external solar shading with wood

EXTERNAL SOLAR SHADING WITH WOOD
external solar shading with wood
a design guide for architects
> brings together architectural and structural considerations
> illustrated with 50 schematic drawings and best-practice details
> researched and written by TRADA Technology, the experts in timber construction
external solar shading
with wood
a design guide for architects
patrick hislop with philip o’leary
patrick hislop with philip o’leary
Areas of glazing are increasing to obtain the maximum advantage from daylight and to benefit from passive
solar gain in the winter. But larger windows increase the risk of internal overheating and glare. UK building
regulations now require solar protection in residential and non-residential buildings.
With its versatility, ease of working and easy maintenance, wood has an important role in the design of
modern shading devices. However, the prime reason for using wood for solar screening is usually its
attractive appearance, whether left to bleach naturally or finished in any colour.
external solar shading with wood
> deals with external screening devices, comparing nine types now in use
> describes screening systems that are wholly or partially wood based
> includes metal support systems and metal hardware necessary for operating louvres.
Patrick Hislop RIBA (author) is a recognised expert in the specification of wood in buildings and is the author
of several TRADA Technology publications on windows, decking and cladding.
Philip O’Leary (technical consultant) is Head of TRADA Technology’s Timber Technology Investigations
Section. He is an expert on timber quality and its characteristics, specialising in the condition, performance
and strength of timber in buildings.
ISBN 978-1-900510-86-8
9 781900 510868
TRADA TECHNOLOGY
TRADA Technology is an independent consultancy company providing a wide range of
commercial and training services to the timber and construction industries. Prior to 1994 it was
wholly owned by TRADA, the Timber Research and Development Association. It is now a member
of the BM TRADA Group of companies and is TRADA’s appointed provider for its research and
information programmes, and for the administration of its membership services.
Bentfield School
Horizontal louvres attached to
building face, comprising
square-edge western red cedar
blades within an outer steel frame,
using mid-span tie rods
Cherry Pool Farm (left)
Horizontal louvres on freestanding
vertical supports, comprising elliptical
western red cedar blades with an
outer steel frame and vertical tie rods
Shires Gateway (right)
Faceted horizontal louvres attached
to the curtain wall system, comprising
elliptical western red cedar blades
Berick Street
Manually operated sliding screens
attached to building face, comprising
English oak louvres and frame
Corsica House (left)
Curved horizontal louvred canopy,
comprising steel-reinforced iroko
blades spanning 2.5 m between
cantilevering laminated timber beams
Glory Park (right)
Horizontal louvres on freestanding
steel vertical supports or supported
on the curtain walling (top) and fixed
between aluminium side arms within
reveals (bottom), comprising elliptical
western red cedar blades
Concorde House
Horizontal louvred canopy, comprising
western red cedar system within
steel frame. Louvre spans of 3.4m
have mid span supports
Langley Academy
Vertical fins attached to the curved
building face, comprising western
red cedar blades on cantilevering
metal brackets
external solar shading
with wood
a design guide for architects
With ever-increasing concern about saving energy, areas of glazing are
becoming still bigger, both to obtain the maximum advantage from daylight
and to benefit from passive solar gain in the winter. UK building regulations
recognise the need to balance the advantage of additional daylight from
large windows against the risk of internal overheating and glare, and now
require solar protection in both residential and non-residential buildings.
Wood has traditionally been used for many forms of solar shading. And
because of its versatility, ease of working and easy maintenance, wood has
an important role in the design of modern shading devices. However, the
prime reason for using wood for solar shading is usually its attractive appearance, whether left to bleach naturally or finished in any colour. Many forms
of sun screening are now becoming an accepted feature in the visual appearance of buildings, but their primary role will always be to moderate the effect
of the sun’s rays on the internal environment of the building.
This publication deals with external screening devices, comparing nine
types now in use. It is limited to those screening systems that are wholly
or partially wood based but does include metal support systems and metal
hardware necessary for operating louvres.
Patrick Hislop RIBA, formerly Senior Consultant Architect at TRADA
Technology, is a recognised expert in the specification of wood in buildings.
He is the author of several TRADA Technology publications on windows,
decking and cladding, and has contributed to many other publications.
Philip O’Leary is Head of TRADA Technology's Timber Technology
Investigations Section. He is an expert on timber quality and timber characteristics and is responsible for all of TRADA Technology’s training courses on
the strength grading of timber. He specialises in the condition, performance
and strength of timber in buildings and also has expertise in wood coatings,
seals and finishes, with several papers published on the subject.
