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Glazing for Thermal Performance

Performance benefits of carefully selected glazing
Performance benefits of carefully selected glazing

The impact of glazing to the thermal performance of a building is significant and complex. We will offer well thought out advice on the selection of glazing options to achieve best results possible for your glazing requirements.

Windows can severely impact on the heating and cooling loads of a building. Up to 40% of a home's heating energy can be lost and up to 87% of its heat gained through the windows. As previously discussed, incorporating passive solar principles at the design stage is the most cost effective way of achieving good thermal performance, looking to design for the climate, orientation, shading, passive solar heating, insulation and in climate zones where there is a requirement for thermal mass, as well as all of those considerations the client may have a specific view they wish to capture.

The implementation of passive solar design can be more challenging on site sites. For example, winter sun might be blocked by neighbouring buildings or trees, or views may be to the south or west, often leading to the inclusion of larger windows with poor orientation. In these instances, selecting glazed elements with improved thermal performance to compensate for aspects of the building that are detrimental to its thermal performance.

The angle that solar radiation strikes glass has a major impact on the amount of heat transmitted. When the sun is perpendicular to the window, if the glass is standard 3mm clear glass, 85% of solar heat is transmitted. Once the angle increases, more solar radiation is reflected and less is transmitted. Once the angle exceeds 55o it falls sharply and the effective area of exposure to solar radiation reduces. Thus the same window can have a hugely different solar gain, depending on the angle of incidence, which is influenced by the position of the sun according to location, season and the time of day in relation to the orientation of the window.

 Looking at the thermal properties of glazing, it is always the case that the lower the Uw-value the better, because its the Uw-value measures the ability to retain heat in winter and cold is summer, regardless of orientation, in hot climates such as Darwin and Brisbane, there is little compromise; a low SHGC is best. In these climates, if passive solar gains are desired in the coolest months they should be obtained from northerly windows only and can have a slightly higher SHGC if they are protected by climate appropriate overhangs or other shading measures.

In selecting the type of glazing, you should be aware that just using a film or heavy tint, whilst reducing solar radiation transmitted through the glass it can also significantly reduce the amount of daylight, to a point where lighting is required and therefore adding to energy costs. The Window Energy Rating Scheme (WERS) rates the energy and energy-related performance of windows, skylights and glazed doors in accordance with AFRC procedures. WERS provides the system U-value and SHGC values as well as air filtration and visible light transmittance. Computer programs such as BERS Pro can calculate your home's heat loss or gain. They take into account local temperature, sunshine, wind & humidity. Offering options and comparisons to optimum performance of each window.

There are a wide variety of glass products available. The thickness of the glass has negligible impact on its U-value and SHGC values. Although it does have a significant effect on noise transmission and the strength and safety of glazing.

Glass products can be divided into several categories:

  • Toned glass has colouring additives included during manufacturing, and available in various colours usually bronze, grey, blue and green. The different colours provide different SHGC values and some variation in TVw (visible transmittance of the window). However, tinting does not change the U-value of the glass, unless a toned glass option called supervened, which has heavier pigmentation that filters solar wavelengths and provides lower SHGC while preserving adequate TVw.
  • Low emissivity glass (low E laminate) has either a vacuum-deposited thin film coating or a pyrolytic coating. As vacuum-deposited coatings are soft, for protection and longevity they must be deployed inside an insulating glass cavity. While the pyrolytic coatings are baked onto the surface during manufacture while the glass is still hot to make it hard and durable. Soft and hard coatings are available in products for use in both high and low transmission applications. Low-E coatings make glass a practical wall material for contemporary living spaces. Low-E coatings can effect a very dramatic improvement in both U-value and SHGC but they must be employed correctly to perform to specification. Low-E coatings can be used in combination with clear, toned or reflective glass.
  • Laminated glass has a plastic glazing layer, called an interlayer, to improve impact resistance. This interlayer is placed between two sheets of glass in order to reduce the danger of glass breaking and forming long dangerous shards. Typical applications include areas like bathrooms, doors, staircase balustrade and in areas of glass close to the floor.
  • Insulated glass unit (IGUs) also known as double or triple glazed windows, are the combination of two or more glazing layers sealed with a gap between each layer. Multiple layers can be assembled with sealed cavities between each sheet. The performance of IGUs depends on the properties of each layer of glass and the width of the cavity (can vary from 6mm to 50mm), seal can be a polyamide thermal break and type and content of the cavities between the glass layers, which can be air or argon gas filled. It is wrongly assumed that IGUs is only for cold climates when in fact, the best performance levels in both U-value and SHGC can only be achieved by using IGUs.

It's interesting to note the star ratings that are achieved in varying combinations of the above glass options and varying aluminium window methods of assembly.

Energy Performance Data Comparison Chart - same size opening, in a standard aluminium frame and doubled glazed in an aluminium frame with a polyamide thermal break;

The more Cooling or Heating Stars the better but keep in mind the Lower the Uw-value and the SHGCs the Better for our climate;

With Standard Aluminium Frame

Glass Option Cooling Stars Heating Stars Uw-value SHGCw TVw
3 mm clear 1 Star 3.5 Stars 6.5 0.65 62%
5 mm grey 2 Stars 3 Stars 6.3 0.45 35%
6.38 mm clear low-e laminate 2.5 Stars 4.5 Stars 4.9 0.43 56%
6.38 mm grey low-e laminate 3.5 Stars 4 Stars 4.9 0.33 33%
Double glazed 3/12/3mm clear with 12mm space with air cavity 2.5 Stars 6 Stars 4.0 0.56 58%

Aluminium frame with Polyamide Thermal break

Glass Options Cooling Stars Heating Stars Uw-Value SHGCw TVw
Doubled Glazed 4/12/4mm clear, 12mm space with Argon Gas in cavity 3 Stars 6 Stars 2.6 0.52 58%
Double Glazed 4/12/4mm low-e clear, 12mm space with Argon Gas in cavity 3.5 Stars 7.5 Stars 1.9 0.45 58%
Double Glazed 4/12/4mm Sol B, 12mm gap with Argon Gas in cavity 6.5 Stars 6 Stars 1.7 0.18 48%

 

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