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Calculate your Heat Loss

Calculate your Heat Loss

Heating (and in warm climates, cooling) are a large part of energy use, and this can be reduced dramatically by installing energy efficient windows and doors which will decrease energy costs and greenhouse gas emissions.

yourhome.gov.au writes that 'Up to 87% of a home’s heating energy can be gained and up to 40% lost through windows' and provides the following calculations:

The amount of heat conducted through a glazed unit (in watts; W) is calculated as Uw × T × A:

  • the U value (Uw)
  • multiplied by the number of degrees difference in air temperature on each side (T)
  • multiplied by the area of the glazing unit (A).


If your home has 70m2 of glazing with aluminium frames and clear glass with a U value of 6.2W/m2 °C, on a winter’s night when it is 15°C colder outside compared with indoors, the heat loss through the windows would be:

6.2 × 15 × 70 = 6510W

That is equivalent to the total heat output of a large room gas heater or a 6.5kW room air-conditioner running at full capacity.

If you choose a window with half the U value (3.1W/m2°C) (for example, double glazing with an argon-filled gap and less-conductive frames), you can halve the heat loss:

3.1 × 15 × 70 = 3255W

or a Paarhammer window with U value of 1.1

1.1 x 15 x 70 = 1155W,

This is one sixth of the first example of single glazing with aluminium frames and is well worth doing for energy cost savings as well as comfort.

It is important to note that installing high performance windows alone is not enough. It is vital to see the whole envelope of a building as one and insulate each part, e.g. walls, underfloor and ceiling. Did you ever sit close to a single glazed window and felt the cold (or heat) coming through the glass? The solution is better insulation including high performance windows and doors which will keep temperatures at comfortable levels during extremes of hot or cold weather and provide thermal comfort.


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