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in focus

secondary glazing

in focus

secondary glazing

Secondary glazing Secondary glazing is second best to double glazing, but it’s cheaper and it can provide good results. Words Michael Green

Windows are wonderful for transmitting natural light. The only trouble is they’re also great at transmitting heat. In a typical insulated home, windows allow more heat gain and loss than any other part of the building fabric. No matter what climate you live in, double glazing can vastly improve the insulation performance of your house. There are a few ways to get double glazing into an existing dwelling: you can remove and replace the whole window frame; replace just the glazing unit; or install some form of secondary glazing – either a dual window system (an extra glass window on the inside or outside) or “add-on” double glazing (an extra pane within the existing window frame on the inside). Unfortunately, all these measures can tear a hole in your hip pocket in the short term (though you’ll save on active heating and cooling costs over the long term). In this article, Sanctuary takes a closer look at the cheaper end of the scale: dual glazing and “add-on” double-glazing units.

Double glazing was featured in Sanctuary 10. You can read the article online at But before you open your cheque book at all, it’s smart to close some other holes around the home. Maurice Beinat, from household efficiency specialist ecoMaster – which produces ecoGlaze “add-on” double glazing – says windows are normally the third priority. “Every home is different, but usually the first port of call is draught proofing. The second one is sufficient ceiling insulation that’s properly installed,” he says. “Then the third stage is a 76


toss up between secondary glazing, window coverings and window shading.” External shading is crucial to prevent radiant heat transfer through any glass that gets direct sun in summer. Internal window coverings such as heavy drapes and pelmets will help cut down the warmth conducted through the glass. But Beinat says many householders aren’t keen on curtains at all – let alone heavy ones. “If you don’t want window coverings, secondary glazing is a good alternative,” he says. So what should you look for? Gary Smith, from the Australian Window Association, says secondary glazing performs two functions – thermal and acoustic insulation. Some products, known as dual window systems, consist of a whole new window – with glass and frame – attached to either the inside or outside of the existing window frame. They’re available from many companies – see the AWA website for a list of members in your state. “You get some benefit thermally but they’re usually installed for acoustic reasons,” Smith says. For soundproofing, the air space between the two windows should be at least 100mm; however, for the best insulation results, the gap between the panes should be much smaller. “In an insulated glass unit you need dry, still air or an inert gas,” Smith says. “The problem with having a big space is that the air moves around inside and it reduces the thermal performance.” He says the thermal insulation value provided by double glazing increases with

gaps of 6mm to about 16mm, and then begins to decline. “When you get up to spaces like 80mm and 100mm, the performance drops off quickly.” By way of warning, Smith says wouldbe buyers shouldn’t accept claims about a product’s performance without independent testing under the Window Energy Rating Scheme (WERS). “If you can’t compare it, you’ve got to be careful,” he says. The other type of secondary glazing system on the market more readily achieves narrower gaps, by using magnets to attach an “add-on” acrylic panel to the existing frame. These systems include Magnetite, MagicSeal and ecoGlaze (by ecoMaster). Adrian Lafleur, from Magnetite, says high quality seals and materials make all the difference. “You have to make sure you get an air-tight seal. As for the frame, PVC or timber will insulate much better than aluminium,” he says. Products are available that will suit most kinds of windows, and still allow them to be openable. Maurice Beinat says acrylic panes work well in retrofitted systems because they’re light, easy to handle and safer than glass. Cleaning, too, is no trouble. “The best way to clean acrylic is with an antistatic solution or a mild detergent and a microfibre cloth,” he says. “Never use ammonia based cleaners like Windex, because they will make the acrylic go cloudy.” Acrylic scratches more readily than glass, so beware of combining low windows with pawing pets and toddlers. Mild scratches can be polished out, or the panels easily replaced.

Would-be buyers shouldn’t accept claims about a product’s performance without independent testing under the Window Energy Rating Scheme (WERS). Although secondary glazing systems are much cheaper than replacing the windows altogether, they’re not cheap. Covering your whole home could cost upwards of $10,000, depending on the product you choose. Clear Comfort is a low cost solution. It’s a kind of plastic wrap stuck onto the frame and shrunk to fit (see case study on the right). For just a few hundred dollars and a little DIY labour, you can double all your windows – but it won’t last as long as the sturdier systems on the market. Another alternative is to replace just the glazing unit, but this requires a certain level of know-how, and it’s still not cheap. You can get started with RESOURCES


window films

Two years ago, Adam Tiller applied Clear Comfort to the southern and western windows of his 1928 Federation bungalow, and he’s hooked on the benefits. “I think it should be compulsory in every house,” he says. “It eliminates the cold draught you feel coming off the bottom of big windows, even when you have heavy drapes.” The transparent membrane looks “like glad wrap” in the roll, but once installed, it’s hard to see at all. He had no trouble fitting it to both casement and double-hung sash windows, with the help of his partner.

Window films are another option if you’re retrofitting. They are applied to the surface of the window: they’re not a secondary glazing product, so they won’t provide any convective or conductive insulative benefits, but they do reduce heat gain. Films range from almost transparent to dark grey and cost between $60 and $100 per square metre, installed. They also come with low-e coatings. See the Window Film Association of Australia and New Zealand for more information:


A typical window is around two square metres. Clear Comfort: $198 for a 10 metre by 1.6 metre roll (about $12 per square metre), together with tape and instructions. ecoGlaze: $300 to $350 per square metre, but more if the windows are oversized, oddly shaped or require scaffolding.

D Magnetite: $380 to $420 per square metre.

“Add-on” double glazing, where an acrylic pane is attached by magnets inside an existing window frame. Photo courtesy MagicSeal



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