The heat insulation of a window is expressed as the U value. The lowest U value provides the best insulation. 
A single glazed metal window will typically have a U value of 5.7. 
A single glazed timber window will typically have a U value of 4.8. 
A double glazed timber or PVC window will have a U value of between 2.7 – 1.8. 
Part L of the building regulations for replacement double glazed windows set a U value of 2.00. 
Standard 4mm glass secondary glazing over an average single glazed window achieves a U value of 2.5. Secondary glazing with 4mm thermal (Pilkington K Glass) will typically achieve a U value of 1.8-1.9 
The Pilkington K glass in secondary glazing reflects most of the heat back into the room. 
Secondary glazing can be fitted to improve thermal insulation as well as eliminating draughts. 
Secondary glazing can improve the thermal insulation of a window by up to 70%. 

Eliminating Draughts 

Draughts through your windows can create cold spots around the room, particularly around the legs and feet, as a result heating is normally turned up higher to compensate. 
The tight tolerances designed into our secondary glazing system keeps cold air and dust out whilst retaining the warm air in the building. 

Air Permeability Test 

This defines the ability of the window to resist air penetration when it is subjected to differential pressure and is a measure of the air which seeps through the test window at given test pressures. 
The secondary met the stringent test results of Pressure Class 300Pa, and was tested by BSI to the following standards. 
BS 5368: Part 1 
BS 6375: Part 1 

Window Styles Available 

Extensive use is made of air-conditioning in today's busy offices, shops and at home. Installing secondary glazing will help to reduce the energy used in cooling the building and stops cool air escaping out through your windows. 
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