Insulating combed cieling

8 Apr 2007
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United Kingdom

I need to insulate an extension unfortunately I have no access into the roof space. The ceiling is combed and looking from outside I don’t think there will be much space between the plaster board and the roof. If I gain entry into roof space would it be possible to insert a Celotex or similar board in between the joists in the sloping part of the roof, ensuring I leave the required space for air to flow? As for the level part of the ceiling, if I gain entry to it would I be able to insulate in between the joists, ensuring a 50mm gap between insulation and roof and then would it be possible to insulate the existing plaster board ceiling below the joists by bonding Celotex to the plaster board ensuring all joints are taped with the relevant jointing tape? The plan was to then clad the ceiling in timber, if that’s possible, by screwing through the Celotex or similar board and into the joist, would that be ok or would it compromise the vapour barrier? To add to the problem there is a feature sandstone wall (the original outside wall) this is uneven and has gaps between the ceiling edge and the wall, is there a product that could be used to fill awkward gaps, i.e. some kind of flexible filler?


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Why would you prefer to insulate the pitch and not the floor? Its going to be a lot cheaper to insulate the floor. But yes you can insulate between (and also over) the pitched roof beams. If you don't mind a bit of mess, you could redo the ceiling and put insulation in at the same time.

Personally, while celotex is a great product I'd prefer a more fire retardant insulation in my ceiling to loft space
The floor is solid concrete, so didn't think that was an option, do you think it might be. Agree with you on the fire risk side of things, what would you recommend?
Thanks for the reply.
When I say floor - I mean floor of the roof void..

why don't you have access to the roof void? Is it because there is no hatch?
Can you not cut a hatch, stuff the void with fire retardant rockwool or similar to at least 300mm and if you don't want a hatch replace the cut plasterboard and get a plasterer to skim the ceiling. The other option is to lower the ceiling by installing insulation backed plaster and getting it re-skimmed - or if you are careful you can just skim the joins.
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The extension is 20 years old,no hatch so plan was to do as you say and cut a hole, install insulation, replace board and as we are in north of Scotland clad the ceiling in timber, which is quite traditional up here. I was a bit concerned about the space between the slopping part of the ceiling and the roof thats why i was thinking of sliding insulation boards into the gap.
I think you'll find a rockwool roll easier to work with than celotex. Celotex is a great product and very thermally efficient, but having insulated my garden room with the stuff, I can tell you, you wont want to be measuring and cutting boards to slot between beams, in a confined space.

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