Build Thermally Efficient and Sustainable Structures€¦ · Web viewFigure 8 Shading devices for North facing windows Vertical shading devices are best used on westerly and easterly

Build Thermally Efficient and Sustainable Structures

Chapter 7 Glazing Produced by Pointsbuild in partnership with the Master Builders

Association of NSW

Supported by the NSW Government as part of the Energy Efficiency Training Program — visit savepower.nsw.gov.au

Copyright and disclaimer The Office of Environment and Heritage and the State of NSW are pleased to allow this material to be used, reproduced and adapted, provided the meaning is unchanged and its source, publisher and authorship are acknowledged. The Office of Environment and Heritage has made all reasonable effort to ensure that the contents of this document are factual and free of error. However, the State of NSW and the Office of Environment and Heritage shall not be liable for any damage which may occur in relation to any person taking action or not on the basis of this document. Office of Environment and Heritage, Department of Premier and Cabinet59 Goulburn Street, Sydney NSW 2000PO Box A290, Sydney South NSW 1232Phone: (02) 9995 5000 (switchboard)Fax: (02) 9995 5999TTY: (02) 9211 4723Email: [email protected]: www.environment.nsw.gov.au

Table of contents1 Glazing........................................................................................................................................................4

1.1 Introduction...............................................................................................................................................4

1.2 Glazing Properties.....................................................................................................................................4

1.2.1 Solar Heat Gain Co-Efficient (SHGC)...........................................................................................................41.2.2 U-Value - Heat Transfer through Windows................................................................................................8

1.3 Glass..........................................................................................................................................................9

1.4 Frames.....................................................................................................................................................10

1.5 Window Coverings (Curtains/Blinds).......................................................................................................11

1.6 Glazing - Reducing Solar Heat Gain..........................................................................................................12

1.6.1 Solar Radiation on Vertical Surfaces........................................................................................................121.6.2 Shading of Windows.................................................................................................................................131.6.3 Eaves........................................................................................................................................................141.6.4 External Shading Devices other than Eaves.............................................................................................141.6.5 Clerestory Windows and Skylights...........................................................................................................161.6.6 Area and Orientation of Glazing...............................................................................................................161.6.7 Interaction with Mass and Insulation.......................................................................................................17

1.7 Windows for Australian Climates............................................................................................................17

1.7.1 Temperate Climate..................................................................................................................................181.7.2 Coastal Sub Tropical Climate....................................................................................................................181.7.3 Hot Climate..............................................................................................................................................19

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Building Thermally Efficient and Sustainable Structures: Chapter 7 - Glazing_____________________________________________________________________________________________

1 Glazing1.1 IntroductionGlazing has a very important role in determining thermal performance of a building. Glazing allows sunlight into the building, which is valuable for daylight and winter heating, but can be a problem if there is too much in summer. In winter, passive heating from the sun that is gained through the glazing must be greater than the heat lost from the house to the cold outside air to be worthwhile.

Glazing in a building can be a major source of heat loss in winter and a source of heat gain in summer when the temperature difference between inside and outside the building is high.

Heat flow through glazing is determined by the combined effect of glass, frame and seals.

There are 3 ways heat flows through glazing :

Conduction - heat flow due to temperature difference from the outside to the inside of the glass Convection - movement of heat energy by air that passes over the surface of the glazing Radiation - heat absorbed by glazing that is re-radiated as infrared heat

The thermal performance of glazing is determined by two main properties - the Solar Heat Gain Co-Efficient (SHGC) and the U-Value. Some manufacturers data refers to the Shading Co-Efficient (SC) of glazing that compares the glass to 3mm clear glass. This property is not often referred to and has been superseded by glazing’s SHGC.

Other properties quoted for glazing that do not impact directly on thermal performance are visible light transmittance and air infiltration.

1.2 Glazing Properties1.2.1 Solar Heat Gain Co-Efficient (SHGC)Sunlight and heat (solar radiation) form the Sun passes through a standard clear glass window very easily. The amount of heat that is transmitted through the window depends on the angle of the sun to the surface of the window. As the angle increases the amount of reflected heat increases and the amount transmitted decreases.

