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Sustainable Building Solutions

Data Sheet:
Windows - Retrofit Data Sheet

Information suitable for:
  • Energy saving
  • Funding available

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 and 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.