Thermal Downloads

Download Our Thermal Test Certificate
Download Our Thermal Insulation Report

The specific performance of secondary glazing is subjective as there are many variables, so the laboratory tests and certifications can only be treated as a guide of what the system are capable of. The condition, size and type of primary window, plus its elevation in relation to its position against the elements, will all impact on the final performance achieved. Other weak spots within the room or building will all factor making it impossible to predict a specific outcome for a specific window. However, it is proven  by our own formally certified tests and those carried out by English Heritage, Historic Scotland and Glasgow University that the Insulation performance will be enhanced between 50% & 70%.

  • Glass thickness has little influence on the thermal performance, thicker glass mainly benefits sound-proofing applications.
  • With most systems when face fixed you are likely to get an approx. 50mm glass-to-glass airgap and subject to window design this could be 80mm, most variations around this  distance are perfectly acceptable for thermal insulation. Our test certification is based on a 80mm gap.
  • The thermal performance can be enhanced using a Low-E coated glass, you can research this via the likes of Pilkington Glass or Saint Gobain.


The thermal performance of a window is calculated as a U-Value, the formula W/m2K.

There is a recognised industry standard that quotes “a single glazed timber window 1.8m2 this has a U-Value of 5.8 W/m2K” the larger the u-value number the lower the performance.

It is a legal UK requirement that a new build property or replacement double glazed primary window must have a U-value of below 2 W/m2K.

Whilst there isn’t a legal performance criteria for secondary glazing. the potential performance to expect would be: 

  • Untreated single glazed window U-Value between 5.5 & 6.0 W/m2K
  • Window treated with Secondary Glazing with 4mm toughened standard glass U-value between 2.5 & 3 W/m2
  • Window treated with Secondary Glazing with 4mm Low-E toughened glass U-value between 1.5 & 2.0 W/m2K


For more details on thermal performance [click here] 

Glass Types [click here] 

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