Although the look of a window is really important and often choosing a colour or frame material is a matter of personal taste. But no matter what type of windows you are planning to use, make sure you spent as much money as you can afford on the windows. Keep in mind, while it is quite likely that you might renovate a kitchen or a bathroom again in 10 or 15 years time, you will most likely never ever change the windows again.

The Window Energy Rating Scheme (WERS) is a program implemented by the Australian Window Council Inc. (AWC) with the support of the Australian Greenhouse Office. The windows are evaluated with stars, the more stars, the better the performance. If buying windows, always check the label before making a decision.
A single-glazed window with a typical aluminium frame has U-values ranging from 7.9 W/m²K to 5.5 W/m²K (according to the indicative ranges of whole glazing element performance values in the BCA). These U-values will make it hard to reach a good energy rating for a building/ built an energy efficient home.

Keep in mind, the lower the U-value the better performing a window. Double glazing windows with timber framing in Australia usually range between a U-value of 3.8 W/m²K and 2.5 W/m²K. But you also can get double glazed units in timber or upvc with u-values ranging around 1.8-1.9.

Sealing and weather-stripping
However, a good U-value is no guarantee for a well performing window. The installation of doors and windows needs to be done according to the manufactures guidelines. All gaps must be sealed and weather-stripped carefully in order to perform to the specified U-value. Unfortunately, the energy rating just states the material U-value of the window and not the end product and common practice often shows incorrect installation leading to thermal bridges around the windows.

 Windows And Double Glazing Overseas
Whereas most countries in Europe require double glazing and even recommend triple glazing, it is not standard in Australia yet. Unfortunately, double glazing is still more expensive than single glazing in Australia, in Europe it’s actually the other way around. Due to the fact that single glazing is not allowed any more, no one is producing it on a large scale making it quite expensive. Double-glazing on the other hand is a standard, and although better performing than common double-glazed windows in Australia, they are available for about a quarter of the price. For instance, the minimum required U-value for windows in Germany is currently 1.1 W/m²K. I trust that with time, double glazing will become more affordable and will become mandatory in Australia to achieve good passive solar design.

Up until then, you, as the client, has to make informed decisions about what glazing you are buying. You can’t just trust a manufacturer stating their glazing is energy efficient. They have to prove the performance to you by showing you the actual u-value of the window system.

One little exercise that I like to do when I going to building expos is that  I walk up to the window manufacturers and ask them if their windows are energy efficient. And no matter which company you ask they will allways answer with: Yes, of course! But then I ask if they can tell me what the best u-value is they can produce.  If they can’t answer this questions it’s probably best to keep walking and look for someone else who can give you a proper answer.

What to look for when buying windows?
YES, double glazing is worth its money. It is the best method to reduce heat loss in winter, as long as it is applied, installed and used properly. The window size should respond to the location and the climate, the insulation around the window needs to be snug fit, in order to prevent thermal bridges. Appropriate window frames need to be used and furthermore, adequate internal and or external covers needs to be applied. All these measurements need to work together, otherwise a window is nothing more than a hole in the wall and will be the major contributor for unwanted heat gain and loss, therefore preventing energy efficiency.

Passive House Standard

When you are thinking about building your home according to the passive house standard you should aim for a u-value of 1.4-1.5 for the Melbourne climate. Having said that. The more glazing you have or if the orientation is not ideal. You might need triple glazing with an u-value of 0.6-0.7. Ideally go for certified passive house windows. To make sure there aren’t any hidden thermal breakes within the frames.

I would recommend not leaving the window performance to chance. And use the PHPP (Passive House Planning Package) tool to determine the required performance.