A simple guide to sustainable windows in COLD CLIMATE ZONES

Zones 6, 7 & 8

Regions included in the cold climate zones are the Australian Capital Territory, Tasmania, most of Victoria and some southern parts of New South Wales and Western Australia.

WHAT WINDOW DO I CHOOSE?

Recommended window and door framing materials:

• Aluminium
• Aluminium, thermally-broken
• Timber
• uPVC
• Fibreglass
• Composite

WHAT GLASS DO I CHOOSE?

Recommended glass types:

• Clear, low emissivity (low-E)
• Clear, insulated glass unit (IGU)
• Clear, IGU with low-E

About the SWA Project The Australian Glass and Window Association (AGWA) are developing an industry-wide strategy to deliver more sustainable residential buildings through energy-efficient windows and doors. The SWA project has run over 4,000 simulations in all eight climate zones defined in the National Construction Code (NCC) using three house types with a wide range of windows to show the impact of high performance windows on the energy efficiency or star rating of the home. All results have been peer reviewed by a panel of Australian and international scientists.

About the Houses

The test houses were specified with R2.0 wall insulation plus reflective foil laminate (RFL) and R5.0 ceiling insulation. Although the insulation is in excess of the current NCC requirements, these insulation levels were implemented so that non-window heat transfer would be minimised, thus accentuating the sensitivity of the houses to window performance.

The results would not be credible if the houses were under-insulated to start with. Except where noted all houses had concrete slab-on-ground floors, brick veneer external walls and plaster-board internal partitions. It is well known that, for a given house, its annual energy performance for heating and cooling depends on its orientation. To account for this, each house was simulated four times with the house rotated progressively around the four cardinal directions. This yielded an average performance values for the climate zone.

About Energy Efficiency

Choosing energy efficient windows will make your home more comfortable, dramatically reduce your energy costs and help to create a brighter, cleaner and healthier environment.

Windows are possibly the most complex and interesting elements in the fabric of our homes. They provide light and fresh air and offer views that connect our interior living spaces with the great outdoors. However, windows can represent a major source of unwanted heat gain in summer and significant heat loss in winter.

Windows can severely impact the heating and cooling loads of a building. Between 46% and 61% of a home’s heating energy can be lost through windows and glazed doors and between 79% and 86% of its heat gained. Improving their thermal performance increases comfort and reduces energy costs and Australia’s greenhouse gas emissions.

Selection Guide

Considerations	Maximise your opportunity to exploit desirable solar heat gain throughout the majority of the year. Reduce heat flowing through windows (in both directions) over the course of the year.
Preferred U-Value	Low
Preferred SHGCw	High (or ideally tuned by elevation)
Other Factors	Season-specific physical shading of windows (for example, eaves over north-facing windows).
Star Impact	SHGCw optimised (clear) - offers no improvement in stars (clear is baseline). U-Value - has the potential to improve up to approximately 0.3 stars for each unit reduction in U-value. Ventilation - little or no star impact.
Heating/Cooling Impact	Each star corresponds to reduction in Heating/Cooling requirements of approximately 20% to 30% on pre improved level. In general Improved U-values act to reduce heating load throughout majority of the year, though may marginally increase cooling requirements in periods of hot weather.
Cost & GHG Savings	Approximately 10,000 to 15,000 MJ of energy saved per star, mostly in heating. Worth about $300 per year and approximately 1.0 tonne of greenhouse gases (GHG). Values based on a 240 m2 house located in Melbourne or Hobart.

NOTE: This information is provided as a guide only. For more specific details, refer to your window or glass supplier, or visit the WERS website.

Terminology

U-Value (Uw)

U-Value (Uw) measures how readily a window conducts heat. It is a measure of the rate of non-solar heat loss or gain through the assembly.

The rate of heat is indicated in the terms of the U-Value of a window assembly which includes the effect of the frame, glass, seals and any spacers.

The lower the U-value, the greater a window’s resistance to heat flow and the better its insulating value.

Solar Heat Gain Coefficient (SHGCw)

Solar Heat Gain Coefficient (SHGCw) measures how readily heat caused by sunlight flows through a window. The SHGCw is the fraction of incident solar radiation admitted through a window, both directly transmitted, and absorbed and subsequently released inward. SHGCw is expressed as a decimal between 0 and 1. The lower a window’s SHGCw, the less solar heat it transmits.

Visible Light Transmittance (VLTw or VTw)

Visible transmittance (VTw) measures how much light comes in through a window. It is an optical property that indicates the amount of visible light transmitted.

VTw is expressed as a decimal between 0 and 1. The higher the number, the more light is transmitted.