The amount of heat transmitted through a window is also affected by shading from the eaves and other shading devices.

The transmission of solar radiation from the sun through standard window glass is illustrated below.

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Unit of Competency: CPCCBC4021A 4


The amount of heat absorbed by the glass is dependent on the thickness of the glass and its absorption coefficient. The absorbed radiation is converted to heat, which increases the temperature of the glass. Often the glass temperature will be greater than either the indoor or outdoor temperature. Some of the heat absorbed by the glass will therefore be re-radiated and convected into the building.

Figure 1 Heat transmission through a window

The diagram shows the total energy gain due to direct solar transmission through clear window glass.

Figure 2 Percentage of energy heating a room through a window

The ratio of this transmitted energy to the total incident energy is known as the solar heat gain coefficient, (SHGC).

Although these diagrams show the incident radiation coming from the sky and making a glancing angle with the glass, the SHGC is actually measured with the incident sunlight perpendicular or at right angles to the glass.

The value quoted for the SHGC is often reduced by 10% to take into account transmission loss due to dirt build up. The information referred to in these notes is for clean glass.

The solar heat gain factor (SHGF) is the solar heat gain coefficient of "standard" 3 mm clear glass. For sunlight at normal incidence this is taken to be 0.88. ___________________________________________________________________________________________



Other types of glass can be compared to the standard 3 mm glass by using the concept of a shading coefficient (SC). This coefficient can be expressed as a ratio of the SHGC of the new glazing system compared to the SHGC of the standard clear 3 mm glass (SHGF) i.e.

SC = SHGC. SHGF

The Solar Heat Gain Coefficients for a number of generic glazing types are listed in the table below. Note that these figures apply to light incident at right angles to the glass.

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Figure 3 Transmitted, reflected, and absorbed solar energy by different glazing types

1.2.2 U-Value - Heat Transfer through Windows ___________________________________________________________________________________________



The other main property of glazing is the U-Value.

Heat energy will be conducted through the glass and frame due to the temperature difference from one side to the other. The greater the difference in temperatures – the greater the heat flow. Different frame and glass materials have different abilities to conduct heat, specified by the U-Value.

The lower the U-value – the less heat is transmitted.

U-Values are described individually for the frame or the glass but industry uses the U-Value by combining the glass and frame which is referred to as the system U-Value. The system U-Value depends on the U-Values of the frame and glass and the proportions of the area of the glazing unit occupied by each, which are referred to as the frame fraction and vision fraction respectively.

The heat transfer through a double glazed window is illustrated below.

Figure 4 Heat transfer through a double glazed window

Windows in Australia are certified for their energy performance by rating organisations who conform to Australian Fenestration Rating Council (AFRC) standards.

In the AFRC system, performance is always certified for the whole system – glazing and frame combined – never the glass or the frame alone.

The Australian Windows Association maintains a website (www.wers.net) that lists the properties of glazing units of its members.

The U-Value is important in both hot and cold climates. Conducted heat flow is relative to the difference between indoor and outdoor temperature. In hot climates it may regularly be 10 or 15 degrees hotter

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http://www.wers.net/


outside than inside, so halving the U-Value will halve the conducted heat gain.

1.3 GlassThere is a wide range of glass available to the builder that can be used to maximise or minimise heat energy gain into the building as required.

Some common glass types are listed below.

Toned or tinted glass has colouring additives included in the glass during manufacture. A range of colours is available with tinted grey being a common choice of builders seeking to reduce solar heat gain into the house. The different colours and tints provide different SHGC and some variation in visible light transmittance. Glass tinting does not change the U-Value of the glass because glass conductivity and emissivity are unaffected by the presence of a pigment in the glass.

Supertoned or heavily tinted glass has higher levels of additives in the glass to filter out the solar near-infrared heat energy whilst preserving a reasonable level of visible light transmittance.

Reflective glass has either a vacuum-deposited thin-film metal coating or a pyrolytic reflective coating added during manufacture. They must be kept clean to work effectively. Local councils may impose restrictions on the use of reflective glass as it often causes glare that annoys neighbours.

