CO2 emissions must be
taken into account in calculations;
2. The u-values for roof windows and roof-lights in this table are based on the
U-value having been assessed with the roof window or roof-light in the
vertical position. If a particular unit has been assessed in a plane other than
the vertical, the standards given in this approved document should be modified
by making an adjustment that is dependent on the slope of the unit following
the guidance in BR 443 [13].
6
Energy Efficiency Building Standards in Nordic Countries have set out U-value limit in the
following table [14]:
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Table 3: Limiting U-value standards (W/m2K) in Nordic Countries
Overall U-values1
Component U-values
Ceiling
Wall
Floor
Windows
Overall
Average
Denmark2
0.15
0.20
0.12
1.5
0.77
0.77
Finland3
0.15
0.24
0.15-0.24
1.4
0.91
1.01
0.18
0.29
0.29
1.7
1.104
0.13
0.18
0.15
1.2
0.70
0.18
0.22
0.18
1.6
0.905
0.13
0.18
0.15
1.3
0.72
Norway3
Sweden
1.
2.
3.
4.
5.
0.80
0.72
Overall U-values are calculated in order to compare across countries. It sums
the U-values from the ceiling, wall and floor, and then adds 20% of the
window value.
The values correspond to requirements for renovations; new buildings have
lower component U-values, but a more stringent energy performance
standard.
The two sets of values correspond to two different ways to calculate
compliance, either based on U-values alone or an overall frame value with
some maximum U-values.
This overall value results when the U-values are combined with heat
recovery from exhaust air and meeting air-tightness requirements.
This overall value results when the U-values are combined with a maximum
energy frame value for the whole building.
Source: Laustsen, Jens, “Energy Efficiency Requirements in Building
Codes, Energy Efficiency Policies for New Buildings”, IEA working paper,
forthcoming.
U-value comparison
Scandinavian countries apply more stringent U-values than in UK, about 50 – 60% of the
UK’s U-values. This implies that different U-values are to be applied to the envelope of
buildings which are located in different climatic environments in order to achieve similar
energy performance.
The U-values for sample buildings in HK and Singapore are shown in Table 4 to compare
with the maximum permissible U-values as set out in other countries’ standards to have an
understanding of the building envelope energy performance.
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Table 4: U-values for three buildings in HK/Singapore
Project Description
Project
Location
A 10-storey hospital
with concrete walls
and external tiles,
concrete roof with
insulation and screed
laid to fall; strip
windows with
double-glazed sealed
units
HK
A 32-storey concrete
frame and curtain wall
office building with 4
podium floors.
HK
A 45-storey concrete
frame double glazed
fully curtain wall
office building with
5-storey podium.
Singapore
U-value
of
Opaque
Wall
(W/m2K)
U-value
of
Window
glass
2.271
2.5
U-value
of Roof
Designed
OTTV values
(W/m2K)
(W/m2)
(W/m2K)
0.404
15.9
Target value is
18.
0.45
(spandrel)
1.8
1.53
(spandrel)
2.48
(glass)
0.31
18.1 (Tower)
23.9 (Podium)
0.3
42.21 (ETTV)
40.05
(RTTV)
In comparison with the European U-value criteria Hong Kong and Singapore buildings tend
to have much higher U-values for opaque walls due to less use of thermal insulation materials.
Normally 100-125mm concrete external walls are used in the opaque wall construction. For the
hospital building which has been designed under a government brief a much lower OTTV has
been achieved through the configuration of the envelope that adopts a lower window–to-wall
ratio.
For medium to high-rise, high-end commercial buildings curtain-wall construction is
generally adopted. On these buildings opaque walls are made of composite spandrel panel
which includes external glazing and thermal insulation layers, whilst curtain walling in general
adopts double-glazed low-e glazing with high thermal performance. The OTTV values are
designed to higher standards of international benchmarking. A further influence is the fact that
most of the developers are now adopting Green Building standards such as HK-BEAM. LEED,
BREEM, Singapore Green Mark and China Three Stars. Another perspective is from the tenant
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requirement to occupy buildings that have high energy performance or low energy consumption
to fulfil their corporate responsibilities.
