Australian Window Association

Windows for Good Energy Efficiency

Introduction

Technological advances in the last 25 years have made it possible to design windows which insulate against heat and cold up to four times better than conventional windows. Dramatic improvements in sound insulation are possible. To a point, similar technologies can serve both needs. This Technical Note summarises the objectives, the technical solutions and the trade-offs that building designers and industry need to understand.

Energy Efficiency

An energy-efficient window is one that helps to minimise the use of artificial heating and cooling in a building. In many parts of Australia, the priority is to keep solar heat out of the home, except during colder months, when ‘free' solar heat gain and retention of warmth in the house become important. Heat gain or heat loss take three forms: radiant heat transfer, conductive heat transfer and heat transfer via air infiltration. Ideally, this calls for strong solar protection on east and west windows, but deliberate use of free sunlight (via clear glass) from northerly windows. This means different window solutions for different orientations. Alternatively, it is possible to use the same uniform high-performance window type on all sides of a home - provided it is correctly chosen. From an energy point of view, this is not quite optimal but it simplifies the specification process and still results in very substantial energy savings and improved comfort, compared to using clear single-glazed windows.

Low U-value

In most climates, windows with at least four heating or four cooling stars, on all sides of the house, will ensure that conductive heat losses and heat gains are minimised. This means the windows must have a low U-value. It is possible to reduce the glazing U-value by means of single glazing having a low-e coating. Low-e coatings are near-invisible. While a low-e coating on simple single glazing reduces that part of the heat transfer which is due to radiation, it does nothing to reduce conductive and convective heat flow.

A much better solution is to reduce all three forms of heat transfer. To do this an insulating glass unit (IGU) is required. The IGU is the vehicle for all high-performance windows, in all climates - hot, cold and mixed. The IGU should preferably have some sort of low-e coating on at least one pane and have argon gas fill between the panes, to give the lowest overall heat transfer. U-values as low as 1.8 are possible, compared with about 5 in the case of a single-glazed clear window. To complement the glazing system, a frame with a low U-value assists in reducing the whole-window U-value. Frames that use thermal breaks or composite metal / timber design, or timber or uPVC frames, outperform standard aluminium windows. This frame performance difference shows up in the WERS stars as an extra half star or so.

The most common filling gas between the panes of an IGU is dry air. The narrowest air gap used is 6 mm but this should be avoided unless there is no alternative. The use of wider gaps (10-20 mm) will improve the U-value and increase the WERS heating star rating by at least half a star. Contrary to common belief, a very wide air gap is not disadvantageous, apart from being impractical for an IGU. Although convection cells are bigger and more active, this is counteracted by the greater thickness of air which provides additional resistance to conduction. The use of (cheap) argon gas instead of air in the IGU space lifts the heating rating by about a third to half a star in most cases. Table 1, the WERS table of Generic Windows, gives indicative performance.

Wider gaps also give better acoustic insulation; see the section on acoustics later in this document.

High versus Low Solar Heat Gain Coefficient: Low-e Coatings

As mentioned, all low-e coatings assist energy efficiency by suppressing radiation heat transfer. This means they act like ‘heat mirrors' and reflect heat back into a room in winter, which reduces the amount lost to the outside. Some low-e coatings are essentially transparent to solar radiation; these mostly take the form of so-called ‘hard' or ‘pyrolytic' coatings. They are best suited to cooler climates or on the north side of a building where ‘free' winter heating can be exploited. Other low-e coatings (frequently referred to as ‘soft', ‘multilayer' or ‘spectrally selective' coatings) are available, which block up to half the invisible, radiant solar heat while still 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 reduces or in some cases eliminates the need for wide eaves or shading systems. When the Generic Table was developed, spectrally selective low-e coatings were unusual on the Australian market. However their performance is well approximated by windows 22 - 27 in the table, which achieve similar results by a different construction.

Good Weather Seals

Heat leaves or enters a home through gaps and cracks around sashes and frames. When a window is shut it should be shut. WERS-rated windows must satisfy Australian Standard AS 2047 for air infiltration performance. Most easily exceed it and achieve air leakage figures below 1 litre per second per square metre of window area. Traditionally, windows with compression seals, as fitted to awning and casement windows, tended to have superior long-term infitration performance. However recent advances in some sliding window seals have reduced the gap. The difference between a window that just meets AS 2047 (5 L/s.m2) and a very tight window is about half a heating star. The effect on cooling performance is much less.

Good/Better/Best Windows...and the BCA

The Australasian Window Council (AWC) has submitted a proposal on energy-efficient windows to the Australian Building Codes Board (ABCB) for adoption in the Building Code of Australia (BCA). The submission has been prepared by the AWC-ABCB Working Group. The idea is to present the designer with a simple, effective selection guide to energy-efficient windows that meets the Deemed-to-Satisfy requirements of the BCA. To achieve this it is necessary to identify window options which offer increasingly better insulation and/or sun protection compared with the starting point of clear single glazing and fixed eaves. In many cases, eaves can be cut back or even eliminated, with no loss of summer comfort. At the same time, insulating glass reduces conductive heat losses and gains. The overriding philosophy is to offer the builder value-added window solutions which are cost-competitive with the conventional approach of single-glazed windows and fixed shading.

At the time of writing, house energy ratings (HERS) attainable using the AWC method are being evaluated. The general target is NatHERS 4 stars or better, since this is the standard applying or pending in Victoria, the ACT, parts of NSW and most likely in SA and WA before year's end. An ideal result for the window and glass industries would be to demonstrate that the majority of project homes, with windows up to about 25% of their gross floor area, could meet or exceed NatHERS 4-star level using high-performance windows and reduced eaves and at a total cost no greater than the wide-eaves, single-glazed house.

To assist this submission, the AWC-ABCB Working Group is seeking accurate prices on complete window schedules for six house designs. The combined window costs and energy savings will be used to improve the Cost-Benefit Tool currently available from the Australian Greenhouse Office.