Table Of ContentDesign for improved solar
shading control
CIBSE TM37
Engineering a sustainable
built environment
The Chartered Institution of Building Services Engineers
222 Balham High Road, London SW12 9BS
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without the prior permission of the Institution.
© April 2006 The Chartered Institution of Building Services
Engineers London
Registered charity number 278104
ISBN-10: 1-903287-57-X
ISBN-13: 978-1-903287-57-6
This document is based on the best knowledge available at
the time of publication. However no responsibility of any
kind for any injury, death, loss, damage or delay however
caused resulting from the use of these recommendations can
be accepted by the Chartered Institution of Building Services
Engineers, the authors or others involved in its publication.
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Cover illustration: The Lowry, Salford, England; Michael
Wilford & Partners, architects; photograph © BRE
Note from the publisher
This publication is primarily intended to provide guidance to those responsible for the
design, installation, commissioning, operation and maintenance of building services. It is
not intended to be exhaustive or definitive and it will be necessary for users of the guidance
given to exercise their own professional judgement when deciding whether to abide by or
depart from it.
Printed on 100%recycled paper comprising at least 80%
post-consumer waste
Contents
1 Introduction 1
1.1 The purpose of this document 1
1.2 Principles of solar overheating control 1
1.3 Other sources of information 1
2 Requirements of the Building Regulations 2
2.1 Approved Document L2A (2006): buildings other than dwellings 2
2.2 Approved Document L1A (2006): dwellings 3
3 Solar control techniques 3
3.1 Introduction 3
3.2 Shading measures 4
3.3 Control of solar shading 5
4 Quantifying solar control performance 6
4.1 Glazing 6
4.2 Blinds and shutters alone 6
4.3 Solar protection devices combined with glazing 6
4.4 Derivation of the effective g-value 7
4.5 Combinations of glazing and shading 10
5 Calculating gains 11
5.1 Calculating solar gains 12
5.2 Calculating internal gains 14
6 Examples of gain calculation 18
6.1 Open-plan office 18
6.2 Bedsitting room 19
6.3 Industrial unit 20
References 21
Bibliography 22
Appendix A:Standard casual gains for different types of space 23
Index 29
1
Design for improved solar shading control
1 Introduction where possible, have main facades of buildings
facing north and south. This makes shading easier
and allows use of winter solar gain where this
1.1 The purpose of this document would be beneficial. Spaces where overheating
would be critical can be placed on the north side of
buildings.
Sunlight is often welcome in buildings but the sun can
also have negative impacts: excessive solar gain can lead to — Limiting window area: solar heat gain is roughly
overheating in summer or high air conditioning loads. proportional to the window area. However,
Glare can also cause problems, particularly in interiors reducing window area can also limit daylight and
with computer screens. In most buildings, careful design restrict the view out.
is required so the benefits of sunlight and daylight can be
— Solar shading: this may include external, internal
enjoyed while any problems are effectively controlled.
or mid-pane shading devices, or solar control
glazing. It is described in detail in section 3.
This document deals with the issue of solar overheating
and the way it can be avoided by appropriate building — Thermal mass: an exposed heavyweight structure,
design. with a long response time, will tend to absorb heat,
resulting in lower peak temperatures on hot days.
In the UK, the control of overheating is a requirement of However, this will also need appropriate night-
the Workplace (Health, Safety and Welfare) Regulations time venting and acoustic requirements to be
1992(1), which state that ‘[d]uring working hours, the taken into account.
temperature in all workplaces inside buildings shall be
— Good ventilation: a reasonable level of ventilation
reasonable’. However the regulations do not specify a
will always be required in buildings to maintain
particular maximum temperature.
indoor air quality. The ability to switch to a much
The Building Regulations Part L (2006 edition)(2,3) higher air change rate can be a very effective way
to control solar overheating. This can be achieved
include a requirement to limit the effects of solar gains in
by wind driven ventilation through conventional
summer. This covers both new dwellings (Approved
Document L1A(2)) and new buildings that are not windows, particularly if cross ventilation is
dwellings (Approved Document L2A(3)) in England and possible; by use of the solar heat itself through
stack effects, venting hot air out at high level; and
Wales. In Scotland the Building (Scotland) Regulations
by mechanical ventilation(3).
