Insulating a dormer window / sloping roof

[nickpowell77]

Hi - I was wondering if I could get some advice on insulating a dormer window with a sloping roof. To cut a long story short we had a leaking window and in order to prove if it was the roof or new window we had to remove plasterboard internally. It's all sorted now but I need to repair my Son's room.

I plan to strip the wall back and then get it re-plastered but I'm not sure what best to insulate it with. When I took some of the plasterboard off it was normal plasterboard with rock wool in the cavity.

All help really appreciated

Thanks, Nick

Untitled

• nickpowell77
• 4 Aug 2014
• 1

 

[freddiemercurystwin]

Are you looking to insulate the whole of the sloping portion or just the exposed bit? If just the exposed bit then there seems little point doing anything other than was there before ie a bit of Rockwool. That's the back of the roof membrane we can see I take it? Do you know if there any vents in the eaves and/or at the ridge? Presumably not?

 

[nickpowell77]

Hi, I am planning on removing the plasterboard from the whole wall and replacing it with thermal insulation unless folks here tell me there is no point.

I can see any vents

Thanks, Nick

 

[freddiemercurystwin]

Is is a modern breathable membrane or is it just old fashioned felt?

 

[nickpowell77]

It does seem to be a breathable membrane, it's definitely not solid old fashioned felt

 

[freddiemercurystwin]

I would be fitting as much Celotex (or any other PIR insulation) into the void as possibly and leaving a 25mm gap behind the membrane. You don't say the size of the rafters (I would guess about 100mm looking at the photy) but you could do worse by then fitting a thinner continuous layer of insulation across the back of the rafters and then board on top of that.

That said, if the rest of the loft/dormer is as poorly insulated then all of these good works might be a bit pointless, if well intentioned.

 

[nickpowell77]

Thanks freddymercurystwin - the rafters are indeed 100mm. The rest of the room is well insulated, it's just this sloping roof on the 1st floor.

I was thinking about stuffing the cavity between the rafters with rock wool and then putting celotex sheets across the rafters and then board onto of that.

Do you think this would be a good way to do it?