2.3
Air-tightness
The concept of air-tightness was introduced into the Building Regulations in England and
Wales in the 2002 revision of: ‘Approved Document L’. In this document, a maximum
allowable air leakage rate of 10m3/h/m2 was introduced for all buildings other than dwellings
greater than 1000m2. The 2006 update to Part L [12] has extended this further. All buildings
must be pressure tested, unless the floor area is less than 500m2, when a default value of
15m3/h/m2 can be used. For buildings over 500m2, a maximum reasonable design limit for air
tightness is 10m3/h/m2 [12]. Air-tightness is measured in m3 of air per hour per m2 of building
envelope at an applied pressure of 50 Pa.
In the UK, air Pressurisation/depressurisation testing [15][16] can be undertaken to
establish leakage rates in:- Whole Buildings: Whole building pressurization tests can establish envelope leakage
rates for comparison with client specification or Building Regulation Part L Standards.
- Components: Components include doors windows, blockwork, cladding systems etc.
samples can be tested to establish overall leakage rates / leakage per sq metre / leakage
per linear metre of junctions and joints.
- Enclosures: Enclosures include computer rooms, switch rooms, clean rooms and floor
voids etc Specialist enclosures which need low levels of or residual air leakage.
Areas of residual leakage can be identified to enable remedial sealing to be undertaken
if necessary.
The recommended air leakage specifications for different building types are shown in the
following table:
Table 5: Air Leakage Index, UK, Air Leakage Index (m3/hr/ m2 @ 50Pa)
Best Practice
Normal
Offices (naturally ventilated)
3.0
7.0
Offices (mixed mode)
2.5
5.0
Offices (air conditioned/low energy)
2.0
5.0
Factories/Warehouses
2.0
6.0
Building Type
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Table 5 contiued
Superstores
1.0
5.0
Schools
3.0
9.0
Hospitals
5.0
9.0
Museums and Archival Stores
1.0
1.5
Cold Stores
0.2
0.35
Dwellings (naturally ventilated)
3.0
9.0
Dwellings (mechanically ventilated)
3.0
5.0
The alternative common way to normalize building airtightness [17] is to calculate the
number of times per hour that the total volume of the enclosure is changed, when the
enclosure is subjected to a 50-pascal pressure difference, n50. The Air-tightness
measurements [17] are classified as N50=1.5, a high air-tightness; N50=1.5-3, an average
air-tightness; N50=3-15, a low air-tightness.
Requirements of air-tightness in different countries are indicated below [17]:
Sweden
n50 < 3.0 1/h
Norway
n50 < 4.0 1/h
Germany
n50 < 3.0 1/h,
n50 < 1.5 1/h if an automatic ventilation system is used
Canada
n50 < 1.5 1/h
Finland
n50 < 3.0 l/h
Stringent air-tightness standards have been applied in the above countries to ensure a
better performance of building envelope, in turn a better building energy performance.
In Hong Kong and Singapore, the similar standard on building’s air-tightness has not
been adopted, however statutory control on curtain wall, window and window wall
systems used in buildings have been in place since 1980s. For commercial buildings fully
enclosed with curtain wall and air-conditioned, the building envelope air-tightness
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performance is controlled by the standards of curtain wall design and ensured by curtain
walling mock-up tests required by Building Legislation [18]. The overall thermal
performance of building envelope of curtain walled building, including OTTV and
air-tightness, can be ensured to a higher level. For low-rise, non curtain wall or partially
curtain-walled buildings there is a need to have a performance standard to ensure high
performance.
Furthermore, in Asia there are design concepts in architecture that deliberately open up
the building envelope to the external environment for the following reasons:
Ÿ Aesthetics.
