Windows might be transparent, but they’re complex. Good windows well placed will help keep your home comfortable all year round. Bad windows in the wrong place will cost you dearly.
In a typical insulated house, they cause more heat gain or loss than any other part of the building fabric. While they’re expensive up front, they’re also an investment in the resale value and day-to-day comfort of your home.
So which windows should you choose? There are hundreds of products and combinations to consider, from the glazing, frames and coatings, to the size, shape and location. The Window Energy Rating Scheme website lists detailed ratings of over 40,000 products.
Two years ago, Alan Kerlin designed his sustainable home in Canberra. Afterwards, he established a consultancy, Solar Flair, to help pass on what he found out. When he was researching windows, he found good advice hard to come by. “It’s a difficult area, but it’s easier if you understand some of the basics behind the science,” he says.
Heat transfers in different ways – for windows, you’ll need to consider conduction and radiation. Conduction refers to the ambient warmth that passes through the glass and the frame. A window’s conduction is measured by its U-value. The lower the U-value, the better its insulating qualities, and the better for your electricity bill.
Radiation, in contrast, refers to heat transferred when sunlight passes through the glass, hits something and warms it up. It is measured by the window’s Solar Heat Gain Coefficient (SHGC); the higher the SHGC, the more radiant heat it lets through.
Passive solar design
Armed with this knowledge, you need to consider the weather where you live and the design of your home. Most Australians live in climates where we want to draw in extra warmth during the cold months and shut it out throughout the hot months. With careful consideration, your windows can help this happen – together other elements of passive solar design, such as shading and orientation.
In Canberra, Kerlin designed his home with a bank of glass to the north – the sun streams in throughout winter, but eaves and shading block the direct rays in summer. Small windows to the south, east and west help reduce the solar access when the sun is low in the sky and passes below the awnings. “But remember: it all depends on where you are living,” he says. “In northern Australia, you never want sun hitting your glass at all.”
Insulating glazing units (IGUs)
No matter your location, there is one constant: double glazing is always preferable to single. For now, nearly every Australian home has single-glazed windows. “They’re like a thermal wound in the building envelope,” says Gary Smith, from the Australian Window Association.
Double and triple glazed windows – known as IGUs – help heal the wound. “Standard double glazing can reduce conducted heat transfer by about half,” Smith says. Triple glazing is common in Europe and North America, but rare here. The window units weigh and cost more, but provide extremely low U-values and excellent sound proofing.
Within an IGU’s frame, the panes of glass are held apart by a spacer. A wider gap gives better insulation – 12 mm is regarded as the best. Likewise, an IGU will prevent even more heat transfer if the cavity is filled with an inert gas, such as argon, rather than air. “With argon, you get about a 15 per cent improvement in U-value,” Smith says.
IGUs also perform strongly in bushfire attack conditions. “Double glazing works really well in the bushfire tests because the insulation barrier stops the radiant heat coming through the glass,” he says. This year, all states and territories will introduce a new standard for windows and doors in bushfire prone areas. So far, few products have been tested to the top levels.
Smith says the extra cost between single and double glazing can be between 50 and 100 per cent, depending on the company and the product. Householders can spend from a few thousand, to tens of thousands of dollars extra. “There’s a huge variance. The best bet is to shop around – there are good deals and really good products out there.”
Glass is no longer just plain old glass. It now comes in a dazzling range of coatings and tints that will help keep your energy bills down.
Low emissivity (low-e) glass has a transparent metallic coating that reduces the pane’s U-value. “Low-e glass can significantly reduce the amount of heat that travels through your windows, keeping your house more comfortable in both summer and winter,” says Jamie Rice, vice-president of the Australian Glass and Glazing Association. It can also curtail UV light and reduce fading in furnishings.
Single-glazed low-e coated glass is a good option for people who want a step up from standard glass but can’t stretch their budgets to double glazing. However, it’s far more effective when placed inside an IGU – it can reduce the U-value of a double glazed window by half again.
Tinted glass cuts the heat transmitted into the home from direct sunlight. Available in a range of colours, tints are especially suited to west-facing windows that receive direct, summer afternoon sun. “The problem with standard tints has been that to improve the performance you end up cutting out light,” says Rice. “But there’s now a more sophisticated product, called spectrally selective tinted glass, which significantly increases solar control and only slightly decreases light transmission.”
Low-e coatings and tints can be used in combination. Together, they reduce both the U-value and the SHGC, making for a window that’s ideal for keeping out the heat.
Most window frames in Australia are made from aluminium. They’re cheap and versatile, but conduct heat very easily, which means they slice the insulating performance by up to 30 per cent. Thermally broken aluminium or composite frames offer better insulation, but they’re much more costly and, for the time being, not widely available.
Timber frames also have significantly lower U-values than aluminium. Edith Paarhammer, from Victorian window manufacturer Paarhammer, argues that although timber is more expensive, it performs better than any other framing material.
She recommends that eco-conscious buyers choose products made from either plantation timber or Forest Stewardship Council certified timber. “It’s also very important that the frames are substantial, not flimsy,” she says. “And make sure they have seals all around, so there are no draughts.”
Another high performing frame is uPVC. Only recently introduced into this country, it has a comparable thermal performance to timber, but is cheaper. Warren Miles from Ecovue says a double glazed uPVC window can cost just 25 per cent more than equivalent single glazed aluminium.
Miles says it’s crucial that buyers look for frames that minimise air leakage. “You need a complete seal between the window and the frame, and also between the frame and the structure of the building. If you can’t achieve that you may as well not worry so much about the glazing.”
Miles says it’s crucial that buyers look for frames that accommodate double glazing while also minimising air leakage. “You need a complete seal between the window and the frame, and also between the frame and the structure of the building. Reducing air infiltration is a significant part of energy efficiency.”
Few businesses are specialist window installers, although some manufacturers can also do the job. You can find them listed on the Australian Window Association website.
If you’re in an existing house and want to improve your windows, you have several options. The most effective and expensive way is to remove and replace the entire window units. In some systems you can replace the glass alone.
It’s also possible to retrofit double glazing, either with glass secondary window systems or cheaper acrylic panes that attach to your window frame using magnets. Cheaper still (but less effective) is Clear Comfort, a membrane that you tape to the window frame and make taut by shrinking with a hairdryer (a 10-metre kit costs only $180).
Films are an efficient way to cut solar heat gain on existing windows. They range from almost transparent to dark grey and cost between $60 and $100 per square metre, installed. They also come with low-e coatings.
Glossary of terms
U-value: the measure of a window’s heat conduction. High insulating windows have U-values from about 3.5 down to 1.4 (the lower the better).
SHGC: Solar Heat Gain Coefficient. The measure of the heat transmitted through the window when the sun strikes it directly; 0.8 is high, 0.2 is very low.
IGU: Insulating Glazing Unit. Double or triple glazed window systems, which have sealed cavities between the glass layers.
Low-e glass: glass with a low-emissivity, metallic coating that improves its insulating qualities. Some low-e coatings also reduce the SHGC.
Spectrally selective glass: glass that allows lots of light in, while cutting out unwanted UV and solar heat gain.
Read this article in Sanctuary Magazine.
See related article: Window coverings and retrofitted double-glazing