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I remember reading years ago that heat loss through single glazed windows was 7% if the rest of the building was properly insulated - I think that it was from the rics ... will see if I can find it.
- Thread starter malcolm
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I get that figure if I use the wrong U value for walls too. 
I don't have much faith in official thinking.
It is quite easy to work out a whole house average U value if the area of external surfaces, difference between internal and external temperatures, and heating costs are known. U value is watts per kelvin divided by area. Mine was 1.1 before insulation and I think it will be 0.7 now the loft and attic are finished. Most of that will be walls and windows so assuming they have windows right it doesn't fit that a brick wall could have a U value anywhere near the 3.5 that official calculators like build desk use.
I don't have much faith in official thinking. It is quite easy to work out a whole house average U value if the area of external surfaces, difference between internal and external temperatures, and heating costs are known. U value is watts per kelvin divided by area. Mine was 1.1 before insulation and I think it will be 0.7 now the loft and attic are finished. Most of that will be walls and windows so assuming they have windows right it doesn't fit that a brick wall could have a U value anywhere near the 3.5 that official calculators like build desk use.