Glazing allows warming sun to penetrate the building envelope, making it an important part of a passive building. Despite this benefit, the incorrect placement and type of glazing can be very detrimental to thermal performance. Glass is transparent to short wave radiation, yet it does not transmit long-wave thermal radiation. This means glass allows solar access to the building while trapping heat inside, which is very important when it is colder outside (Crocombe, 2007)(Reardon et al., 2005, Wrigley, 2005).
The location of glazing on a building is important for more than just visual and lifestyle purposes. Glazing must be located to maximise sun during the winter, yet not allow overheating during the summer (Al-Azzawi, 1991, Hollo, 1995). Wherever possible, larger areas of glazing should be situated on the northern side of the building, and glazing to the east, west and most importantly south should be minimised (Al-Azzawi, 1991, Crocombe, 2007, Reardon et al., 2005). Windows on the eastern and western sides can also be effective at capturing morning and evening sun, however excessive
unnecessary heat loss. Windows facing east and west can also expose the building interior to excessive insolation in summer mornings and evenings when the sun is at a lower angle, resulting in excessive heat and additional air conditioning requirements. Shading of eastern and western glazing can help to mitigate this overheating.
Windows on the southern side of buildings should be kept to a minimum, however they can be important in retaining suitable levels of amenity by allowing light into darker areas, or providing access to attractive vistas. Intelligently placed smaller windows; windows shaded from the sun at lower levels by foliage, louvers (see Figure 3.8), courtyards or other shading devices; or windows with coloured or shaded glass are appropriate in such instances. Ideally, wherever glazing is on the northern side of the building, the east, west and south facing glazing should be kept to a minimum (Al- Azzawi, 1991, Hollo, 1995, Reardon et al., 2005, Smith and Pitts, 1997).
Attitudes differ with regard to the area and placement of glazing. While it may seem preferable to include glazed areas as large as possible in building design, and calculations based on energy criteria alone show benefits in glazing areas of as much as 50% of floor area, this excess of solar gain can cause thermal discomfort during the summer and on days with higher levels of solar radiation, as well as increasing other problems in direct gain systems. The Building Code of Australia (BCA) Clause J2.3 specifies how much glazing can be used in buildings under a wide range of conditions, and the Australian Building Codes Board (ABCB) provides online calculators to determine if glazing meets the requirements set out in the code.
3.8.1. The importance of shading and insulating glazing
Glazed areas such as windows and skylights are areas of low insulation, making them a source of heat loss (Crocombe, 2007, Lerner, 1998). This loss can be minimised by the use of double-glazing, insulating curtains and pelmets. One of the best ways to minimise heat loss due to glazing is to minimise the use of southern, eastern and western glazing that does not provide solar gain, but rather acts as a location of heat loss (Al-Azzawi, 1991).
Glazing can also result in overheating during the summer, and it is important to shade windows to prevent this, even in temperate and cool temperate regions. External devices such as external shutters, eaves, louvers, pergolas and blinds are effective methods of shading, as is the planting of suitable trees (Lerner, 1998, Windust, 2003, Wrigley, 2005). Solar pergolas and shutters are particular desirable as they can open and close when required to provide valuable external reflection of insolation or alternatively allow the penetration of warming sunlight (Hollo, 1995, Wrigley, 2005).
External shading is generally more effective, as it reflects energy before it enters the building envelope. Unlike external shading, internal blinds will be warmed by sunlight, and allow window frames and architraves to be similarly warmed (Reardon et al., 2005, Wrigley, 2005). Despite this, they can be quite effective and frequently easier and more cost effective to install (Reardon et al., 2005, Wrigley, 2005).
Intelligently selected and located plantings are another method of externally shading a building. As a general rule, the northern side of a building should feature deciduous trees, allowing the penetration of sun during the winter when the trees have shed their
is thick (Hollo, 1995, Reardon et al., 2005). Planting evergreen trees and tall bushes to shade glazing on the eastern and western sides of a building can prevent direct sunlight from penetrating to the glazing yet allow diffused light and retain amenity levels. Appropriate plantings are illustrated in Figure 3.9.
