Windows and glazing: U-value, SHGC, frames and the installation details that stop leaks

Windows do more work than any other part of a house envelope. They let in light and views, provide ventilation, and connect the inside to the outside. They are also the weakest point in the thermal envelope, the part most likely to fail an energy rating, and one of the most common sources of water leaks. For builders, getting windows right means understanding both their performance and how to install them.

This guide covers what the performance values mean, how frames and glass compare, why window substitutions break energy compliance, and the installation details that keep water out.

Why windows matter so much

A square metre of window lets far more heat through than a square metre of insulated wall. In winter, heat escapes through the glass and frame; in summer, sun pours in through unshaded glass. So the size, orientation, shading and performance of windows have an outsized effect on a home’s heating and cooling needs, which is why they dominate NatHERS assessments and BASIX thermal performance in NSW.

Windows also affect comfort directly. Standing next to a large, cold window in winter feels cold even if the room air is warm, and a sunny unshaded window in summer can make a room unbearable. Better windows make a home more comfortable, not just more efficient.

The performance numbers

The two numbers that matter for energy ratings are the U-value and the SHGC. Both describe the whole window, frame and glass together, as tested. That is important: the same glass in a different frame gives a different U-value.

The U-value measures how easily heat passes through the window by conduction. A lower number means less heat loss in winter and less heat gain through the glass and frame in summer.

The SHGC measures how much of the sun’s radiant heat hitting the window ends up inside. A higher SHGC lets in more solar heat. That can be good on north-facing windows in cooler climates, where winter sun helps heat the house, and bad on west-facing windows in hot climates. That is why an energy assessor may specify different glass for different elevations of the same house.

In Australia, window energy performance is commonly rated through the Window Energy Rating Scheme (WERS), which publishes certified values for window systems. Energy assessors use those values in their modelling. The structural and water performance of windows is covered by the Australian Standard for windows and external glazed doors, and glass selection and installation, including safety glazing, by the Australian Standard for glass in buildings.

Glass options

• Single glazing is the traditional option: one pane. It performs poorly thermally and is becoming harder to use in new homes that must meet current energy requirements, though low-E coatings improve it.
• Double glazing uses two panes separated by a sealed space, sometimes filled with argon gas. It significantly lowers the U-value and reduces condensation and noise.
• Low-emissivity (low-E) coatings are thin metallic layers that reduce heat transfer. Different coatings are designed to keep heat in, keep solar heat out, or balance both.
• Tinted and reflective glass reduce solar gain and glare but also reduce daylight.
• Safety glass, toughened or laminated, is required in many locations, such as doors, low windows, bathrooms and near stairs. Laminated glass also improves security and acoustic performance.

Frame options

Standard aluminium frames are the most common in Australian housing, but aluminium conducts heat readily. That lowers the whole-window performance and causes condensation on the frame in cold weather. Thermally broken aluminium, with an insulating barrier within the frame, uPVC, timber and composite frames all perform better. The choice balances cost, appearance, durability, maintenance and performance.

Substitution: the compliance trap

This is where many builders get caught. The energy assessment, NatHERS or BASIX, is based on the specific window values the assessor modelled. A window supplier offers an alternative that is cheaper or faster, the same size and style. But its U-value or SHGC is different, and the home may no longer meet the rating or the BASIX certificate.

In NSW, the certifier must be satisfied that the BASIX commitments, including window performance, have been met before issuing the occupation certificate. A substituted window can hold up the occupation certificate until a revised certificate is obtained or the windows are changed.

Installation: where leaks come from

Water leaks around windows are one of the most common defects in new homes. Surprisingly often, the window itself is fine. The water gets in around it, because of how it was installed and flashed.

The principle is simple: water that gets past the outer face must be directed back outside, never inwards. The sill flashing or tray collects any water that gets past the window and drains it out. The head flashing sheds water running down the wall over the window. The wall wrap must be lapped so that water always runs over the next layer down, never behind it. And the window’s own drainage must be kept clear.

Common installation faults include missing or incorrectly lapped head flashings, sill flashings without upturned ends, sealant blocking weep holes, windows fixed hard against the structure with no allowance for movement, and wall wrap cut without being folded into the opening.

Wind and water ratings

Windows are rated for the wind pressures and water penetration they can resist. The required rating depends on the site’s wind classification and the window’s location on the building. On exposed coastal and elevated sites, and on upper storeys, the required performance is higher. Make sure the windows ordered are rated for the site, and that the rating appears on the label.

Bushfire-prone areas

In bushfire-prone areas, windows must meet the construction requirements for the site’s bushfire attack level. Higher levels require specific glass, frames, screens or shutters. Windows are one of the most expensive parts of building to a higher bushfire level, so the level should be known before windows are specified.

Condensation on windows

Condensation on windows in winter is usually a sign of cold glass or frames meeting humid indoor air. Double glazing and thermally broken or non-metal frames reduce it significantly. But even the best window will show condensation if indoor humidity is very high, which is why ventilation and exhaust matter as much as the window.

Shading: the cheapest window upgrade

External shading changes how a window performs more than almost any glass upgrade. Eaves sized for the orientation keep high summer sun off north-facing windows while letting lower winter sun in. Pergolas, awnings, external blinds and deciduous planting do the same for other elevations. East and west windows are hardest to shade because the sun is low when it strikes them, which is why they often need lower SHGC glass or adjustable external shading.

Shading is part of the energy assessment too. If the design relies on eaves or a pergola to achieve its rating, removing or shortening them during the build can change the result, just as a window substitution can.

A worked example: the west-facing living room

A client wants a wall of sliding doors opening to the backyard, which faces west. The energy assessment shows the room overheating on summer afternoons, and the design struggles to meet its thermal target.

The assessor tests options. Upgrading to double glazing with a low SHGC coating on the west doors cuts solar gain significantly. Adding a deep pergola with adjustable louvres over the doors cuts it further and gives the client an outdoor room. Keeping standard clear glass on the north windows lets winter sun in. The combination meets the target and makes the room comfortable. The builder prices it that way, and the window order lists different glass for different elevations.

Months later, the window supplier offers a cheaper standard door for the west wall. The builder checks the label against the assessment, sees that its SHGC is much higher, and declines. That check is the difference between a comfortable room and a failed BASIX commitment at the occupation certificate.

Lead times and ordering

High-performance and custom windows can have long lead times. Order once the design is final and the energy assessment is complete, confirm the performance values on the order, and check deliveries against the order before installation. Store windows upright, protected from damage and weather, until they are installed.

Talking to clients

Clients often focus on window size and style. Help them understand that performance matters too: better windows mean a more comfortable, quieter home with lower bills and less condensation. Explain that the energy assessment locks in certain values, so a change of window supplier or style during the build may mean a revised assessment and a cost.

The bottom line

Windows carry more of a home’s energy performance, comfort and leak risk than any other element. Understand the U-value and SHGC, choose frames and glass to suit the orientation and climate, never substitute without checking the certified values, and install every window with the flashing and wrap details that send water back outside. That combination avoids both failed energy ratings and the leak callbacks that follow a poor install.

Questions builders ask

• BASIX
• Energy efficiency
• Passive design
• Waterproofing
• Windows and glazing

Published 16 August 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.