Insulation into external timber frame

14 Oct 2009
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United Kingdom
The complete rear section of the 2nd floor of our house is timber framed and is very cold in the winter. It consists of foil backed plasterboard, a thin covering of mineral wall within a 100mm cavity, membrane and then weatherboard.

We are going to replace the weatherboard with tiles and fill the 100mm cavity with insulation. We are currently thinking of retaining the foil back plasterboard > 100mm Insulation boards > 20 mm OSB or wood fibre boards attached to the outside of the frame > breathable membrane > batten and tile.

Can anybody advise if this construction would be OK and breathable? OSB or wood fibre insulation boards? I was thinking of a foil based breathable membrane are these worth it over the tyvek types?

I am a complete newbie so please excuse an errors/stupid questions.

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25mm void (ie laths)
Breather membrane (normal type not foil)
12mm OSB/ply sheathing
100mm Celotext/Kingspan between frame with taped joints

This will meet current b/regs, but does not allow for services

Add another 25mm (preferably 50mm) insulation board across the inside of the frame before plastering, and it will be much better/warmer inside
Not if he uses celotex/kingspan [foil coated] and tapes the joints ;)
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Thanks for all the replies.

Do I tape the oustide of the celotex/kingspan foil? (E.g. the foil side under the OSB/Ply boards) Or should it be on the side next to the plasterboard (which for the moment I was hoping not to removed).

Good idea about using insulated plasterboard on the inside as well.

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