2004* and the supporting guidance provided in Section 6
of the Technical Handbooks apply, and in Northern — Reducing internal gains: for example, by specifying
Ireland Part F (Conservation of fuel and power) of the energy efficient equipment, lamps and luminaires,
Building Regulations (Northern Ireland) applies. or controls to switch off lighting and other equip-
ment when it is not required. Occupancy levels
This document provides additional explanation of the and small power loads will be dictated by the
solar overheating guidance in Approved Document L2A. intended use of the space, and this will need to be
It has been endorsed for this purpose by the Office of the agreed between the client and the design team.
Deputy Prime Minister (ODPM).
— Mechanical cooling or air conditioning.
1.2 Principles of solar overheating
1.3 Other sources of information
control
This document is intended to be read in conjunction with
Solar overheating can be reduced or avoided by the
other industry guidance on building design. In England
following techniques:
and Wales, Building Regulations Approved Document
— Planning the layout of buildings and rooms to maximise L2A(3) is the primary source of guidance on compliance
the benefits of sunlight and minimise the disadvantages: with the regulations. In Scotland and Northern Ireland
provisions are given in the relevant Technical Handbooks
and Technical Booklets, respectively. BRE Report BR364:
* At the time of publication, the Scottish Executive is considering
Solar shading of buildings(4) gives detailed advice on solar
proposals for amending the energy standards in the Building (Scotland)
control. BRE Trust Report FB9: Summertime solar
Regulations 2004 and the supporting guidance provided in Section 6 of
the Technical Handbooks performance of windows with shading devices(5) gives
2 Design for improved solar shading control
information on how to calculate the solar control perform-
ance of glazing with and without blinds and external
shading. The British Blind and Shutter Association has
also published guidance on the different shading types
and giving a list of suppliers(6).
For offices and similar side-lit buildings, the BRE
Environmental Design Guide(7)includes tables that show the
impact of window area (and other design parameters) on
peak temperatures, and other useful design guidance.
2 Requirements of the
Building Regulations
Part L of the Building Regulations for England and Wales
(coming into effect on 6 April 2006) includes a Figure 2.1 Ventilation stacks at the BRE Environmental Building
requirement ‘limiting heat gains and losses through
thermal elements and other parts of the building fabric’.
Approved Document L2A(3) gives guidance on meeting storage areas which only occasionally have someone
this requirement in England and Wales. working in them, tea making areas used for only a few
minutes each time, toilets.
At the time of publication the Scottish Executive is
considering proposals for amending regulations under the ADL2A describes three possible design strategies:
Building (Scotland) Regulations 2004 and the associated
— appropriate combination of window size and
guidance on ways to comply with the building regulations
orientation
in Scotland. In Northern Ireland, Part F (Conservation of
fuel and power) of the Building Regulations (Northern — solar protection through shading and other solar
Ireland) and the associated guidance applies. control measures (Figure 2.2)
— using thermal capacity with night ventilation.
2.1 Approved Document L2A (2006):
For school buildings in England and Wales, Building
buildings other than dwellings Bulletin 101(8) specifies the overheating criterion and
provides guidance on methods to achieve compliance.
Approved Document L2A* (ADL2A) covers new non-
domestic buildings and large non-domestic extensions For other building types the AD gives two specific ways to
(with floor area greater than 100 m2 and greater than 25% comply with the requirement in a space:
of the floor area of the existing building). The solar gains
— limit solar and internal casual gain
requirement does not apply to small extensions or other
work being carried out in existing buildings. It applies to — show that the space will not overheat.
occupied, naturally ventilated spaces so as to avoid the
retrofit of cooling systems in naturally ventilated These are alternatives, so only one of them need be used to
buildings that overheat. demonstrate compliance for a particular space.