Ÿ 24 hour public right of way that pass through the building.
Ÿ Practicality to allow the mass movement of people such as the underground rail
entrance / exit.
Ÿ As a means to encourage pedestrian entry such as retail malls.
Designs for such buildings have adopted alternative energy measures using door air
curtains to stop air transfer from outside to inside. Alternatively some designs will
over-pressurise the internal space to prevent the entry of hot and humid outside air, this
will inevitably cause leaking cool air into the street and consequently increasing the
operational energy use.
Considering a cubic metre of outside air at a condition of 32o C and 80% relative
humidity the total energy (enthalpy) to reduce the temperature and the moisture content to
a supply condition of 22 oC is 48.5 kW, of which 36.5kW is used for reducing the
moisture content and 12kW for the temperature. Air leakage into the buildings through
openings in the building envelope generates a high consumption of energy. Therefore
sealed buildings are more environmental friendly in terms of performance and energy
conservation.
3.
CONCLUSIONS
The thermal performance of the building envelope is generally measured by two standards,
OTTV method adopted in hot climates and U-value method in cold climates. OTTV is a
measure of the average heat gain into a building through the building envelope and consists of
three major components: Conduction through opaque walls, Conduction through window
glass, and solar radiation through window glass. The usual practice is to have two sets of
OTTV – one for the exterior walls and the other for the roof. An OTTV standard limits the
maximum allowable thermal transfer value, i.e. heat gain in cooling-dominant locations, in
W/m2. The OTTV method has been used in a number of Asian countries. There is a trend to
form a set of comprehensive Building Energy Codes with use of the existing prescriptive
codes including the OTTV code and the performance-based building energy code, which will
address energy efficiency requirements of buildings in a systematic and holistic way.
For the UK and other European countries the thermal performance of the building
envelope is measured by U-values. Scandinavian countries apply more stringent U-values
119
than in UK, about 50 – 60% of UK’s U-values. This demonstrates that different U-values are
to be applied to the envelopes of buildings which locate in different climatic environments in
order to achieve similar energy performance.
This paper has demonstrated from the designed parameters (U-values and OTTV values) of
practical projects in Hong Kong and Singapore that buildings tend to have much higher
U-values for opaque walls due to less use of thermal insulation materials. However for the
top-tier high-end commercial buildings in these places international standards and
benchmarking have been applied to achieve better and more efficient thermal and energy
performance of the building envelope. There is a big space for the Asian countries to adopt the
latest trends in sustainable building design and develop performance based and
comprehensive legislation to drive sustainability in the construction industry.
Air-tightness, in reality, has a great influence on energy efficiency of the building envelope.
The air-tightness standard has been implemented in the UK’s latest building legislation so do
the other European countries. In the UK, air pressurisation/ de-pressurisation testing must be
undertaken to the buildings built to comply with the relevant national and international
standards. These will not only ensure the design to the respective standards but more
significantly the practical construction and workmanship to the performance requirements.
Hong Kong, Singapore and other Asian countries that have not adopted the air-tightness
criteria in their building legislation shall take prompt action to be more pro-active and
responsive to energy-saving and sustainable development as the trend worldwide nowadays.
An integrated sustainable building requires better thermal and energy performance of
building envelope. Efficient interaction of envelope choices with other building systems shall
be pursued to achieve the ultimate performance and comfort of the building.
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on
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Thermal
Performance
for
Buildings,
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[13] BR 443: Conventions for U-value calculations, UK.
[14] http://www.iea.org/Textbase/nptable/2008/Finland2007_t6.pdf
[15] CIBSE TM23 – “Testing for air leakage”
[16] The Air Tightness Testing and Measurement Association (ATTMA)’s Technical Standard (TS1)
[17] http://www.euroclad.co.uk/technical/air_tightness.html
[18] Practice Note No. 106 for Authorised Persons and Registered Structural Engineer,
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