Windows are great, they serve to provide us with natural light and natural heating via solar heat gain. Windows also allow us to see outside and they facilitate natural ventilation when operable.
As good as windows are they are really bad at one thing – ‘insulating.’
When we consider our thermal fabric, (floor, walls, ceiling) the weak point is where this fabric is compromised. Windows represent a wound in the thermal fabric.
The NCC minimum thermal requirement, (R-value) for external walls is *R2.8. The R-value of a typical aluminium framed, single glazed window is R0.15, (Uw6.5). This 18.7 times less thermally efficient than the wall! Even a typical aluminium framed, double glazed window at R0.25, (Uw3.95) is 11.2 times less thermally efficient than the wall.
Now that we recognise that windows are a major source of heat loss, what do we do about it?
When designing your house consider where the major heating load is, (typically in the living areas/ kitchen etc.) and ensure that the windows in these areas are thermally efficient in order to minimise heat loss. Double glazing is a starting point. Good thermally broken, double glazed units are the best choice to minimise heat flow between inside and outside. Drapes with pelmets also add insulation to windows, (makes a big difference on cold nights).
How do you know what a good thermally efficient window is? The important thing to look for when choosing a window is the U-value of the whole window. This figure appears as Uw-.-. The lower the U-value the better. Typical Values:
- Uw6.50 = aluminium, single glazed.
- Uw3.95 = aluminium double glazed.
- Uw.2.3 = aluminium framed, thermally broken, double glazed.
The U-value is measured as W/m2K. Lets look at the aluminium framed single glazed unit as noted above.
Uw6.50 = 6.5Watts per 1m2 of window area per 1 degree of temperature difference, (temperature difference between inside and outside).
Now lets plug this U-value figure into a house with 30m2 of window area with a temperature difference of 12 degrees, (thermostat set to 21 degrees inside with an outside temperature of 9 degrees).
6.5 x 30m2 x 12 = 2730Watts of energy loss.
Putting this figure into perspective, 2730Watts or 2.73kW is equivalent to running 27 **Television sets. That’s a lot of energy.
Running the calculation again with the thermally broken window gets us down to 8 TV’s.
2.3 x 30m2 x 12 = 828 Watts of energy loss.
It’s not all bad news as of course windows also act to provide solar heat gain. This is heavily dependant on careful consideration when placing the windows. It’s important to get the orientation and shading right to maximize winter heat gain and summer shading to ensure a warm house in the winter and a cool house in the summer.
When you are looking for windows for your new home, or more importantly when you are at the design stage of your new home, have a careful look at where the windows are, which way they’re facing and address them appropriately. As a rule of thumb, north facing windows to living areas with well designed shading is optimal. Large expanses of south facing windows are primarily a heat loss issue.
Remember, the U-value is the measure of how much heat is transferred through the window. The lower the U-value the better the insulation properties of the window – the better it is at keeping the heat or cold out. In all cases regardless of climate zone a window with good insulation properties will help to improve the comfort of your home.
Note: The latest version of our glazing calculation includes window heat loss data, (measured in Watts) for the total window area of the house. This data is on the Window Analysis page and has scenarios for temperature differences from 6 – 21 degrees.
*R-value based on NCC Climate Zones 6 & 7
**Television sets assumed at 100W.
Example data from spread sheet below. Figures based on 30m2 total window area and Uw2.3 window units.