Some buildings have stacks (Figure 2.1), or atria, to drive
air movement. The guidance does not apply to them
unless they are occupied. If an atrium, for example,
contains a reception area or restaurant where people work
for a substantial part of the day, then it is an occupied
space. Spaces that are only occupied on a temporary basis,
such as circulation spaces, do not count as occupied.
Generally, the following activity areas would not count as
occupied and no calculation would be required:
bathrooms, changing facilities, circulation areas (unless
they contain permanent workstations as described above),
IT equipment areas (without desk-based staff), perform-
ance areas (stages), plant rooms, public circulation areas
(unless staff members are permanently working there),
* For England and Wales, the text of Approved Document L2A(3),
including the legal technical requirements, can be viewed on the website of
the Office of the Deputy Prime Minister (ODPM) (www.odpm.gov.uk).
Requirements for Scotland are published by the Scottish Building
Standards Agency (www.sbsa.gov.uk) and for Northern Ireland by the
Department of Finance and Personnel (www.buildingregulationsni.gov.uk). Figure 2.2 Brise soleil at the Scottish Office building, Edinburgh
2 Design for improved solar shading control
information on how to calculate the solar control perform-
ance of glazing with and without blinds and external
shading. The British Blind and Shutter Association has
also published guidance on the different shading types
and giving a list of suppliers(6).
For offices and similar side-lit buildings, the BRE
Environmental Design Guide(7)includes tables that show the
impact of window area (and other design parameters) on
peak temperatures, and other useful design guidance.
2 Requirements of the
Building Regulations
Part L of the Building Regulations for England and Wales
(coming into effect on 6 April 2006) includes a Figure 2.1 Ventilation stacks at the BRE Environmental Building
requirement ‘limiting heat gains and losses through
thermal elements and other parts of the building fabric’.
Approved Document L2A(3) gives guidance on meeting storage areas which only occasionally have someone
this requirement in England and Wales. working in them, tea making areas used for only a few
minutes each time, toilets.
At the time of publication the Scottish Executive is
considering proposals for amending regulations under the ADL2A describes three possible design strategies:
Building (Scotland) Regulations 2004 and the associated
— appropriate combination of window size and
guidance on ways to comply with the building regulations
orientation
in Scotland. In Northern Ireland, Part F (Conservation of
fuel and power) of the Building Regulations (Northern — solar protection through shading and other solar
Ireland) and the associated guidance applies. control measures (Figure 2.2)
— using thermal capacity with night ventilation.
2.1 Approved Document L2A (2006):
For school buildings in England and Wales, Building
buildings other than dwellings Bulletin 101(8) specifies the overheating criterion and
provides guidance on methods to achieve compliance.
Approved Document L2A* (ADL2A) covers new non-
domestic buildings and large non-domestic extensions For other building types the AD gives two specific ways to
(with floor area greater than 100 m2 and greater than 25% comply with the requirement in a space:
of the floor area of the existing building). The solar gains
— limit solar and internal casual gain
requirement does not apply to small extensions or other
work being carried out in existing buildings. It applies to — show that the space will not overheat.
occupied, naturally ventilated spaces so as to avoid the
retrofit of cooling systems in naturally ventilated These are alternatives, so only one of them need be used to
buildings that overheat. demonstrate compliance for a particular space.
Some buildings have stacks (Figure 2.1), or atria, to drive
air movement. The guidance does not apply to them
unless they are occupied. If an atrium, for example,
contains a reception area or restaurant where people work
for a substantial part of the day, then it is an occupied
space. Spaces that are only occupied on a temporary basis,
such as circulation spaces, do not count as occupied.
Generally, the following activity areas would not count as
occupied and no calculation would be required:
bathrooms, changing facilities, circulation areas (unless
they contain permanent workstations as described above),
IT equipment areas (without desk-based staff), perform-
ance areas (stages), plant rooms, public circulation areas
(unless staff members are permanently working there),
* For England and Wales, the text of Approved Document L2A(3),
including the legal technical requirements, can be viewed on the website of
the Office of the Deputy Prime Minister (ODPM) (www.odpm.gov.uk).