High transmission Low emissivity (low-e) glass has a thin metallic coating added during manufacture that allows daylight from the sun to pass into the house but reduces the amount of the longer wavelength infrared heat (radiated by heated objects within the house) that can escape through the window.

Low transmission low-e glass has a coating added during manufacture which reduces the amount of solar heat gain while still maintaining good levels of visible light transmittance and may provide an improvement in both U-Value and SHGC of glazing units.

The thickness of glass has little impact on its U-Value and SHGC. It does though, have a significant impact on noise transmission and the strength and safety of the glazing.

Glass may be provided as a single sheet, or as two sheets with a polymer laminate between the glass. The performance of laminated glass is determined by the type of glass in each layer.

It is often wrongly assumed that double glazing is only for cold climates. In fact, the best performance levels in both U-Value and SHGC can only be achieved by double-glazing, particularly in air conditioned homes. Multiple layers of glass can be assembled with sealed cavities between each sheet.

This is commonly called double or triple glazing but is now increasingly referred to as an Insulating Glazing Unit (IGU).

Insulating Glazing Unit: The insulating value of an extra air space to increase the resistance to heat flow. Improvements in winter performance is due to this decrease in conductance, while the increase in

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summer performance is due to a combination of decreased conductance, and decreased transmittance

of solar radiation through the second pane.

Hermetically sealed units usually have a desiccant in the edge spacer between the glass panels to stop internal condensation. The gap between the two panes can be filled with a gas that is less conducting than air such as argon or krypton.

Glass has an emittance of about 0.9 that leads to a significant amount of heat being radiated across the gap in double glazed systems. Thin low emittance films can be deposited onto the internal surfaces of double glazing. These films reduce the radiated component of conductance across the air gap. Windows incorporating these films are referred to as being "low E".

Commercially available gas filled double glazed windows with one low emittance transparent metallic coating have a claimed conductance of one quarter that of single glazing, while still maintaining a high light transmittance.

Window films can be an cost effective option for significantly reducing solar heat gain through existing windows.

Applied to existing glass, some window films can halve the overall SHGC of the window by means of absorption and/or reflection of solar radiation. They may also cause an equal reduction in visible light transmittance which must be considered when choosing a film.

Window films do not generally have significant impact on the glazing U-value because they do not add thermal resistance nor reduce the emissivity of the glass.

1.4 FramesAluminium frames are common in many windows used in Australia but conduct the largest amount of heat per unit area. To improve the performance of the window, use frames with a low conductance such as timber, PVC, or aluminium incorporating a thermal break to reduce heat loss through the windows in winter and heat gains in summer.

Even though the frame is a small fraction of the total window area, nominally 10%, the impact on the conducted heat flow through window as a whole may be large and varies with type, size and material of the frame.

However, it is the high transmittance of heat through glass that is of more concern. Six times more heat is conducted through glass than an equivalent area of wall with resistance R 1.0.

Commonly available frames in Australia include :

Aluminium window frames are light, strong, durable and easily extruded into complex shapes, but aluminium is a good conductor of heat and can decrease the insulating value of a glazing unit by 20 to

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30 percent. Aluminium frames, especially dark coloured ones in full sun, absorb a lot of solar heat and conduct it inside.

A thermal break is often used to reduce the heat conducted through aluminium frames. It separates the exterior and interior pieces of the frame using a low- conductivity component (typically urethane or other low-conductivity polymer).

A large amount of energy is used to make aluminium but it can be recycled at the end of its use. Some manufacturers may be able to provide aluminium frames made from recycled material which uses far less energy to produce. Powder-coated aluminium never needs painting, which significantly reduces its resource impact.

Timber frames are a good insulator but requires more maintenance than aluminium. Timber frames may require larger tolerances in openings, which can result in gaps that allow air infiltration, unless good draught sealing (weather-stripping) is provided.

Timber absorbs carbon dioxide as it grows and retains that carbon until the wood is burnt or decays. Timber species must have naturally high durability or be treated to prevent decay and deformation. It is important to check that the timber is sourced from a sustainably managed forest. There are currently Australian hardwood window frame manufacturers that use timber certified by the Forestry Stewardship Council (FSC). Plantation-grown hoop or radiata pine can be treated with LOSP (light organic solvent preservative) and painted which provides another option apart from FSC-certified durable hardwood.

