Greensaver: Top of the glass

Reported by Susan Hely

By Susan Hely

Money magazine, August edition

Susan Hely is battling to stay cosy at home

The windows in my house are pretty basic and nothing like the hi-tech ones that have been developed over the past 25 years. Windows now insulate against heat and cold up to four times more effectively than conventional windows, according to the Australian Windows Association (AWA). With power bills climbing – mine are up 18% this year – I’m considering either putting a coating on the old windows in my living room to keep the heat in, or replacing my windows with energy-efficient ones such as double-glazed windows.

Is it worth it? Apparently your home loses between 46% and 61% of its heating while between 79% and 86% of its heat is gained through windows, according to the AWA. With the right windows, you can cut your power costs and your greenhouse gas emissions while making your living much more comfortable.

If you are building a new home or renovating, it makes good financial sense to put in energy-efficient windows. There are different sorts of glass to choose that carry codes and ratings to help you understand their benefits and suitability. Look for windows with at least four heating or four cooling stars. This means that heat losses and heat gains are minimised. Also understand the “U-Value” of a window. This measures how a window conducts heat and it includes the frame, the seals and any spaces. The lower the U-Value, the better its insulating value.

According to the AWA, you need different sorts of windows depending on your climate zone. See www.awa.org.au for a map of Australia and an explanation of the best windows.

You can save the most money if you live in cold climates such as Victoria, Tasmania, ACT and southern NSW and Western Australia, explains the AWA. It estimates you can save around $300 a year and around one ton of greenhouse gases (GHG) with the right windows. This is based on an example of a house of around 240 square metres with wall and ceiling insulation.

The savings are marginally less if you live in mixed climates such as in Sydney, Perth and Adelaide – around $250 a year or 0.4 ton of GHG.

If you live in northern Australia the AWA recommends windows with a tint or clear low E or clear IGU. You can save $150 to $250 and 0.35 ton of GHG. But before you go to the expense of retrofitting windows or employing someone to coat them (it’s hard to do it yourself), make sure that all the draughts are plugged. Curtains can help keep the heat in. The curtains must have pelmets and sweep the floor to be most effective.

Window coating

A low-e (low emissivity) coating on your window cuts back heat moving out of the room. According to AWA, it acts like a heat mirror reflecting heat back into a room in winter.

Other low-e coatings known as soft, multilayer or spectrally selective coatings can block up to half the invisible, radiant solar heat while preserving daylight. Windows with spectrally selective low-e coatings reduce the solar heat gain coefficient (SHGC) by up to 60% compared with clear 3 mm glass. This is good if don’t have wide eaves or shading ariund the house.

Use low-e on double glazing for maximum effect. Double glazing is also called an insulating glass unit (IGU).The most common filling gas between the panes is dry air. Get some advice about the optimum gap between panes. The use of wider gaps (10-20 mm) will improve the U-value. Wider gaps also give better acoustic insulation.

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