Exterior Envelope Insulation
Overview for Office Building
Better insulation of the external envelope can help to reduce heat gain
during summer months and reduce the demand for air-conditioning,
which will help to reduce energy use. During winter, heat loss through
the external envelope is also reduced, which helps to reduce demand
for heating of the interior space.
The main component of the external envelope is the solid wall and the
glass area. Here are some examples of how better insulation of the
external envelope can be achieved
1
:
• Window to wall ratio
• Glass properties
• Envelope material
Benefits of Green Office Building
• A good building envelope will save
on energy consumption and reduce
investment costs
• Lower initial investment cost of air-
conditioning systems
• Reduce heat gain during summer
months and reduce the demand for
air-conditioning
• During winter, heat loss through the
external envelope is also reduced,
which helps to reduce demand for
heating of the interior space
• Higher thermal comfort because of
warmer surface temperatures on
the interior surfaces in winter and
lower temperatures in summer. This
also results in a lower risk of mold
growth on internal surfaces
Green Strategies for Office Building
z
Better insulation of exterior
wall reduces heat gain and heat
loss and reduces demand for
air-conditioning and heating
Window to Wall Ratio
• Opaque walls generally resist
heat transfer better than glass.
Lowering the window to wall
ratio can reduce the amount of
outside heat being transferred to
the interior
• Balance the view and daylighting
requirement, glazing should only
be placed where appropriate
• The north facing façade should
have the largest glazing area
since it receives the least solar
heat gain
z
Use of double glazing or an
integrated glass unit (IGU) helps
to provide better insulation in
glass areas and window areas
Glass Properties
Office buildings often have large glazing areas. Glass properties affect
insulation performance. With high performance glazing, thermal insulation
can be significantly increased. Solar heat gain in the summer will be reduced,
so that less energy is consumed by air-conditioning. Heat loss in winter will
be reduced so that less energy is needed for heating.
The glass properties are quantified by the following factors:
• Shading coefficient (SC) can portray how well a product blocks heat caused
by sunlight. The lower the glazing’s SC, the less solar heat it transmits into
a building
• Visible light transmittance (VLT) is an optical property which indicates the
amount of visible light transmitted through the glass
• External reflectance measures how much light from an external source is
reflected by the glazing product
z
Adopt /effec t i ve use of
s u n - s h a d i n g
d e v i c e s
(case example on
,
)
Envelope Material
Good insulating material installed on the roof and façade
improves the overall performance of the building. The insulating
property is measured by U-value. Material with a lower U-value
has better insulating properties and has a great impact on
heat gain inside the building. Having a building with enough
insulation is like a person wearing a ski jacket in cold weather,
and having an umbrella on a hot sunny day.
Solar Reflectance Index (SRI) is an indicator of the material’s
ability to retain solar radiationbymeasuring the solar reflectance
and emissivity. A standard black roof is defined as having an SRI
of 0 while a standard white roof is 100. A cool roof usually has
a higher number.
When not taking emissivity into account, the measure used is
albedo, which is the proportion of incident radiation reflected
by a system. A perfect absorber has an albedo of 0 while a
perfect reflector has an albedo of 1.
z
Motorised sun louvres (case
example on
)
z
in the
external envelope during the
design stage (case example on
,
)
Figure 3.1.16 Sunshading
(Source: Hong Kong Science and Technology Parks Corporation)
Figure 3.1.17 Exterior Envelope
DESIGN STAGE
3.1.4 Exterior Envelope Insulation
U-Value
(W/m
2o
C)
SC
Window Fixed:
Max. 6.93
Operable:
Max. 7.21
Min. 0.25
Skylight Max. 7.72 Min. 0.25
Wall
Max. 3.3
NA
Roof
Max. 0.39 NA
Table 1 Inputs for reference building
envelope
Criteria
Basic Requirements
Thermal
Insulation
Performance
Single Glazing U-value: ≤ 5.8W/m
2
K
Double or triple glazing U-value: ≤
3.3W/m
2
K
Shading
Coefficient
(SC)
Shading Coefficient: ≤ 0.90
Solar heat gain coefficient: ≤0.78
Visible Light
Transmittance
(VLT)
VLT ≥50%
External
Reflectance
(ER)
External Reflectance: ≤20%
Table 3 Hong Kong Green Building Council HK G-PASS
Assessment Standard for Glazing (version 1.0)
)
Measure
Advantage
Disadvantage
Use double-glazed units Lower U-value compared
to single glazing
Less heat transmission
through conduction
Increase cost and
structural loads
Use glass with lower
Shading Coefficient
Less solar heat gain
Usually darker
appearance
Less opportunity for day
lighting
Specify different glass
according to function
and orientation
Balanced budget – spend
on high performance
glass where it is most
beneficial
More difficult to keep
track of various glass
types for a single façade
or window unit
Table 2 Adva
DESIGN STAGE
3.1.4 Exterior Envelope Insulation
Office
BUILDING
Office
BUILDING
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