What is double glazing?
A Double-Glazed Unit refers to two pieces of glass separated by an air space, or some type of spacer, or gas in a sealed unit. Common spacing of double glazing in Australia is 6mm or 12 mm.
What is the U Value?
The U value or Uw is a measure of transmittance and allows comparison of thermal behaviour between different window units. It is a measure of conductivity of the whole window (glass and frame included) in Watts per square metre per Kelvin difference: W/(m2K).
The lower the U value the better the thermal insulation, therefore the more energy efficient the window performance. In simple terms, the Uw indicates how well a window or door will retain heat in the winter and reject heat during summer.
Aluminium conducts energy more readily than timber or uPVC and as a result, aluminium products will have a higher Uw than timber or uPVC with the same glass. The glazing also has an impact on the Uw of the product, when a frame is fitted with double glazing or low emissivity glass the Uw will fall making the product more energy efficient.
Thermal Conductivity of different frame materials (Uw value)
(calculated according to DIN EN 10077-2)
Hardwood Window 1.9 W/m2K
Aluminium without thermal break 6.6 W/m2K
Aluminium with thermal break (polyamid 25 mm) 2.2 W/m2K
uPVC Window 1.9-1.3 W/m2K
Some uPVC window systems achieve Passive House suitability as defined under the European Passive House standard, with U values of <1 W/(m²K).
What is a Solar Heat Gain Coefficient (SHGC)?
The SHGC measures the amount of heat that can be gained through your windows at times of direct solar contact. In summer, the SHGC can add large amounts of unwanted heat to your house, whereas in winter, the SHGC can provide free heat from the direct sun entering the home. thus the SBGC is important to know as it indicates the amount of direct solar heat that will penetrate your house.
The SHGC is mainly affected by the glazing type and orientation of the building windows.
What is a thermally broken aluminium window?
Preventing thermal bridging is important to improve insulation. A Thermally Broken aluminium window frame is one which includes a thermal break. Because aluminium as a metal conducts temperature, it has poor insulation without a thermal break being added. The thermal break is a non-conductive element that has been applied in the centre of the framing system to stop the flow of energy (thermal bridging) through the product. Thermally breaking the frame lowers the U-value. uPVC is sometimes offered as a cladding to aluminium products to improve their insulation.
Video Resources:
High Level Energy Efficiency (YouTube)
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The Science of Energy and Energy Efficiency (YouTube)
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Selecting the Right Glazing for Comfort (YouTube)
Australian Glass and Window Association
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