Requirements for Scotland are published by the Scottish Building
Standards Agency (www.sbsa.gov.uk) and for Northern Ireland by the
Department of Finance and Personnel (www.buildingregulationsni.gov.uk). Figure 2.2 Brise soleil at the Scottish Office building, Edinburgh
Solar control techniques 3
2.1.1 Limiting gains (c) gains from hot water storage, distribution and
consumption
One way to comply is to show that the combined solar and
(d) heat loss through the fabric
internal casual gain on peak summer days (corrected for
geographical location) would not be greater than 35W per (e) natural ventilation
m2of floor area in each occupied space. The basis for this
(f) average external temperatures (depending on
guidance was CIBSE AM10: Natural ventilation in non-
location within the UK)
domestic buildings(9), which states that good practice natural
ventilation design should be able to cope with (g) thermal capacity of the building.
30–40W·m–2of total heat gains.
The calculation does not include solar gains through the
Gains are averaged over a 07:30–17:30 BST time period. fabric. In practice these may offset some of the heat losses
The solar gains are given as the entry for July in the table through the fabric, particularly through roofs which may
of design irradiances in Table 2.30 of the 2006 edition of be fully exposed to the sun. As thermally separated
CIBSE Guide A(10). conservatories are exempt from the solar overheating
provisions of Part L, they are not included in the
Section 5 of this document explains how to carry out the calculation.
calculation of gains.
The calculation gives a threshold internal temperature T
in degrees C. The methodology then classifies the
2.1.2 Overheating calculation
likelihood of high internal temperatures in hot weather
into four different bands. The four overheating bands are:
Compliance is also possible by showing that the operative
temperature in the space does not exceed an agreed — not significant: T< 20.5 °C
threshold for more than a reasonable number of occupied
— slight: 20.5°C <__T< 22.0 °C
hours per annum. An exact definition of what constitutes
overheating is not given in the AD. The threshold — medium: 22.0 °C <__T< 23.5 °C
temperature, and the maximum number of hours that it is
to be exceeded, depend on the activities within the space. — high: T>= 23.5 °C
The 2006 edition of CIBSE Guide A(10) contains some
guidance on this issue. Dwellings which are in the first three bands are viewed as
complying with the solar gain provisions of the 2006
Building Regulations Part L1A.
This is intended to provide a completely flexible method.
It could be used, for example, in spaces with night cooling
and thermal mass, or where innovative natural ventilation The calculation attached to the SAP gives an overall figure
techniques, such as stack effects in tall spaces, are used. for the entire dwelling. This is reasonable because in
The AD does not specify a calculation procedure except to many cases people leave internal doors open during the
state that the building be tested against the CIBSE Design day. In some dwellings there may be a potential risk of
Summer Year(11)appropriate to the building location. Any localised overheating, however, for example in a small
reputable calculation technique could be used. room with large south facing windows.
Further information about techniques to avoid over-
2.2 Approved Document L1A (2006): heating in dwellings can be found in Reducing overheating
— a designer’s guide(13).
dwellings
The 2006 Approved Document L1A(2) for England and
Wales also contains a requirement to limit the effects of 3 Solar control techniques
solar gains in summer. The requirement applies to all new
dwellings, even those where full air conditioning or
comfort cooling is already planned. It does not apply to 3.1 Introduction
extensions or work in existing dwellings.
Compared to the alternative of installing air conditioning,
The Approved Document explains that ‘provision should
solar shading can be a highly cost effective way to control
be made to limit internal temperature rise due to excessive overheating. A BRE study(14) estimated that installing air
solar gains. This can be done by an appropriate combi-
conditioning in a typical 1960s open plan office would
nation of window size and orientation, solar protection require an extra 55kW·h·m–2per year, resulting in overall
through shading and other solar control measures, running costs for the air conditioning of £15 per m2 per
ventilation (day and night) and high thermal capacity.’
year.
