Installing good quality windows is vital in an energy efficient retrofit project. Typically a house will 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.

Technical information

Replacing old windows for new ones can vastly improve the thermal efficiency of a dwelling as modern window frames and glazing have been designed to achieve much lower U-Values than were previously achieved, especially if the existing windows have single glazing.

New windows should be positioned on a bed of mortar with the rear of the frame in the same location as the existing window's reducing the amount of disruption required within the dwelling. Once positioned, fix through the frame into the brickwork ensuring packers are placed between the frame and the wall at the fixing locations to offer maximum support.

Any remaining gaps between the frame and wall should be filled with expanding foam to give support and to form an insulating barrier reducing any thermal bridging. Once the foam has set and been cut flush with the frame face, sealant should be applied around the whole window frame. To finish internally either fix painted softwood or uPVC cover strips/quadrant.

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.

Hints & Tips

When replacing windows make sure there are no planning or heritage constraints placed on the dwelling.

When measuring up for replacement windows be careful to ensure each window will fit as new windows come square and true but old openings may not be.

If there are large gaps around the frame on the outside it may be better to either use a mortar fillet or install painted softwood of uPVC cover strips/quadrant.

All properties lose heat through their windows. But energy-efficient glazing keeps your home warmer and quieter as well as reducing your energy bills. That might mean double or triple-glazing, secondary glazing, or just heavier curtains.

Some window manufacturers show the energy efficiency of their products using an energy-rating scale from A to G. The whole window (the frame and the glass) is assessed on its efficiency at retaining heat. The scheme is run by the British Fenestration Rating Council (BFRC).

Energy efficient glazing should generate:

  • Smaller energy bills.
  • Smaller carbon footprint.
  • More comfortable home: energy-efficient glazing reduces heat loss through windows and means fewer draughts and cold spots.
  • Peace and quiet: as well as keeping the heat in, energy efficient-windows insulate your home against external noise.
  • Reduced condensation: energy-efficient glazing reduces condensation build-up on the inside of windows.
  • The costs and savings for energy-efficient glazing will be different for each home and each window, depending on its size, material and the installer you choose. Double glazing should last for 20 years or more.