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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.

Regulations

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

Windows must achieve an area weighted average U-value of 2.2, but this can rise to a maximum of 3.3 if other elements of the building offer higher than average insulation.

Replacement windows to existing dwellings are required to achieve a U-value 2.0 and to buildings other than dwellings 2.2.

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.

Why Secondary Glazing helps Building Energy Performance

Thermal Testing and Results