Glass Thermal Calculation????

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BI came round today and plaeased with the 3mx7m extension Im building so far. However he did say he wanted a Thermal calculation done because Im installing new Bi Fold Windows on the back wall which are 4.1m x 2.1m.

The lounge size will be approx 32sqm when completed.

He mentioned about calculating all the window/glazing areas and a 25% figure. Not too sure what he was on about and so I thought' well, there's more money for more calcs to be done'!

So, is it something I can do? Anyone care to help me and explain what the % of glass has to do with thermal calculations?

Please advise!

b
 
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Generally, you are permitted to have openings in your extension which are no greater in area than 25% of the new floor area plus any existing openings removed due to the extension

So if your new floor area is 20m2 and the extenion is being built over an existing window of say 2m2, then your new openings should be 25% of 22m2

If the openings are more than 25%, then you need to prove that the extension is as thermally efficient as a similar extension with only 25% of openings. This would require u-value calcs on the glass and frames

In reality it means either using a more efficient glass unit (argon filled and coated), or increasing insulation elsewhere to compensate for the extra loss through the doors

If your loft insulation is low, then the BCO may accept that you increase this to compensate, and this should not require calcs
 
Thanks Woody- very helpful.

So, my new floor area is 32.49sqm.

The old windows and door openings were 6.60sqm.

I add these totals together and get 39.09sqm.

25 % of 39.09sqm = 9.77sqm.

The new doors/windows are 4.1m x 2.1m = 8.61sqm.

So, if I have done this right then I'm within the limits?

Do I simply just show the BI this calculation on a piece of paper?

Kind regards

B
 
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Yes just scribble it down or cut and paste the above and print

Add your address and app number and give it to him. Or just email it
 

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