How to get the most out of windows

G Magazine

Windows are not just dressing to a house; they also make a big difference to how much heat and energy your home uses. So how do you make your windows work for you?


Credit: iStockphoto

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We think of windows as providing some protection from the outside world as we gaze out - indeed they do, but not as much as you might think. Even though they protect us from rain and wind, they also let about one third of a home's heat escape.

"Windows are the weakest thermal element in a house," says Leigh Duxton, advisor for Canberra-based Home Energy Advisory Team.

So what can we do to prevent our home's warmth from rushing out the window?

Build efficiently

The most cost-effective way of creating an energy-efficient home is to consider these issues when you're in the design stage. As well as the location and size of windows, the orientation of the house and type of insulation and materials should all be factored into the home's design.

Ideally, you want most of your windows (if you're in the Southern Hemisphere) facing north, with one or no windows on the west and east, and only a few on the south. This arrangement will make the most of northerly sun for winter warming while limiting how much hot westerly sun penetrates in summer.

The size of the windows and where you plan to put them depends on a number of factors, not least the angle of the sun, which changes with the seasons and latitude. So it's a good idea to find an architect with an understanding of home energy rating tools to advise you when the time comes to renovate or build.

Choose efficient windows

With thousands of windows on the market and 27 different basic types of glazing to choose from, buying a window can be quite bewildering.

Basically, there are two issues to consider - the U value, which is how easily heat moves through a window, and the SHGC (solar heat gain coefficient), which measures how well a material blocks heat from sunlight. A low U value is recommended in all climates, but the ideal SHGC will vary depending on where you live.

If this all sounds a bit confusing, don't worry - the Window Energy Rating System (WERS) is a star rating system to simplify the decision process. The label is split in half, with blue stars to describe which windows work well in winter and red stars for summer.

WERS administrator, Michael Palin, suggests "Choose at least a 2.5-star rated window for a mixed climate", meaning a climate which is both hot and cold, such as Canberra. "For more extreme heating or cooling climates such as Hobart or Darwin, aim for at least 3.5 stars."

Double glaze

While nearly 60 per cent of Australian homes are insulated, surprisingly only four per cent of homes have high-performance glazing.

"It is a myth that in Australia we don't need double glazing," says Ian Frame, executive director of Window and Film Association of Australia New Zealand.

Double glazing is an extremely effective way of insulating a window during cold weather. In fact, it is essential for preventing heat loss if you have skylights or a great view you don't want to block.

Bear in mind, though, that double-glazed windows don't prevent heat gain in the summer, so they will need some shades on the outside to prevent your home from turning into a giant oven throughout the warmer months.

While a single-glazed window doesn't prevent any heat from escaping, a double-glazed window will cut down on heat loss by 30 per cent. Double-glazed windows trap air in the gap between two panes of glass, providing more insulation.

The gap can also be filled with argon gas, which is 20 per cent more efficient as an insulator than air. Another option is double glazing with 'low-e' (low-emissivity) glass, which lets in the light while blocking the sun's heat. These windows also reduce heat loss by 20 per cent compared to standard double glazing.

While double glazing is not cheap, the benefit in terms of comfort and reduced energy bills is huge. "In some areas the investment in double glazing can pay back within three years" says Frame.

Selecting your frame

Choose your window frame carefully since heat can just as easily sneak past a window frame as it can glass. The standard aluminium frame is the worst offender, although heat loss can be halved if the aluminium frame has a thermal break (a good insulating material).

The best insulating frames are wood or PVC. Wood is attractive, but it's expensive to purchase and maintain. PVC on the other hand is durable - but there are serious environmental concerns about its use, and the Green Building Council of Australia recommends avoiding it.

A good compromise is a composite frame, which has the insulation of wood on the inside and the durability of aluminium on the outside.

Leigh Duxton says "Thermally, they are nearly as good as wood but have the advantage of longevity."

Affordable window treatments

Don't be concerned if installing double glazed windows doesn't suit your budget - there are other options such as secondary glazing or using shrink plastic film. Not only are these approaches cheaper, but they also reduce the need for new raw materials because they make use of the existing window.

Secondary glazing is achieved by installing another pane of glass in front of an existing one. Because it uses the existing window, the cost of secondary glazing knocks about 30 per cent off the price of conventional double glazing.

Using colour-matched timber and sealants makes the result similar in appearance to factory manufactured double glazing. Because this treatment makes the glass is heavy it doesn't work with sliding doors, says Maurice Beinat, chief technical officer for EcoMaster.

Perspex, a synthetic made from acrylic, can also be used in secondary glazing. These types of windows not only improve window insulation, but can significantly reduce noise pollution. Perspex is lightweight, but unlike glass, it can become scratched.

The most affordable way to simulate double glazing is to use shrink plastic, at a cost of just $20 for a 120 x 80 cm window. It works exactly like glass double glazing, but instead of adding a second pane of glass, a sheet of thin plastic film creates a gap between it and the window.

While it's designed to be permanent, if applied with removable tape shrink plastic could even be used in a rental home.

Internal window coverings

Unlike windows, curtains and blinds have no energy rating system, but there are some essential principles to keep in mind. Like double-glazed windows, the aim of a good window covering is to create a seal around the window that will insulate your home from the elements outside.

Timber Venetian blinds might be fashionable, but they won't do much to retain heat in winter. Aluminium Venetians and vertical blinds are just as ineffective for the same reason - all those slats let air escape. Better alternatives are insulated Roman, Holland or Austrian blinds.

If the timber look appeals, try wood or polystyrene shutters, which are twice as effective as Venetians at preventing heat loss.

Shutters can be installed either inside or outside the home but in either case must fit snugly against the window frame, with the louvres closed. Ideal not just for insulation and privacy, they also offer an elegant solution for the security-conscious.

Coming in a close second to shutters are closely woven lined curtains with a pelmet (a cover for the curtain rod), which surprisingly rate slightly better than standard double glazing.

They allow a heat loss of 63 per cent of heat compared to double glazing's 67 per cent. To be effective, curtains must either fall all the way to the floor or be installed so they fit inside the window frame.

You can improve the seal even further by applying velcro tape to the outside edges of the curtains and attaching these edges to the wall.

For those who like the look of sheer material covers or other styles of window treatments that are poor insulators you could use a combination of treatments. For example if you can't live without your timber Venetians, double glaze or tuck a Holland blind in behind them.

Combining window treatments reduces heat loss even further when coupling double glazing with heavy lined curtains and pelmet, which can get your heat loss down to just 47 per cent.

Of course, the amount of heat lost depends on how well the windows themselves are sealed. You can either do this yourself or get a professional to fill the gaps between the window and frame.