3
Contents
1 Introduction to solar protection
7
2 Types of solar protection
9
2.1 Horizontal louvred canopy
2.2 Arrays of horizontal louvres on freestanding vertical supports
2.3 Arrays of horizontal louvres attached to building face
2.4 Vertical freestanding fins
2.5 Vertical fins attached to building face
2.6 Sliding screens freestanding from building face
2.7 Sliding screens attached to building face
2.8 Hinged or folding screens freestanding from building face
2.9 Hinged or folding screens attached to building face
3 Advantages of wood for solar protection devices
21
3.1 Performance
3.2 Choice of species
4 Design and detailing of louvres and fins
25
4.1 Architectural design principles
4.2 Factors affecting size, shape and spacing of louvre and
fin sections
4.3 Horizontal louvres
4.4 Vertical fins
5 Connections, fixings and operating devices
31
5.1 Connections
5.2 Fixings
5.3 Operating devices
6 Structural design
35
6.1 Structural design principles
6.2 Horizontal louvres
6.3 Vertical fins
6.4 Hinged, folding or sliding screens
7 Finishes and maintenance
37
References
39
Further reading
39
5
1 Introduction to solar protection
In the tropical parts of the world there are many traditional ways of eliminating or reducing the effects of the sun, which is generally regarded as
‘hostile’ in these areas. These include the use of thick walls, small windows,
lattice screens and external awnings or foliage to provide some shade.
Even in less hot climates such as southern Europe or the southern parts of
North America, devices such as verandas, shutters and blinds have traditionally been used to temper the effects of the sun, generally combined with
good ventilation.
On the other hand, in more temperate climates, the sun has generally
been regarded as beneficial, with buildings planned to gain the maximum
advantage of sunshine. However, in these areas the increased popularity
of large windows, or even totally glazed walls, has frequently resulted in
internal overheating and excessive glare. This is generally only mitigated by
the use of some form of internal blind, often added as a remedial measure
against glare, or air conditioning to reduce unwanted internal heat.
UK building regulations recognise the need to balance the advantage of
additional daylight from large windows against the risk of internal overheating and glare, and now require solar protection in both residential and
non-residential buildings. Meeting these requirements is now part of the
Standard Assessment Procedure1 (SAP) calculations. BS 8206-2 Lighting for
buildings2 also recommends shading devices to prevent overheating and
glare.
Now, with ever-increasing concern about saving energy, areas of glazing are
becoming still bigger, both to obtain the maximum advantage from daylight
and to benefit from passive solar gain in the winter. The latter gain may be
directly (by using the sun’s radiation) or by storing this heat in the thermal
mass of the building, which may reduce the heating load in the winter.
Because the sun’s rays are beneficial in the winter, any form of sun screening
should be designed to allow sufficient penetration of the sun’s rays, while
Short wavelength
radiation passes
through glass
Glass is partially opaque to
long wavelength ‘heat’
radiation (some radiation is
absorbed by the glass and
re-radiated to outside)
Conduction
heat loss
Heat is emitted from
surfaces as long
wavelength radiation
Solar radiation is absorbed
by the structure (the amount
depends on surface
reflectance and thermal mass)
7
Figure 1.1: The greenhouse effect
2 Types of solar protection
Chapter 1 mentioned a number of traditional methods of mitigating the
effects of excessive sun and touched upon various modern versions of these
methods.
This chapter compares nine modern types of sun screening, describing their
efficiency in controlling unwanted solar gain, while accepting that solar gain
is welcome in the winter months for reducing the heating load. The difficulty
is in benefitting from this solar gain while avoiding the risk of overheating
or excessive glare.
External shading devices have the advantage that in the summer they
prevent both solar glare and energy entering the building, but can allow
for passive solar gain in the winter. With careful detailing and orientation,
external screening devices do this by allowing in the lower-angle winter sun,
while excluding high-level summer sun.
Figure 2.1: A thermal mass strategy
1. High thermal mass is positioned in
sunpath to absorb solar radiation for
background heat.
2. Lightweight fast response fabric is
used in non-collection areas to allow
rapid warm-up as required during
occupancy.
3. Temperature zoning and good
heating controls are important for this
strategy.
4. Care is required to avoid summer
overheating.
Besides controlling the entry of sun into a building, external solar shading
affects various secondary functions such as structural implications, penetration of daylight, compromising the view out, the cleaning or replacement of
glass and security.
The importance of these secondary functions will vary depending on such
factors as whether there will be energy saved by using daylight rather than
artificial lighting. This may depend on building usage and occupation, and
whether the building is likely to be mechanically rather than naturally ventilated. The orientation of the facade will also largely dictate whether vertical
fins are preferable to horizontal louvres because, for instance, low-angle sun
from the west might be more of a problem than high-angle sun on a southern
elevation.
