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Aside from other things going on, chez P is currently having an extension built!
It seems the building inspector has flagged up that we're looking at 'overglazing' the extension. Hmm...
I appreciate the basic calc is that we can have (the current glass area the extension is replacing) PLUS (25% of the new floor area in M2) as glass.
However, in order to allow MORE glass in the extension, we can take into account U-values.. i.e newer/better glass can have a U value of 1.0.
My question is: "what is the basic/standar U-value the Building inspector classes glass to be (double glazed)?"
I.e if they assume standard glass is the MINIMUM of 2.0, then a u value of 1.0 means we can have twice as much, no?
Also, one of the windows we are removing (for the extension) is single glazed. I believe the U-value of this is 4.8, right?
Cheers!!
DrP
Sounds suspiciously like you're building  an orangery.
You're one step away from going full "media room"
Sounds suspiciously like you’re building  an orangery.
LOLZ!
4x5m extension, with bifolds down one side and a 2x3m roof lantern....
No oranges will be grown, but we like light!
I kind of jsut want to know what building regs class as 'normal glass' - I'l email our local BR guy..
DrP
It's a bit more complex than just use glazing with half the u-value. What he is looking for is a calculation to show that the total heat loss from your extension is equal to or less than an extension with the minimum acceptable standards. It's a fairly simple calculation if you know how to do it. Give your local energy assessor the details and ask him what the simplest upgrade is that will make it comply. The usual solution is to add more insulation in the roof of the extension or if that's not enough, a trade off by adding more in the roof of your existing house.
He is likely to want a calculation to prove it anyway. You could heavily insulate everything and get really expensive windows that have an excellent u-value. That will most likely satisfy him but cost you thousands. Get a calculation and forget the guess work, it's cheaper in the long run.