How Important is U Value in Choosing Windows?

April 30, 2015

Windows lose and gain heat by conduction, convection, radiation and air leakage. This heat transfer is expressed with U-values.

As a rating of energy efficiency, U-value can be used to consider performance of windows, exterior doors, skylights and all other exterior building components, including exterior walls.

In windows, the U-value measures how well heat is transferred by the entire window, that is the frame, sash and glass combined. 

The lower the U-value, the more insulated the window unit and therefore the better the window will be in retaining the interior’s heat in the winter and keeping heat out during summer.

U-values are important because they form the basis of any energy or carbon reduction standard.

A significant part of the thermal energy transmission can be through the frames, or profiles, depending on how well the profile material conducts heat. Metals are generally good conductors which is why they feel hot to touch when exposed to the sun. This makes them poor thermal insulators. 

Timber and plastics such as uPVC are poor conductors which therefore stops the heat transfer from the heat-exposed side to the non-exposed side. PVC and timber window frames are approximately thermally equivalent and provide a high level of energy efficiency compared to a standard aluminium frame.


 

Window UnitU-value 

A typical single glaze aluminium window generally 7.0

A typical double glazed aluminium window4.5

A typical uPVC double glazed window1.8

A typical uPVC triple glazed window0.7

 

The above U-values indicate the potential to save valuable heating and cooling energy being lost through your windows.
 

According to the Australian Windows Energy Rating Scheme , the U-value ratings for Australian products generally fall between 2.0 and 10.0. A typical double glaze uPVC window rates better than this at 1.8.

Assessing the impact of radiation


A second element used to measure the energy efficiency of a window system is the Solar Heat Gain Coefficient (SHGC) – a measure of solar radiation going through the glazing into a building and therefore warming the interior.

The SHGC is therefore an important factor to consider in the hotter climate zones across Australia.


Australians are now using more energy to artificially cool their homes in summer than to heat them in winter. However, the use of energy efficient windows and doors can provide significant energy savings.


In climates where reliance on heating and artificial cooling is significant, choosing a window unit with a lower U-value and a lower SHGC is important.

 

 

AUSTRALIA’S CLIMATE ZONES

 

Cooling Climates 
In cooling climates, the main goal is to keep unwanted heat from entering the home and to reduce the reliance on artificial cooling systems. Choosing double glazed uPVC windows with low SHGC measures limits solar heat gain and can therefore reduce artificial cooling requirements and so, energy consumption. 

Heating Climates
In heating climates, the aim is to retain heat and to maximise the penetration of solar energy in winter. uPVC window profiles provide excellent insulation, ensuring heat loss from inside to outside is minimised but solar radiation through glazing is good. uPVC windows with a low U-value and moderate to high SHGC are preferable in cooler climates such as Tasmania, Southern Victoria and some parts of New South Wales and South Australia.

Mixed Climates
In a mixed climate, the goal is to stop heat from entering the home during summer and from escaping during winter. In these climates, windows which offer a good balance between U-value and SHGC are required. Southern Western Australia, parts of New South Wales and South Australia, and northern Victoria are mixed climates. 

uPVC double glazed windows provide excellent insulation for your home, offering protection from the outside elements. When choosing windows for your home, be sure to choose based on the Solar Heat Gain Coefficient for your climate and the U-value.

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