OPINION: If you're looking to cool your house in summer or keep it warm in winter, the most critical element to get right is your windows.
More than insulation, or the lack of thereof, windows are the prime cause discomfort in most Kiwi buildings. Too hot or too cold but rarely in the Goldilocks zone of being just right.
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The majority of homes in New Zealand are single glazed, and a single pane of glass cannot retain heat in a building effectively except when it's hot outside. When the sun is out, those windows magnify the sun's heat inside the building (also known as solar heat gain).
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There are measures you can take to improve the performance of your windows.
To improve the thermal performance of timber framed single glazing, look for a window film that acts as secondary glazing. 3M does a good one, and it's around $25 from most DIY stores.
The pack can upgrade about five windows, it takes an hour or so to install and is almost invisible. It reduces the heat lost out the windows by about 50 percent. Combine this with sealing any gaps around the window by using something as simple as painters tape. The return on a $30 investment and a bit of faffing would be recovered in a few days of winter power bills.
To reduce the solar heat gain, rather than relying on curtains, stop the heat getting in by external shading. A simple meshed fabric or geotextile hung outside the window can prevent much of the heat from the sun being magnified by the glass without preventing you from seeing out.
Newer buildings have double glazing. Double glazing works by the insulating effect of the air gap between the two panes of glass.
Unfortunately, double glazing isn't much better than single glazing in New Zealand because of the use of aluminium.
Aluminium is the seventh best conductor of heat known to science after diamonds, silver and gold (at least we can say we use a material that is more cost-effective than diamond-encrusted silver window frames).
Fifty percent of the heat lost in a new build house is lost through window frames despite taking up a vanishingly small area of the outside of a building.
Paying an extra five percent for a thermal break in that window frame (and positioning it correctly in line with your insulation) will have a noticeable impact on the comfort of your building and your energy bills.
A thermal break is a small bit of plastic that separates the inside and outside window frame. This bit of plastic prevents the heat from being, very efficiently, conducted in or out of the building.
In my opinion, aluminium joinery without thermal breaks should be made obsolete. There would be a minimal cost from this change and a considerable performance benefit. The only reason it costs more at the moment is that it isn't a standard offering. All the major window companies want to change this.
Low E coatings to your windows reduce the solar heat gain from the sun - they can be also be fitted retrospectively. The coating does not have to be tinted, so if you don't want to live in the dark, make sure you avoid the tinted versions.
Windows are essential to get right if you want to be comfortable in your home. This is often overlooked when building new and retrofitting old buildings.
This oversight is a significant mistake in my view. Especially as improvement measures do not need to be costly, just $30 for some window film and painters tape.
- James Powers is director of Oculus Architectural Engineering
