I've often seen "official" statements that 30% of heat may be lost from houses through windows, but I've never seen any evidence that this is so.
I'm wondering if it is possible to get a firm estimate by taking the inner and outer temperatures of the windows? (The current discussion about a Lidl infra-red thermometer put the idea in my mind.)
Well if they are single glazed that would let you estimate the heat conducted throught the glass. What about conduction through frames or drafts through the gaps?
There are a *lot* of factors which affect house total heat loss (damp on the walls, clearness of night sky...), single numbers don't really tell the story.
It's easy enough to do a calculation based on the U values of your type of glazing and an estimated U value of the wall construction
The U value is the number of watts that will pass through 1m2 of the material with a 1 Kelvin temperature differential.
You can then factor in the area of your wall vs the area of the glass to get a reasonable estimate.
Official or not, it patently isn't universally true - and will vary from building to building, depending on wall, floor, roof, etc. construction and the relative area of glass to other materials - not to mention the type of glazing itself. Your 'official' statement only says "may be" anyway - leaving plenty of scope for alternative values!
No! That will tell you the inside and outside temperatures and nothing else. It won't tell you how much heat is flowing through the glass, and certainly not how much is flowing through the walls, floor and roof. Without this information, how are you going to calculate the proportion of the overall loss which is attributable to the glass?
The only way to do it for a give building is to do a proper heatloss calculation - taking account of the areas and U-values of all the outer surfaces of the building. You'd need to that anyway - both overall and for each room - if you were sizing a heating system for the building.
Windows often have poor thermal performance, however they only make up a relatively small percentage of the total wall area of the property.
I would guess that for "normal" walls - (i.e. not insulated to modern standards) you will lose some 30% - 40% of the total heat loss from the building, through the walls including the windows. So the loss through the windows alone will be significantly less.
The big variable is air leaks. An airtight window is much better overall than one that leaks air. To the point that an airtight single glazed window may well outperform a poorly fitting DG one.
If you have look at:
It works through an example that you should be able to copy for your own house.
I've not been actively involved in this since 2008 but running a typical 6m x 9m 2-storey semi-detached house built to 2006 standards through my SuperHeat program I get the following breakdown:
Walls: 22.5% (UV: 0.41) Ground floor: 8% (0.23) Roof: 5.5% (0.16) Doors and windows: 26% (2.03 avg) Ventilation: 38%
Would be interesting to compare with a 1950's build with single glazing, uninsulated cavity, and perhaps 2" retrofitted rockwool loft insulation. Probably many more of those than there are 2006 homes.
Just for you I ran it again very quickly changing walls to brick/brick cavity, uninsulated solid floor, 50mm roof ins and SG windows and the comparison is
1950: Fabric loss 286W/K + ventilation 78 = 3642006: 112 + 59 = 171
The SAP 2006 indicative heating costs are £290 and £106 with the same boiler - not in the same ratio as the heat loss as the extraneous gains would be much the same.
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