Glazing of both windows and doors is what allows the outside environment to interact with the interior of the house, a relationship that can be both good and bad. It’s good when we open doors and windows up to allow cool summer breezes inside, but it’s bad when all that glass sucks the warmth out of a room on a frosty winter evening, or lets the hot afternoon sun heat up the internal temperature to unbearable levels.
Glazing is usually the Achilles heel of a building’s performance and should be one of the very first things to go under the microscope when considering a building upgrade. An otherwise well-insulated house can suffer considerable unwanted heat loss or heat gain through single-pane glass, which has almost no insulating ability – around R0.15.
The Australian Window Association (AWA) estimates that up to 40 per cent of a home’s heating energy can be lost through windows and up to 87 per cent of its heat gained through them. Choosing high-performing windows and placing them appropriately can reduce energy costs significantly and improve thermal comfort. The art is in knowing how different windows will interact with the design of your home.
But where do you start to work out which glazing system or treatment is the best solution for you? It’s a complex task even for a switched-on homeowner. The AWA has made things easier with the Window Energy Rating Scheme (WERS).
Window performance measures
WERS simplifies window comparison by rating the performance of residential windows using a star rating system, much like star ratings for appliances. The star ratings are based on the window’s basic performance measures: U-value and solar heat gain coefficient (SHGC). Windows receive a rating for both heating and cooling performance. WERS has three climate types for the whole of Australia (and New Zealand fits one of these): heating, cooling and mixed. The zones indicate whether most energy will be dedicated to heating or cooling to maintain thermal comfort, and mixed means just that – about equal shares of both.
The whole-window U-value (Uw) measures how readily a window conducts heat. The lower the U-value, the greater a window’s resistance to conductive heat flow and the better its insulating value. WERS gives comparative ratings for frames and glass combined in a functioning window or door. If we use old-school aluminium frames plus single glazing as a benchmark, modern aluminium frames, thermally broken frames and some uPVC frames plus double glazing (insulated glazing units or IGUs) can reduce internal to external thermal transfer (conductivity) by 50 to 75 per cent or even more. Timber frames (softwood) are about equivalent to thermally broken aluminium frames; steel frames are not proven high performers.
The other important factor influencing window performance is its whole-window solar heat gain coefficient (SHGCw). This measures the window’s ability to control heat transfer from solar radiation. This coefficient is expressed as a number between 0 and 1 – the lower the number, the less solar heat the window transmits.
Real-world U-values normally fall between about 8 (worst case) down to 1 (best case). Real-world SHGCs range from about 0.75 down to 0.15. Unlike U-value, we don’t tend to label high SHGC as ‘bad’ and low SHGC as ‘good’ as the judgement depends on the climate where the building is located. In almost all cases, a low window U-value is better in all climates.
A window allowing air movement through it even when closed will be a poor insulator, no matter what its materials and construction. Windows that have moving sections should have good seals between the moving sash and the fixed frame. Most modern commercially made windows have a reasonable seal, but their effectiveness depends on how well the window is designed and manufactured. The amount of air that passes through an area of window under a given pressure is known as the infiltration rating – the lower the value, the better.
Compression seals (as seen on awning, casement and tilt-and-turn windows) usually seal better and last longer than brush seals commonly fitted to sliding or double-hung windows.
A good seal between the window frame and the wall is also very important. It is not uncommon to see windows with gaps and considerable air leakage between the frame and the wall – this means that energy saved with improved glazing can be lost through the frame.
Single, double and triple glazing
In general, all windows will benefit from having better insulating properties – that is, from double or triple glazing or from insulating window coverings. These technologies slow down the conductive heat flows in both directions.
Double-glazed windows are far superior to single-glazed windows for insulating your home. If you’re wondering whether the added cost of improved glazing is worthwhile, consider that compared to single glazing, in cold and mixed climates a double-glazed window could cut your window heat loss by 60 per cent or more, reducing heating loads.
Some people may simply choose to replace leaky old windows with double glazing, with good outcomes. But for anyone currently in the process of assessing quotes and specifying energy-efficient windows, you will already know there’s a lot more to consider than multi-pane glazing to achieve optimal results.
Double glazing retrofit and window film
Other than entirely new windows, there are ways to achieve better window performance. There are several aftermarket double-glazing products which replaces or improves the glass with a double-glazed unit which are designed to improve but not replace existing single-glazed windows.
Another way to improve the performance of your existing windows is through the application of film. Film can be considered to be a lot like a factory-fitted coating on the glass, but instead it comes on a roll and should be fitted by a professional. Films come in many types, including spectrally selective and low-e like Solar Gard Ecolux, and vary in performance and what they actually do – the trick is to select a film that does what you need it to do, depending on the local climate, the orientation and location of the window, and the performance of the rest of the house. Choosing the window film option offers a quick and unobtrusive installation at a fraction of the cost of window replacement.
Source: Sanctuary Magazine: Lance Turner and Dick Clarke