Minimum insulation level for attic conversion

2 Jul 2008
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United Kingdom
I'm In the process on having an attic conversion (1 bedroom and on suite).

Had a brand new roof with veluxes, breathable membrane, and 150mm rafters.

Building inspector has had a look and so far so good.

So I'm about to install the 100mm insulation in the rafters, with 50mm air space. I ask him do I install 25mm insulation over the 100mm to counter air bridging, and then cover the 25mm with 12.5mm foil back plasterboard. Basically he said just to cover the 100mm with normal 12.5 plaster board, and not even foil backed. He reckoned that the current building regs were way over specced and that he would sign off 100mm foil backed insulation covered with 12mm plasterboard.

Is he right or should i ignore him and do the 25mm insulation and foil backed plasterboard. I would be nice if he was right as it would save me time and money.

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Strictly, in that situation you are looking at a total of around 125 insulation board.
But if he has said he will sign it off with 100, what's the problem? Leave it at that.
I was requested by BR to put 100mm and silver multi foil on the non room side. On the dormer it was 96mm above roof and 25mm below.

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