Appendix P of SAP 2005(12) contains a procedure that The same study showed that comfort could be achieved at
enables the designer to check on the likelihood of solar zero cooling energy consumption, with a combination of
overheating. The method takes into account the following solar shading (either mid-pane or external) and night-time
factors: ventilation. The extra cost of such measures will usually
be substantially less than that of installing cooling. The
(a) heat gains through windows
calculations also showed that, even in a building where
(b) internal gains from lighting, appliances, cooking cooling had already been fitted, the shading could pay for
and people itself in less than five years.
Solar control techniques 3
2.1.1 Limiting gains (c) gains from hot water storage, distribution and
consumption
One way to comply is to show that the combined solar and
(d) heat loss through the fabric
internal casual gain on peak summer days (corrected for
geographical location) would not be greater than 35W per (e) natural ventilation
m2of floor area in each occupied space. The basis for this
(f) average external temperatures (depending on
guidance was CIBSE AM10: Natural ventilation in non-
location within the UK)
domestic buildings(9), which states that good practice natural
ventilation design should be able to cope with (g) thermal capacity of the building.
30–40W·m–2of total heat gains.
The calculation does not include solar gains through the
Gains are averaged over a 07:30–17:30 BST time period. fabric. In practice these may offset some of the heat losses
The solar gains are given as the entry for July in the table through the fabric, particularly through roofs which may
of design irradiances in Table 2.30 of the 2006 edition of be fully exposed to the sun. As thermally separated
CIBSE Guide A(10). conservatories are exempt from the solar overheating
provisions of Part L, they are not included in the
Section 5 of this document explains how to carry out the calculation.
calculation of gains.
The calculation gives a threshold internal temperature T
in degrees C. The methodology then classifies the
2.1.2 Overheating calculation
likelihood of high internal temperatures in hot weather
into four different bands. The four overheating bands are:
Compliance is also possible by showing that the operative
temperature in the space does not exceed an agreed — not significant: T< 20.5 °C
threshold for more than a reasonable number of occupied
— slight: 20.5°C <__T< 22.0 °C
hours per annum. An exact definition of what constitutes
overheating is not given in the AD. The threshold — medium: 22.0 °C <__T< 23.5 °C
temperature, and the maximum number of hours that it is
to be exceeded, depend on the activities within the space. — high: T>= 23.5 °C
The 2006 edition of CIBSE Guide A(10) contains some
guidance on this issue. Dwellings which are in the first three bands are viewed as
complying with the solar gain provisions of the 2006
Building Regulations Part L1A.
This is intended to provide a completely flexible method.
It could be used, for example, in spaces with night cooling
and thermal mass, or where innovative natural ventilation The calculation attached to the SAP gives an overall figure
techniques, such as stack effects in tall spaces, are used. for the entire dwelling. This is reasonable because in
The AD does not specify a calculation procedure except to many cases people leave internal doors open during the
state that the building be tested against the CIBSE Design day. In some dwellings there may be a potential risk of
Summer Year(11)appropriate to the building location. Any localised overheating, however, for example in a small
reputable calculation technique could be used. room with large south facing windows.
Further information about techniques to avoid over-
2.2 Approved Document L1A (2006): heating in dwellings can be found in Reducing overheating
— a designer’s guide(13).
dwellings
The 2006 Approved Document L1A(2) for England and
Wales also contains a requirement to limit the effects of 3 Solar control techniques
solar gains in summer. The requirement applies to all new
dwellings, even those where full air conditioning or
comfort cooling is already planned. It does not apply to 3.1 Introduction
extensions or work in existing dwellings.
Compared to the alternative of installing air conditioning,
The Approved Document explains that ‘provision should
solar shading can be a highly cost effective way to control
be made to limit internal temperature rise due to excessive overheating. A BRE study(14) estimated that installing air
solar gains. This can be done by an appropriate combi-
conditioning in a typical 1960s open plan office would
nation of window size and orientation, solar protection require an extra 55kW·h·m–2per year, resulting in overall
through shading and other solar control measures, running costs for the air conditioning of £15 per m2 per
ventilation (day and night) and high thermal capacity.’
year.