Composite frames use thin aluminium profiles on the outer sections with either a timber or uPVC (unplasticised polyvinyl chloride) inner section. These provide the low maintenance and durability of aluminium plus improved thermal performance.

uPVC frames are petroleum derived products which are relatively new in Australia but common in Europe and North America. Their insulating properties are similar to timber and they can be moulded into complex profiles that provide excellent air seals. The colour range is more limited than powder coated aluminium.

Fibre-reinforced polyester (FRP) frames are used overseas and are generally the most thermally efficient high-strength framing materials available.

(Your Home Technical Manual - Fourth Edition as amended - 2010)

1.5 Window Coverings (Curtains/Blinds)The conductance of most window types is quite high so window coverings (curtains/blinds) may be used to reduce this heat flow. The best results are achieved by using drapes and pelmets that do not allow air to escape out of the cavity formed between the glass and drapes.

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Figure 5 Curtains can reduce heat flow

Window coverings are also used to restrict the amount of sunlight entering the room if it starts to overheat.

1.6 Glazing - Reducing Solar Heat GainIn regions around Australia (generally the warmer and hotter climates), where no winter heating is required, the heat gained through glazing in all orientations should be minimised.

In all other regions, windows facing north should have glass with a high SHGC to allow for heat gain in winter, whereas the east and west facing windows should have glass with lower SHGC values to minimise overheating in summer. In sub tropical regions, south facing windows only receive diffuse and reflected radiation (except for short periods at the beginning or the end of the day in summer), so they don’t contribute to overheating to the same extent as east and west facing glazing.

The easiest way of reducing solar heat gain is to reduce the area of glazing in the wall. If this is not possible or desirable then windows can be shaded. Alternatively, tinted or reflective glass can be used to cut down on the solar heat gain.

1.6.1 Solar Radiation on Vertical SurfacesThe width of the arrows in the diagram below shows the relative amounts of average daily solar radiation incident on walls and windows facing each of the four cardinal orientations in summer and in winter.

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Figure 6 Example of average daily solar radiation on vertical walls

North facing vertical windows receive considerably more solar radiation from the sun in winter than in summer. East and west facing windows receive more radiation in summer than in winter. South facing windows also receive more radiation in summer than in winter, but much less so than for windows having east and west orientations.

This means that only north facing windows behave in a way that compliments the seasonal heating and cooling requirements of a building. Window placement can be optimised by putting as much of the glazing in the north wall as possible but this is obviously undesirable from a day lighting perspective so non-north glazing area should be kept to a minimum required for light and views.

South facing windows will receive less winter sun, but also less summer sun than east or west oriented windows.

Although east and west facing windows receive about equal amounts of solar radiation, it is better to have windows facing east rather than west. The sun enters the east windows in the morning when the house has cooled off during the night, whereas it enters the west windows in the afternoon when the building will be reaching its maximum temperature.1.6.2 Shading of WindowsAlthough sunlight penetration through windows is desirable in winter in non-tropical locations, it is to be avoided in summer. Simply selecting a window that reduces solar transmittance by absorbing or reflecting much of the solar radiation will enhance summer performance, but the trade off is reduced winter performance. Well-designed shading devices that are appropriate for the glazing orientation can reduce summer gain and still maintain useful sun penetration in winter.

A shading device is most effective when it blocks the direct sunlight before it reaches the outside of the building. In this way the solar energy that is absorbed by the device will be converted to heat away from the building without heating it. The device may even shade some of the ground in front of the house, reducing the amount of sunlight reflected in through the windows.

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1.6.3 Eaves Eaves are only effective in blocking direct sunlight, and do little to reduce diffuse radiation and are ineffective at blocking reflected radiation. However, it is the direct component of radiation that carries the most energy, and contributes most to summer overheating. The judicious use of eaves can be a cost effective way of controlling unwanted solar gains.

Figure 7 Use of eaves to exclude the sun

Eaves are more effective above north facing windows than windows in other orientations. East and west facing windows need much wider eaves to achieve the same degree of shading as north facing windows and additional vertical devices may need to be considered on these elevations.