Natural shading from deciduous trees can provide some protection from
summer sun but combining deciduous foliage with solar shading devices
can be a more effective way of controlling the penetration of the sun in the
summer months.
There is a tendency among architects to screen the whole facade with
horizontal louvres whatever the aspect, which may unnecessarily reduce
the penetration of daylight and view out, and may not be effective on nonsouth-facing elevations. Limiting any screening device to the upper part of
the window (above the line of sight) may well provide adequate protection
9
External solar shading with wood: a design guide for architects
2.1 Horizontal louvred canopy
Figure 2.5: Horizontal louvred
canopy variations
Figure 2.4: Horizontal louvred canopy
Function
Performance
Solar protection
Provides complete protection from high-angle sun. Limited
penetration of low-angle or flanking sun. Can also be used with
deciduous foliage to improve summer shading.
Structure
Canopy cantilevered from building: No separate supports or
foundations, but considerable wind and gravity loads (including
foliage weight) transferred to building structure. Limited projection
from building face.
One side on separate vertical supports, other side on building
structure: Outer columns require separate foundations. Some
loads transferred to building, including wind loads. Projection can be
reasonably wide.
Freestanding canopy on separate vertical supports: Requires
separate foundations, independent of building structure. No loads
transferred to building. No limits on width.
Transparency
No restriction on horizontal view out.
Ventilation
No restriction.
Security
No implications, except may provide unwanted access to upper
floors.
Cleaning or
replacing glass
Some restrictions on cleaning from outside if below window head. On
multi-storey buildings, can be designed to provide cleaning access to
glazing. No restriction on replacement.
Improved insulation No advantage.
Daylight
The canopy alone has a minimal effect on daylight. But adding
foliage could substantially reduce daylight.
Privacy
No effect.
12
External solar shading with wood: a design guide for architects
4.2 Factors affecting size, shape and spacing
of louvre and fin sections
1:4
Apart from aesthetics, designers must consider (generally in this order):
1:6
Figure 4.5: Width-to-thickness ratio
1:4
Width to thickness ratio
1:4
Figure 4.6: Edge lamination permits
wider louvres
Edge-glued section
1. effectiveness as a shading device at critical times for both high- and
low-angle sun
2. resistance to wind forces
3. resistance to deflection
4. resistance to distortion due to moisture movement
5. adequacy of fixings to supports
6. durability and weathering performance
7. choice of appropriate species to satisfy all these criteria.
Problems can arise when:
•
•
•
•
the louvre system is not robust enough
fixings are inadequate for the predicted load
width-to-thickness ratio is excessive
wood distorts in the ‘wrong’ direction due to moisture movement.
4.3 Horizontal louvres
Louvre heartside up, bowing
Louvre heartside down, cupping
1/4 sawn
Figure 4.7: Cupping behaviour of
rectangular sections
Rectangular section distortion
Assymetrical section heartside-up
bowing
The efficiency of any external solar protection will depend largely on the
spacing, size, pitch and profile and, to some extent, the colour of louvres.
In practical terms there are limits on the size, thickness and shape of any
wood louvre. The wider, or deeper, a profile is, the more liable it is to distort
or twist compared to a smaller section. Ideally no wood louvre should have a
width-to-thickness ratio exceeding four, but with denser more stable woods
this can be increased to six. Any louvres wider than this should be edge
laminated from pieces that themselves are within these ratios. However,
although this may reduce the risk of the boards cupping (forming a concave
surface) it will not necessarily prevent a whole section from twisting. For this
reason it is always preferable to use horizontal louvres of limited width, even
if this means closer spacing of the louvres.
Ideally louvres should be made of ‘quarter-sawn’ sections but this may prove
unacceptably expensive. Where the grain is tangential to the section, ensure
that louvres are installed with the ‘heart side’ of the wood uppermost which
will reduce the risk of the upper surface cupping and holding water.
It is important that water is shed quickly off the upper surface of horizontal
louvres because any water allowed to lie on the top surface could eventually
cause distortion of the section. A relatively flat upper surface will also result
in slower drainage of surface water which, in turn, can lead to increased
discoloration of the wood due to increased moisture absorption. It is preferable to increase the pitch of the louvre, rather than taper the profile to the
leading edge. This is because, if the profile is not symmetrical, there is an
increased risk of the section distorting.
Increasing the pitch of louvres has other advantages:
Heartside-down cupping
Figure 4.8: Cupping behaviour of
asymmetric sections
•
•
angled profiles are less likely to deflect under their own weight
increasing the slope on the underside will also prevent water tracking
back, possibly causing staining towards the back of the louvre below.
26