A brief guide

Windows are essential to our houses. They give us natural light, a way to ventilate the home without depending on mechanical systems, a means of emergency exit, and offer an alternative way to heat the house free of charge through passive heating.

By Denis Boyer

But they are also holes in our wall structure, making them by far the weakest part of the thermal envelope of a home.  So how should we select our windows to reduce energy bills all year round?

Choosing the wrong windows will work against efforts of adding more insulation to the walls and roof to improve energy efficiency. To illustrate this point, let’s imagine that a home has about 1,800 sq. ft. of walls which are insulated to R50. If the house was fitted with just one 3 feet by 3 feet window with an R-value of 1, the R-value of all of the walls combined would drop to R40 even though the poor-performing window occupies only 0.5% of the total wall area.

If the house actually had two such windows, the R-value would drop further to R33; and if there were one on each façade, the overall effective insulation level would be a mere R25, half of the desired value but with only 2% of the whole wall area covered with windows. The same principle applies to all other parts of the thermal envelope. For instance, there is no need to insulate the roof to R100 when the walls are just R30.

If, however, the windows performed better, the overall R-value of the wall system could be much improved. The table below gives an idea of what could be achieved with different window surfaces and R-values for that same wall insulation level of R50.

Having more window area, on the other hand, guarantees more solar gain [except for North-oriented windows] and natural light and, thus, can lead to even lower energy bills.

Denis Boyer, P. Eng. M. Eng. Is Energy Efficiency Co-ordinator at Ecohabitation in Montreal, www.ecohabitation.com .

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