When incorporating windows into a new home or renovation, it is natural to consider the aesthetic positioning and the contribution that the windows will make to natural light inside the home. But windows have a very big impact on the Energy Efficiency of a home.  

Why does Glazing make such a difference to Energy Ratings?

The Energy Efficiency measures incorporated into the National Construction Code over the last decade aim to reduce Greenhouse Gases arising from actively heating and cooling buildings by mandating a minimum acceptable level of Energy Efficient building design. The software that calculates the Star Ratings calculates the temperature on every day of the year, in every room of the house, to determine the relative amount of energy that will be required to actively heat or cool the home to a comfortable temperature.

The windows form part of the ‘building envelope’ and are an important measure for the Energy Rating. If you think about it, you can spend a lot of money insulating your walls and ceiling to a good level, but if your windows cannot retain the conditioned air temperature inside your home, then the effectiveness of your wall and ceiling insulation is going to be greatly diminished.

There are three main ways windows and glass doors contribute to the Energy Efficiency of a building.

1.  Air-Flow

The software used to calculate the NatHERS Energy Efficiency Ratings takes into account the position and open-ability of windows. This is so that the cooling effect of breezes in the hotter months can be incorporated in the calculation. Louvre and bi-fold windows have the highest openability (around 90%), followed by awnings, casements and then sliding windows (45%). Window positioning that allows cross-breezes in a room, and that can ‘pull’ air through the building will help a home feel cooler in the hot months, and so lead to less of a need for artificial air cooling.

Note that the Australian Building Codes Board (ABCB) recently introduced a requirement that all windows in new homes and apartments that are more than 2m off the ground must be fitted with window locks to stop the window being opened more than 125mm, or they must have reinforced screens, to prevent children from falling from a height. This ruling will effect normal window open-ability.

2.  Heat Conduction – U Value

The second way windows contribute to the energy efficiency of a home is the same as any other building envelope material. If the air is warmer on one side of the material, the temperature will slowly equalise depending on the material’s resistance to heat flow. This is measured for all building materials by their R-Value (think of R1.5 or R3.0 bulk insulation). Glass also has an R Value, but conventionally for windows it is expressed as a U-Value, where the lower the number, the better at resisting heat flow it is (ie, inverse of R-Value). The U-Value of a window or U_w, is usually between 2 and 8.

What many don’t realise is that the U-Value of the whole window system (glazing and frame) is taken into account when measuring the energy-efficiency of a window. You can have double-glazed tinted windows, but if you have uninsulated aluminium frames, then heat will transfer from or to the building through the frames, thus reducing the total U-Value of the window system.

3.  Solar Heat Gain Co-efficient

While all materials have an R Value, windows have an additional measure of energy efficiency, and this is the SHGC, or Solar Heat Gain Co-efficient. This is because heat is able to enter a building through a window in a way that it won’t through other materials. The SHGC is the fraction of incident solar radiation (from direct sunlight) admitted through a window. This heat transfer arises from the energy from the sun’s rays hitting the glass and entering the building as solar energy, both directly transmitted as well as absorbed by the window and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window’s SHGC, the less solar heat it transmits. Note that the measurement again is the SHGC of the total window system, expressed as SHGC_w.

What are the different terms that you might hear in relation to windows?

a.  Tint

Windows can be tinted to reduce the SHGC. This is used in hotter climates, or sometimes in Western-facing windows in moderate climates. Tinting doesn’t effect the U Value, as the material and thickness is still the same. However, tinting will reduce the light that enters the building, unless the glazing is ‘super-toned’ (a specialised glass that reduces SHGC by allowing certain wavelengths of light through, but blocking others such that the light is barely effected).

b.  Double-glazed

Double-glazing increases the U_w, and the gap between the two panels of glass can be filled with air or argon to further increase the insulating value of the window (as heat is transferred very poorly through air).

c.  Low-E

Low-emissivity (or Low-E) glazing enables a window to perform better in terms of both U-Value and SGHC, by the application of a special coating. There are two main types of low-E glass for different climates:

• High Solar Gain/Low-E allows SHG in during the day, but then reduces the heat that can escape at night or when it is colder outside.
• Low Solar Gain/Low-E reduces the SHGC of the window while still allowing natural light to enter through the window (thus making it preferable to dark tinting)

Note that each of these elements above can be combined (for example, double-glazed, low-e, tinted etc.), and then produced with different window styles (for example louvre or sliding) and frame type (for example timber, aluminium, uPVC) by different window manufacturers, leading to tens of thousands of different window options to choose from.

Note also that the needs of a home in both cooler and warmer months need to be balanced. For example, a high solar gain low-e double glazed window will have a slightly lower SHGC than a single-glazed window, but should still be preferred in a cool to moderate climate for a North facing window due to the much lower U-Value of the glass.

How should you choose your windows for your new home or renovation?

The answer will depend on personal style preference, house design, cost factors, manufacturer choices, and of course energy efficiency. But remember that spending a bit more on good quality, appropriate windows for your build will lead to continuing cost savings in terms of heating and cooling costs.  A good Energy Assessor will be able to help you work out which windows in your property will have the greatest effect on the home’s thermal comfort, and therefore which windows it is worth spending more money on.  

Performance requirements from energy efficiency reports are normally given to the window supplier, who will match or exceed the U_w and SHGC_w with their products. In some cases, it will be easier to select the windows first then supply the performance data to your Energy Rater.

Where to from here:

The Australian Window Association has some excellent resources on their website, especially the 6 factors in the “Things to Consider when Selecting Windows” section.

Windows have a Star Rating System, much like electrical appliances and homes. The Window Energy Ratings Scheme (WERS) has a database of the thousands of windows available in Australia. If you know the U_w and SHGC_w that you want in your home, you can search these products here.

The thermal performance of a window system can be offset by external or internal window coverings. Have a look at a separate BERA blog entry titled: “The Cooling Effect of External Awnings and Shutters“.

If you are still confused by all the options, or seeking an Energy Rating Report, contact BERA.