Glazing

The stakesWhat windows cost a house in heat

Windows can account for up to 40% of a home's winter heat loss and up to 87% of its summer heat gain. An insulated wall resists heat flow many times better than even good glass, so each square metre of window is a deliberate trade: daylight, view and winter sun in exchange for heat.

The type of window decides how bad that trade is. Here is the heat lost through a single 2 m² window on a Canberra winter night, with 20°C inside and 0°C outside:

Multiply by every window in the house. The best of these loses less than a third of what the first one does. That difference is heating you pay for every winter night for the life of the building.

The U-values are NatHERS default values for each type, which are deliberately conservative. Certified products often do better.

The numbersU-value and SHGC

Every window has two numbers that matter. Always ask for both as whole-window values, for the glass and frame together.

• U-value (Uw) measures how easily heat flows through the window, in watts per square metre for each degree of temperature difference. Lower is better. Real windows range from about 8 at the worst to about 1 for the best triple glazing.
• Solar heat gain coefficient (SHGCw) measures the share of the sun's heat that gets through, from 0 to 1. Real windows range from about 0.75 down to 0.15. Lower keeps summer sun out. Higher lets winter sun in. Neither is always better: it depends which way the window faces.
• Visible light transmittance (VLT) measures how much daylight gets through. Dark, heavily tinted glass can cut heat at the cost of a gloomy room.

Whole-window values in Australia are certified under the Australian Fenestration Rating Council's rules and listed on the Window Energy Rating Scheme (WERS). Figures for the glass alone always look better than the finished window, because the frame performs worse than the glass. Overseas ratings use different methods and can't be compared directly.

FramesHalf the story

• Standard aluminium is strong, slim and durable, but it conducts heat very well. On a frosty night the inside of the frame gets cold enough to collect condensation. Dark frames in full sun also heat up and pass that heat indoors.
• Thermally broken aluminium has an insulating strip between the inner and outer sections. It's the ideal starting point for a new Canberra home if you want aluminium, and on more complex designs it becomes the minimum. Manufacturers have responded in recent years with better-performing frames, so the choice keeps improving.
• Timber insulates well naturally. It needs good seals and regular maintenance.
• uPVC insulates about as well as timber and needs little maintenance.
• Composite frames have aluminium outside for durability and timber or uPVC inside for insulation.

Double glazing in a standard aluminium frame is a common compromise, and it's a weak one. The frame undoes much of what the second pane achieves.

By elevationThe right glass for each side

In a cool climate like Canberra's, the U-value drives most of a window's year-round energy cost. The right SHGC depends on which way the window faces:

Low-E coatings come in two kinds, and they go on different sides of the house:

• High-transmission low-E lets the sun's heat in but stops heat radiating back out. It belongs on the north.
• Low-transmission (solar control) low-E blocks much of the sun's heat while still admitting daylight. It belongs on the east and west.

Putting the same glass on every side is simpler to order, but it gives up either winter heat on the north or summer protection on the west. It's often cheaper to buy the best windows for the elevations that need them, rather than mid-range glass everywhere.

More north glass isn't automatically better either. Past a point, a large window loses more heat overnight than it gains during the day. This is especially true if there's no thermal mass in the room to store the day's sun.

SealsA window that leaks doesn't insulate

A window that lets air through when it's closed insulates poorly, whatever its glass and frame.

• Awning and casement windows close against compression seals, which stay tight for years.
• Sliding windows and doors rely on brush seals, which wear with every opening. They're convenient, but they leak more as they age.
• Louvres must seal fully when closed. Many don't.
• The gap between frame and wall must be sealed during installation. Otherwise the energy saved by good glass leaks out around it.

WERS listings include an air infiltration figure for each product. Lower is better.

On siteMaking sure you get what was rated

The energy rating is only as good as the windows that are actually installed.

• The window schedule in the drawings should list each window's size, opening type, frame, glass, whole-window U-value and SHGC.
• The NatHERS certificate lists the windows that were modelled. Where default values were used, the certificate shows the range the installed windows must fall within.
• In NSW, the BASIX certificate states a maximum U-value and an acceptable SHGC range for most upgraded window types. The supplier's documents must show they comply.
• Substitutions are where good designs quietly fail. A cheaper window that looks identical can perform very differently. Any change should be checked against the certificate before it's ordered.