external solar shading with wood

Transcription

1 external solar shading with wood a design guide for architects patrick hislop with philip o leary

2 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 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 Langley Academy Vertical fins attached to the curved building face, comprising western red cedar blades on cantilevering metal brackets

3 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

4 Contents 1 Introduction to solar protection 7 2 Types of solar protection 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 Performance 3.2 Choice of species 4 Design and detailing of louvres and fins 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 Connections 5.2 Fixings 5.3 Operating devices 6 Structural design 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

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 Procedure 1 (SAP) calculations. BS Lighting for buildings 2 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

6 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

7 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 Solar protection Structure Transparency Ventilation Security Cleaning or replacing glass Improved insulation Daylight Privacy Performance 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. 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. No restriction on horizontal view out. No restriction. No implications, except may provide unwanted access to upper floors. 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. No advantage. The canopy alone has a minimal effect on daylight. But adding foliage could substantially reduce daylight. No effect. 12

8 External solar shading with wood: a design guide for architects 1:4 1:6 Figure 4.5: Width-to-thickness ratio 1:4 1:4 Figure 4.6: Edge lamination permits wider louvres Louvre heartside up, bowing Louvre heartside down, cupping 1/4 sawn Figure 4.7: Cupping behaviour of rectangular sections Assymetrical section heartside-up bowing Heartside-down cupping Figure 4.8: Cupping behaviour of asymmetric sections 4.2 Factors affecting size, shape and spacing of louvre and fin sections Apart from aesthetics, designers must consider (generally in this order): 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 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: 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