As the latitude of the location increases, the altitude of the sun decreases, so wider eaves are required to produce effective shading. Low latitude tropical locations will require wider eaves above south facing windows, because the sun spends more time in the southern part of the sky.

1.6.4 External Shading Devices other than EavesSome examples of common shading devices are illustrated below.

Horizontal shading devices are best used above north facing windows.

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Figure 8 Shading devices for North facing windows

Vertical shading devices are best used on westerly and easterly orientations where the sun is too low to be effectively blocked by a horizontal projection.

Figure 9 Shading devices for East and West facing windows

Vertical devices will tend to obscure the view from the window if they are mounted directly in front. Vertical fins at the sides of the window will restrict the view to that side.

Retractable or Adjustable Shading offers a better solution. If a building is designed with oversized north facing windows in order to enhance solar gain in winter then the summer performance will suffer. Using a shading device that can be closed to reduce solar gain through windows when overheating would otherwise occur can overcome this.

The main problem with these devices is ensuring that they are adjusted correctly; otherwise both summer and winter performance can suffer.

1.6.5 Clerestory Windows and Skylights___________________________________________________________________________________________



Highlight, or clerestory windows, located with eaves of a reasonable width, can be used to allow daylight into rooms if required. A significant amount of heat can be lost through these highlight windows in winter, so well fitting curtains should be considered. Double glazing will reduce heat loss, and should be also considered as an option.

North facing highlight windows may contribute to solar heat gain in winter but may produce glare. South facing glazing will admit light without producing glare in locations south of the tropics.

Skylights are not easily shaded and so usually permit considerable solar gain that can make the room too hot in summer. Some inbuilt control to minimise this heat gain is desirable in warmer climates.

Figure 1 Highlight windows can enhance solar access

Skylights can be a source of heat loss in winter when the warm air inside the room rises to the glazing where heat is transferred to the outside. Units that are ventilated to avoid fogging may contribute to draughts. Double glazed units can reduce this heat transmission while avoiding condensation.

Small tubular skylights such as Solatube present less of a potential problem. Often a reflective well is used to direct the sunlight downward. Diffusers should be fitted to these skylights to reduce glare.

1.6.6 Area and Orientation of Glazing Whilst glazing is important for allowing light into a building, for views, and to provide a feeling of being connected to what is outside, it is a potential source of discomfort in both summer and winter. Conventional windows have the highest solar transmittance and the highest heat conductance of all window systems and building elements.

Good building design needs to take advantage of the transmission of light through glass, while minimising heat loss in cold conditions and controlling heat gains during the summer months.___________________________________________________________________________________________



Ensuring the area of glazing for the orientation of the glazing is the most effective way of limiting too much heat gain or loss through windows.

Shading of glazing, using low U-Value and SHGC glazing and frame materials enable a larger area of glazing to be used if desired and may be possible without impacting on the comfort inside the building or increasing the need to run a heater or air conditioner.

1.6.7 Interaction with Mass and InsulationHouses with significant thermal mass are able to store much of the excess heat produced by solar radiation entering through windows, and to release it later when room temperatures drop. Lightweight buildings tend to respond to solar gain by producing a more rapid rise in air temperature, which can quickly produce uncomfortably hot conditions. Consequently, heavyweight buildings are able to take advantage of larger window areas, which would cause overheating in lightweight structures.

The use of low conductance windows is only warranted if the rest of the building is well insulated. Comfort conditions are easier to maintain when all the elements that make up the envelope of the building have similar thermal conductance, rather than having some very well insulated elements while others have poor resistance to heat flow.

1.7 Windows for Australian Climates Window selection should be based on the climate of the location, the area and orientation of the glazing, the mass of the building, the insulation in the building, and any shading either deliberate or due to trees or structures.

Conventional windows facing directions other than north will reduce the level of thermal comfort in buildings located anywhere within Australia. These windows include single and double glazed as well as tinted and reflective types.

This loss of comfort increases as the area of non north facing glass increases.

In warmer climates, even north facing windows contribute towards discomfort in summer, although not as much as windows in other orientations.

