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glasswindows.com.auWhat a window is glazed with, and why
Guide 3 of five · heat through the window

What the gap in double glazing does between the panes of glass

Your Home says both single and double or triple glazed units will allow good solar heat gain, but the double and triple units are better at preventing heat loss. The measure that catches it is the U value, which describes the conduction of the whole window, glass and frame together.

  • Single, annealed In doors, annealed glass only within the code’s limits: for 3 mm and 4 mm glass, no more than 0.1 m² and no more than 125 mm wide, and none in kitchen, bathroom, ensuite or spa room doors, or the like, where the glass comes lower than 2.0 m.
  • Laminated Grade A safety glass, which the code asks for in doors, side panels within 300 mm of a doorway opening, panels whose lowest sight line is under 500 mm from the floor, and other places it names, with exceptions it sets out.
  • Toughened Grade A safety glass in the same places, and marked by permanent etching or by a label that cannot be reused once removed.
  • Double glazed Two or more layers of glass sealed into a frame, with a gap between them, and better at preventing heat loss, Your Home says.
  • Coated A low-e coating is either a pyrolytic coating or a thin metal film, and the solar heat gain coefficient measures how readily the sun’s heat flows through the whole window.

Not to scale between types.

Why it matters

What a window costs in heat

Your Home uses the word glazing for the glass and frames in the windows of a home, external doors and skylights, both openable and fixed. Its key point on the subject is a pair of figures: up to 40% of a home’s heating energy can be lost through windows, and up to 87% of its heat can be gained through them. The same page puts the risk the other way round as well: inappropriate or poorly designed glazing can be a major source of unwanted heat gain in summer and significant heat loss and condensation in winter.

The gap

What is between the panes

Your Home defines double or triple glazing, also known as insulated glass units or IGUs, as the combination of two or more layers of glass sealed into a frame with a gap between the layers. The gap is not just air left there: the cavities must be dry and well sealed to prevent moisture getting in, and manufacturers have special dry rooms for IGU assembly.

What the second pane and the sealed gap buy is stated plainly in the same page: both single and double or triple glazed units allow good solar heat gain, but double or triple glazed units are better at preventing heat loss.

The worked example

One living-room window, read through the U value

Take one window in a living room, as an example. Your Home defines the number that follows it around: the U value for windows describes the conduction of the whole window, glass and frame together, and the lower the U value, the greater a window’s resistance to heat flow and the better its insulating value. The steps below walk that window through the changes the page describes, using nothing but what it says each change does.

  1. The opening as it is, single glazed. One pane in an aluminium frame. The U value describes the pair of them, not the glass alone, and aluminium is a very good conductor of heat: Your Home says it will decrease the insulating value of a glazing unit unless the frame is specifically engineered to reduce this.
  2. The same opening, double glazed. Two or more layers of glass sealed into a frame with a gap between them, a cavity that has to be dry and well sealed. What changes is the heat loss: Your Home says double or triple glazed units are better at preventing heat loss, while both single and double or triple units allow good solar heat gain.
  3. The frame, changed while the glass stays. uPVC frames provide excellent thermal performance, Your Home says, often better than timber or thermally broken aluminium. On the same page, awning windows and casement windows, which seal by compression, control air leakage much better than sliding windows and doors, whose seals tend to lose their shape and wear out gradually from constant friction.
  4. The inside face, at the end of it. Water condenses onto cool surfaces; the inside of efficient glazing assemblies will stay at the same temperature as the room, Your Home says, preventing condensation and the risk of mould.

If a full second unit is not the answer, the same page describes secondary glazing: single-glazed windows retrofitted with a transparent acrylic or glass sheet attached to the inside of the frame or openable sash, with a secondary frame or with magnetic strips.

The rated number

Where a window’s figures come from

Your Home says all glazed window and door systems in Australia are rated according to guidelines recognised by the Australian Fenestration Rating Council, and that the testing conditions and documentation procedures the council recognises are based on the procedures of the US National Fenestration Rating Council. The scheme the same page names is WERS, the Window Energy Rating Scheme, which rates the energy and energy-related performance of windows, skylights and glazed doors in accordance with those procedures.

The U value describes heat flow through the window. How much heat from direct sunlight comes through it is a separate figure, the solar heat gain coefficient, which is the ground of the next guide: tinted, coated or clear glass and the sun.

Where to go next

Two questions the example does not answer

Which glass the code asks for in the places a person can fall into it is the first guide’s ground: where the code asks for safety glass. And if the window is on land mapped as bushfire prone, the glass question changes altogether, which is the last guide: glass in a bushfire-prone area.

This guide puts no U-value figure on any window, because the U value describes a particular window, glass and frame together. The figure for a window comes from its own rating, under the schemes above.