insulating pitched roof

7 Nov 2009
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United Kingdom
hi there, i have a question about insulating pitched roof in a midterraced house small house.

at the moment there is no insulation on the roof and loose insulating material between joists in the floor, underneath chipboard.

I'm not sure of the costruction of the roof but by the looks of it there are tiles on the outside laid on the wood boards which are fixed to the rafters. I don't think there is any membrane on the outside of the wood boards but there may very well be. Because I'm not sure I'm assuming there isn't one.

Rafters are 100mm deep and space between rafters is 400mm.

My idea is to fit celotex or kingspan 50mm insulation as per this guide:



It is not going to be habitable area so I don't think I'm going to plasterboard it but I'd like to create a "warm roof" setup to minimize heat escaping through the roof.

Will 50mm of celotex or kingspan will be enough insulating material and do I need to fix this vapour barrier or is that not required? or maybe its already included in celotex/kingspan sheet?

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why insulate the whole roof????.cheaper to just insulate the ceiling area.
anyway,if you wanna insulate the roof,why not just screw the cellotex to the underside of the rafters????.
ceiling area is the whole roof.

It goes from front of the house, highest in the middle and going down to the back again. Obviously usable space is roughly until 1.5 meter away from the front and end due to hight but I still want to do the whole roof to the end just to make it clean and usable.

havent actually thought about fixing insulation on the rafters instead of between. I think its better to do it between to avoid using any usable headroom.
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so would 50mm insulation be enough and is the vapour barrier/membrane needed with kingspan?
You can not build a 'warm deck' unless you remove the roof tiles battens and membrane as a warm roof describes the insulation sitting wholly above the rafters.

However, it is possible to retrospectively create a cold deck (vented) from within the roof space.

This method does rely on their being ventilation above the insulation either via a vented air gap or breathable membrane plus air gap. the air gap ideally should be 50mm.

The required thickness of pir foam board insulation is 100mm between the rafters and 40mm under draw - total 140mm.
last job i did.100mm between rafters(200mm rafters) and 40mm below but you arent regulated by BC so its really up to could go 50mm between (cos you only have 100mm rafters)and see how you get on,if not happy,add another 50mm below the rafters.
not sure you would need a vapour barrier if there is no roofing felt.
If you buy foil backed insulation boards and use foil tape on all the gaps and joins then this will be your vapour barrier.

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