In cold climates, north facing glazing is valuable if the heat gain from solar radiation exceeds the heat losses through the glass. This means that standard single glazed windows will always produce a net loss. Low conductance windows such as sealed double glazing in PVC frames, or even double glazing incorporating low E surfaces, and perhaps low conductance gases, must be used before there will be a net thermal benefit from the solar gain through north facing glazing.

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In the milder climates experienced by most of Australia’s population, standard single glazing to the north will be beneficial in winter, although it may contribute to overheating in summer.

The following sections present information on the performance of glazing in a range of climates. Performance is measured here in terms of heating and cooling energy required to maintain thermal comfort in a building.

1.7.1 Temperate Climate The use of low conductance windows with a lower U-Value with frames such as timber or uPVC

instead of aluminium can reduce heating and air conditioning energy requirements by about 7%. North facing windows may require a higher SHGC with a lower SHGC to the east and west facing

windows. The use of double glazing in low conductance frames can reduce energy consumption by about

17%. Significant areas of glazing in any orientation other than to the north decreases performance. North window areas below 12 m² can decrease energy consumption of the standard building by

up to 10%. High mass houses are able to achieve greater gains in performance through the use of areas of

north facing glazing than lower mass buildings. West facing glazing, and to a slightly lesser extent, east glazing, decreases thermal performance. An eaves width between 450 mm and 900 mm will suffice above tall north windows. The

performance of south windows is not affected by eaves width. Wide eaves should be used above east and west windows.

Fitting retractable external blinds or other adjustable shading devices to north facing windows can enhance thermal performance, particularly if there is no other shading of the windows. Houses with adjustable shading devices fitted above 12 m² of north window area require about 20% less energy than if fixed eaves have been provided.

1.7.2 Coastal Sub Tropical Climate With the exception of north windows in a high mass house, increasing the window size in any

orientation reduces performance. Tinted and reflective glazing in non-north facing walls can reduce the impact of summer overheating.

The best summer performance is from north, and to a lesser extent, south facing windows, rather than from windows facing east or west. Good cross ventilation, which is important for comfort, requires openings on opposite sides of the building, so it is better to have most of the glazing openable, and facing in the north and south, rather than east and west.

Winter performance of non-north glazing is almost independent of glass area and eaves width. This indicates that solar gains through these windows in winter are balanced by heat losses. Lower conductance clear windows can enhance performance.

South window performance deteriorates with an increase in glazing area. The deterioration in performance due to increases in glazing areas in both east and west

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windows is at a minimum when eaves are very large, as is effectively the case with a verandah. Low mass buildings suffer the most loss of performance from an increase in window areas,

particularly if the eaves are narrow.

1.7.3 Hot Climate If there is no need for passive heating in winter, then any heat gain at other times can lead to

overheating. Window areas should be kept to the minimum required for day lighting and views. External shading of all glazing by eaves, verandas, etc is necessary. Slightly larger glazing areas can be considered if tinted or reflective glass is used.

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Acknowledgements

Figure 1 Your Home Technical Manual - Department of Climate Change and Energy Efficiency -

Fourth Edition as amended - published 2010

Figure 2 Dr Holger Willrath - The Thermal Performance of Buildings - Short Course Notes









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Questions - Glazing

1. A window has a SHGC of 0.86. This is most likely to beA. ClearB. Double glazedC. Tinted or tonedD. UV resistant

2. A window has a SHGC of 0.65. This is most likely to beA. ClearB. Double glazedC. Tinted or tonedD. UV resistant

3. Select the most correct statement for temperate locations A. Horizontal shading devices work best above north facing windowsB. Horizontal shading devices work best above east and west facing windowsC. Eaves work best above East and West facing windowsD. Adjustable shading devices work best in front of south facing windows

4. A window suitable for a cold climate requires A. High SHGC high U value B. Low SHGC high U valueC. High SHGC low U value D. Low SHGC low U value

5. The most important property for a window in a hot climate is A. High SHGC B. Low SHGCC. High U value D. Low U value

Documents

Build Thermally Efficient and Sustainable Structures€¦ · Web viewFigure 8 Shading devices for North facing windows Vertical shading devices are best used on westerly and easterly