Appendix P of SAP 2005(12) contains a procedure that The same study showed that comfort could be achieved at
enables the designer to check on the likelihood of solar zero cooling energy consumption, with a combination of
overheating. The method takes into account the following solar shading (either mid-pane or external) and night-time
factors: ventilation. The extra cost of such measures will usually
be substantially less than that of installing cooling. The
(a) heat gains through windows
calculations also showed that, even in a building where
(b) internal gains from lighting, appliances, cooking cooling had already been fitted, the shading could pay for
and people itself in less than five years.
4 Design for improved solar shading control
Solar shading can be particularly effective as part of a
package of passive measures to limit overheating. These
include night-time ventilation and the use of more
efficient equipment and lighting controls to limit internal
heat gains. Solar shading is of particular value in such
situations because it can reduce the swings in temperature
that can occur on sunny days.
Selecting the correct form of solar control is important.
Sometimes energy and cost savings can be less than
expected because the solar control system blocks incoming
daylight, increasing the use of electric lighting. Some fixed
systems can obstruct useful solar gain in the winter,
increasing heating needs. However some kinds of blind
can reduce heat loss through the window to some extent. Figure 3.1 Awnings shading windows at the Royal Albert Hall, London
(photo courtesy Deans Blinds and Awnings Ltd.)
Also, some types of shading can impede natural venti-
lation. BRE Report BR364: Solar shading of buildings(4)
contains detailed advice.
This document concentrates on the thermal aspects of
solar shading. Glare is also an important issue for many
building occupants, and glare control is a legal
requirement where workers regularly use display screen
equipment. Glare can be caused by the bright sky or by
reflections from buildings outside, but most commonly by
direct sunlight. If direct sunlight causes glare, transparent
shading devices such as tinted glazing usually do not help
much because the sun is so bright.Translucent shading,
such as a thin light coloured fabric blind, gives some
protection but may itself become uncomfortably bright
under sunlight. Opaque shading is best. Various BRE
publications(15,16)give guidance on glare control. Often the
best option is to control overheating using one technique,
e.g. an external shade or solar control glazing, and provide
a separate system to reduce glare, e.g. internal blinds.
3.2 Shading measures
Figure 3.2 Roller blinds used for external shading (photo courtesy James
3.2.1 External shading Robertshaw Ltd)
The most effective way to control overheating is to
prevent sunlight from reaching the window. External
shading is particularly appropriate for heavily glazed
buildings where solar heat gain would otherwise be a protection in summer but an overhang projecting by only
major concern. half the window height will help, particularly on a south-
facing window.
Simple overhangs can be highly effective at blocking high
angle summer sun. They work particularly well on south A variety of external blinds are available(6) and these
facing windows. In brise soleil form (Figure 2.2), they can provide more flexibility in use than a light shelf or
be fixed to an existing façade. They do not hinder opening overhang. External horizontal slatted blinds and other
of the windows and a full view out is retained. types of external controllable louvre have the lowest
shading coefficient of any system. External roller blinds
(Figure 3.2) can also be retrofitted. Where the outward
The light shelf is a form of overhang installed part way up
view is important, retractable blinds with occupant
a window, typically just above an occupant’s head height.
control are the best option. An open-weave fabric blind
Extra daylight can enter the space by reflection from the
can give a view out even when lowered, though may allow
top of the shelf, passing through the glazing above it.
glare from the sun.
Having an internal shelf as well as an external one helps
control glare for the occupants.
When designing external shading, access for window
cleaning should be planned.
In a range of building types, an awning can be an
attractive, and simply installed, way of providing a
retractable overhang (Figure 3.1). 3.2.2 Glazing and films
The extent of solar protection depends on the ratio of the A wide variety of solar control glasses are available and
projection of the overhang (or brise soleil or awning) to they have been extensively used in commercial buildings.
the height of the window. A ratio of 1:1 gives good There are two main types: absorbing glasses, which are
Solar control techniques 5
body tinted, and reflective glasses, which have a special
coating. Reflective glasses are usually slightly better at
rejecting incoming solar gain. Absorbing glass heats up
more when sunlight falls upon it and some of this heat can
reach the inside of the building.