Figure 3.9: Plantings for Appropriate Light & Shade
While glazing is an important source of insolation, it can also be the location of unnecessary heat loss (Lerner, 1998). Glazing should be insulated wherever possible to minimise this energy transfer, and windows should be oriented towards the north (Lerner, 1998, Reardon et al., 2005). There are a variety of methods that can be employed to prevent unnecessary heat loss through glazing. Heavy curtains are an effective and relatively inexpensive method. The air space between the glazing and the curtain works as an insulating barrier. Drapery is most effective when reaching down to floor level and in combination with heavy pelmets. At the present time there is cultural resistance to curtains, with many people preferring a modern curtain free look, in many cases to the detriment of their thermal comfort or resulting in unnecessary energy expenditure.
Another method of reducing heat loss through glazing, and a possible solution to people‟s desire to not employ curtaining, is the installation of double-glazed windows. Double-glazing is the practice of installing two parallel separate sheets of glass in a window, separated by a space of 12-20mm. The thin air space reduces the likelihood of
convection currents, which move cold and warm air across the air space, increasing undesired heat loss or gain. This space can be filled with air, or alternatively gases such as argon, or a vacuum to further reduce the transfer of energy across the space. Double-glazed windows require specially fitted frames and can present considerable cost to home builders or retrofitters, but the finished product will generally look identical to regular windows; provide increased thermal comfort and light; and in many cases the resultant heating and energy savings can quickly offset the additional cost of purchase and installation (Wrigley, 2005). Double-glazing is particularly effective in colder climates, and when used in conjunction with heavy curtain insulation can almost remove extra heat loss of glazed areas (Reardon et al., 2005).
The use of reflective films can also further reduce heat loss, to prevent excessive insolation during summer, or to reduce glare (Hollo, 1995). Low e glazing is coated with a film that reflects long wave radiation either back into or out of the building. In this way it can help to keep heat either in or out of the building envelope. The film is generally placed on the inside surface of double-glazed windows (Hollo, 1995, Reardon et al., 2005). More information on the insulation of glazing is given in section 3.10 on insulation.
Skylights allow natural light into dark areas that are inaccessible to conventional windows (Crocombe, 2007, Lerner, 1998, Wrigley, 2005). If used inappropriately, however, skylights can exacerbate problems in regulating the thermal environment. Skylights can allow valuable insolation into a building, but if not properly shaded, excess solar energy can cause overheating during warm months (Lerner, 1998, Reardon et al., 2005). If not properly insulated, skylights can result in significant heat loss at night and in colder months. Ideally a skylight should comprise insulating properties at
ceiling level, preventing heat from rising up into the roof cavity, and have mobile external shading to prevent unnecessary sunlight from entering the building. A more suitable alternative to skylights are clerestory windows. These are vertical windows positioned between two roof planes, frequently on skillion roofs (Figure 3.10). Vertical positioning allows the implementation of eaves to shade from summer sun, yet winter sun is still allowed to penetrate. They can be insulated similarly to other windows, using double-glazing or internal curtains and shutters (Hollo, 1995, Reardon et al., 2005, Wrigley, 2005).
Figure 3.10: North Facing Clerestory Windows in a Skillion Roof to Allow Solar Penetration
Glass houses and conservatories on the northern side of the house can be another source of solar gain, particularly when coupled with dark interior surfaces and sufficient thermal mass (Wrigley, 2005). In temperate areas featuring hot summers however, these types of solar capturing rooms will need to be shaded as with other glazing to prevent overheating and unnecessary use of air conditioning.
There are a variety of types of window frames available, with differing thermal properties and maintenance requirements. Metal window frames will conduct heat across the building envelope, however they generally require less maintenance than wooden frames, particularly in the case of aluminium. Some metal window frames feature an insulating barrier inside the frame to prevent such heat transfer (Reardon et al., 2005).