WINDOWS

When you buy a window, it’s not like buying a washing machine – which will perform the exact same no matter where it is. The performance of a window is almost temperamental, depending on climatic conditions, orientation, shade, user behaviour and the type of building it’s in.

Without losing the run of ourselves, we need to be aware that the one-fits-all approach is definitely not for windows. Choices need to be made about the type of frame as well as the type of glazing, and there are many combinations of both. Double-glazing is almost taken for granted these days; the talk has now turned to triple-glazing – although not everyone is as enthusiastic about this as you might expect (more about that later). Glazing can also be coated and/or tinted, and it can come in layers with gases (argon or the more expensive krypton) instead of air between them. Frames can be wooden, aluminium or PVC, or combinations of these.

Windows are the weak spots in the overall thermal efficiency of a house and a concerted drive has been taking place in building regulations to lower their U-value (their heat-loss rate). The SEI (the independent agency, Sustainable Energy Ireland) says windows can strongly influence a building’s overall energy performance. Their distribution, area and quality affect solar gains and heat losses. In the EU, the presence of windows accounts for as much as 25-30% of space heating energy consumption.

Developments are constantly being made to window design, especially in the context of energy saving. One of the latest advances is a special coating on one of the panes of double-glazing, which further reduces heat loss below that of ordinary double glazing. This coating is known as low-e (for low-emmissivity) such as Pilkington’s K-Glass. There is also tinted glass, which is used for more than just keeping out the sun. The tinting, in either grey or bronze, does cut down on the light coming into the home but it also keeps heat in.

And triple-glazing? That’s three panes of glass with two air/gas spaces between them. It is widely used in cold climate countries such as Sweden and Norway, where its greater efficiency in energy performance is much appreciated. However, critics say that triple glazing doesn’t make sense in a climate like ours and that the additional energy saving is miniscule in any case. While it reduces heat loss, it can also have the effect of reducing solar gain. Someone even suggested we could get as much benefit by going back to the old-fashioned way of drawing the curtains across our windows after dark!