Solar control films can be easily added onto flat glazing.
Although less durable than glazing, they are easy to
retrofit into existing buildings.
As with fixed systems, both glazing and solar films will
reduce useful winter solar gain and daylight. A US
study(17) suggests that if the light transmittance of the
window is less than around 35%, then people start to find
the view out dull (see also reference 18).
Advanced glazing is now available that can control over- Figure 3.3 Reflective window film blind (photo courtesy Reflex-Rol
heating while admitting reasonable levels of daylight. The (UK))
glass has a spectrally selective coating that reflects infrared
radiation while admitting visible light. The coating also
reduces heat loss in winter in the same way as a
conventional low emissivity glass. Some forms of solar
control film also have this type of coating. be reduced where tinted glazing or window film is used,
since heat reflected by the shading could be absorbed or
Where the façade is being refurbished, one option is to reflected back in by the glazing.
reduce window area. Insulated cladding panels are
available that can match the appearance of glazed units.
3.3 Control of solar shading
Daylight and view out will be reduced, but so will heat
loss in winter.
In most buildings the need for shading changes
throughout the year. It can vary according to:
3.2.3 Mid-pane blinds
— seasonal requirements: overheating is a problem in
summer, but winter solar heat gain can be
In double glazed units, mid-pane blinds can be an
welcome;
unobtrusive way to help control solar heat gain. Generally
the solar performance of a mid-pane blind will be — daily weather: on dull days there is often little need
intermediate between that of an external and an internal for shading devices;
system. In a sealed unit, a mid-pane blind will get dirty
— occupant requirements: for some activities people
less quickly than an external or internal blind. Various
options are available(6) to allow mid-pane blinds to be need extra privacy, or extra control of glare.
controlled from the inside of the building.
For all these reasons adjustable shading is often the best
option. However it has the disadvantages of being more
3.2.4 Internal blinds costly and harder to maintain as moving parts can fail.
Internal systems can contribute towards solar heat control Where only seasonal requirements change, it is possible to
but tend to be less effective than their external or mid- use fixed shading which varies its performance from
pane counterparts. Incoming solar gain can be absorbed winter to summer, by means of its geometry. An example
by the shading device and convected or re-radiated into is an overhang or light shelf, which blocks high-angle
the interior. Fabrics with a solar reflecting coating or a summer sun but lets through low-angle winter sun.
metallised finish on the reverse will help reflect this solar
heat and offer improved solar shading performance If adjustable shading is used it should be easy to control.
compared to conventional fabrics. Thus systems that The choice of manual or automatic control should depend
incorporate reflective materials usually have lower heat on the needs of the space. In buildings such as offices
transmittances. people expect to control the shading themselves, and tend
to resent automatic shading. In spaces such as atria,
Reflective roller blinds (Figure 3.3) look similar to a entrance halls, airport buildings, swimming pools and
heavy-duty window film. They can give good rejection of sports halls buildings, users do not expect to control the
solar gain but, unlike conventional window films, can be shading and automatic control will normally be best.
withdrawn when daylight and solar radiation are required.
Opaque types are available and also those that are Manual controls should be easy to find, reach and operate.
transparent and darken, but do not obscure, the view. The Ideally they should be self-explanatory otherwise the
transparent types will not completely control glare from occupants need to be instructed in their use. Controls
the sun and the opaque types are better if this is a should operate quickly and give feedback to users.
problem.
If control is automatic then manual override should be
Fabric roller blinds are also available with a metallised considered, e.g. for blackout purposes. Sometimes a
backing which helps to reflect solar heat. The performance combination of manual and automatic control is best. A
of internal shading systems with reflective properties may BRE Information Paper(19)explores the various options.
