Selectaglaze secondary glazing
Selectaglaze Ltd, Alban Park, Hatfield Road, St Albans AL4 0JJ
Phone 01727 837271   Fax 01727 844053
The leading secondary glazing specialists

Regulations and Tests

What are u-values?

U-values indicate the rate of heat flow across a material - the lower the value, the better the insulator.

Single glazed windows are poor insulators and heat loss is exacerbated if the window does not have effective seals.


Approved Document L of the Building Regulations sets insulation standards for materials used in new buildings and those undergoing significant alteration.

Historic buildings need only improve their energy efficiency where it is practically possible and also where it will not prejudice the character of the building.  English Heritage guidelines advocate the use of secondary glazing.

Calculated U-values

Selectaglaze has worked with the Centre for Window Cladding and Technology (CWCT) to calculate U-values for their full range of secondary windows in combination with existing single glazed windows constructed from different materials and at various air spacings (cavities).  For comparison purposes, the calculations are based on the British Fenestration Rating Council's standard window size - 1230mm wide x 1480mm high.


  • Secondary Glazing with Standard glass in combination with a primary window can reduce heat loss by around 50%.
  • Secondary glazing with low emissivity glass exceeds the performance required by Part L of the Building Regulations.
  • Secondary Glazing with sealed glass units offers significant insulation levels.
  • The effect of glass thickness is negligible whilst larger air spacing will marginally reduce the insulation level.


 Notes to table

Note 1:  A U-value of 3.3 is permitted if other elements of the building offer higher than average insulation ensuring that the total energy loss from the building is within regulation.

Note 2: A U-value of 2.2 is the area weighted average for windows in new-build dwellings and non-dwellings and the standard for replacement windows in non-dwellings.

Note 3: A U-value of 2.0 is the standard for replacement windows in dwellings.


 Why Secondary Glazing helps Building Energy Performance


of secondary glazing
Benefits to:
• Specifiers
• Contractors
• Building owners
• Trade installers