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How windows help your house stay warm

The size of your windows and where they are positioned in your house is a huge contributor to the amount of heat that is gained from solar radiation (heat from the sun). So designing the placement of the windows, buying the right type of windows and most importantly – making sure they are installed correctly!! – are critical decisions when building or renovating your home.

Solar radiation enters glazing (the glass part of the window) by direct, diffuse and reflected paths before being absorbed by elements in the room, like the floors, walls, etc, which in turn heat up. These elements will then give off heat which can be positive during winter (but may lead to overheating during summer).


Solar radiation entering a building through a window.

Solar radiation entering a building through a window.

To avoid overheating in the summer, shading along with specialised types of glazing can be used to control the rate and impact of solar gains, as well as helping to maintain the optimum amount of daylight for natural lighting. Different kinds of glazing can be used to draw the sun’s heat into the interior, reject it, or allow interior heat to escape.

The number of panes of glass, internal coatings, colour tints, gaps between panes and gasses between panes all affect daylight and radiation transmittance.

It is important to note that windows responsible for large solar gains (bringing in lots of heat) during the daytime and during summer may also be responsible for significant heat losses during night time and during winter, so thinking about window coverings is also essential.

In Ireland, solar gain is optimised by positioning living area windows to be south facing, which maximises the potential heat gain where it can be most useful. Shading can then be used to block excess sun at warmer times and allow the sun’s heat in at cooler times.

Illustration of shading between summer and winter.

Illustration of shading between summer and winter.

To help you get the most benefit from your windows, your designer or builder should work to maximise solar gain within your building in the winter to reduce space heating demand and to control solar gain in summer so that there is limited overheating.

For more articles about energy efficiency and quality building, go to our articles archive.
Article adapted from QualiBuild Train the Trainer course manual

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