Installing good quality windows is vital in an energy efficient build. A house may go through up to four boilers before the windows need replacing, so it's important to get them right. All dwellings should have at least double-glazing, and fitting them properly in a air-tight manner is vital. Triple glazing is the very best option, but the incremental returns in terms of energy efficiency are not that great compared to incremental costs.

Technical information

Windows should be positioned with their front face 20mm past the back of the brickwork for improved thermal efficiency. When installing vertical and horizontal cavity closers around window openings, ensure all joints are square and free from gaps and screw fix the internal flange to the inner leaf of the wall.

Over openings, to reduce the possibility of heat loss, use an insulated cavity lintel such as Catnic Cougar.

As the chart shows, the most cost effective glass to use within the window is a double glazed argon filled unit with planitherm glass, giving a U-Value of 1.2 W/m²K. Another factor which may influence this is when the construction of the external wall changes.

How to install

This animation illustrates the Travis Perkins plc recommended installation of a new build window with a good balance between cost of the solution and thermal efficiency, using traditional building materials.

The construction has been thermally modelled by the BRE and accredited by LABC and delivers a 57% improvement over SAP Appendix K default values.

Watch video

Hints & Tips

Ensure cavity closers are installed the correct way around with the full plastic side against the external skin.

Vertical cavity closers should have 75mm of their flanges trimmed at the bottom allowing the insulated centre section to continue down into the cavity forming a complete butt connection with the horizontal cavity closers insulated section.

Windows should be secured in place using fixing tabs attached to the sides of the frame and screw fixed to the inner leaf of the wall. This removes the need to fix the window through the frame improving thermal performance.

Ensure the window is fully sealed around the external face using silicone frame sealant.

Double-glazed windows have two sheets of glass with a gap in between, usually about 16mm, to create an insulating barrier that keeps heat in. This is sometimes filled with gas. Triple-glazed windows have three sheets of glass, but aren’t always better than double-glazed windows. To choose the most energy-efficient window look for the BFRC rating.

Energy-efficient windows come in a range of frame materials and styles. Performance criteria vary according to the following: How well they stop heat from passing through the window. How much sunlight travels through the glass. How little air can leak in or out around the window.