Flat roof insulatinon

[Deejay]

Morning all

I have a repair to do. My daughter has a 1970's house with flat roofed dormer windows. A leak developed which damaged the plasterboard in one of the bedrooms. The roof on the dormer has been repaired.

The majority of the ceiling sits beneath the pitched part of the roof, but I'd like some advice on insulating the roof of the dormer window. The existing structure is what it is. The joists, probably 3 x 2, support a deck made up of planks.

Should I completely fill the space between the underside of the deck and the plasterboard with insulation and fit a vapour barrier to prevent moisture getting through the plasterboard?

Cheers

Dave

 

[9fingers]

I believe you need air flow across the top of your insulation from the gutter/fascia/soffit through into the loft area.
With 3" joists that does not leave much but you can get 45 and 50mm celotex or similar boards to fit in. Whist you have the ceiling down then yes I would always put in a vapour barrier of thick ish polythene as belt and braces.
Humans exhale pints of water vapour overnight and that is best kept in the room and not finding its way to the cold side of the insulation and risking condensation.

Bob

 

[Mike G]

It's often (in fact almost always) condensation rather than leaks which cause damp problems in dormers. Both the cheeks and the roof are generally too thin to achieve the same insulation value as in the roof surrounding them, and as a cold spot they are the site, naturally, of condensation. It's a pity that the roof has been repaired, because the best place for additional insulation is above the joists, which would turn the roof into a warm roof which doesn't require cross-ventilation.

So, it's a cold roof. That means it needs venting above the insulation, as Bob said. That means 2" Celotex between the joists, leaving an inch air-gap over (with vents at the eaves to allow air in and out). If there is any space below the joists (there generally isn't), then plant an inch of Celotex across the whole of the underside, and then definitely either foil-backed plasterboard or a plastic vapour barrier and ordinary plasterboard.

 

[Lurker]

It's often (in fact almost always) condensation rather than leaks which cause damp problems in dormers.

I didn’t know that, but it is entirely logical.
Shows the worth of talking to experts.

I helped a friend take off the flat roof and replace with a sloping roof with tiles that matched the main roof. We got the tiles from a house up the street that was being re roofed. The lead we removed paid for all the other materials.

 

[Deejay]

Thanks for the replies.

The leak was apparently a bucket job, but I take Mike's point about condensation. A warm roof would have been better, but the dormer is on one of a pair of semi's. It stretches across both houses and the neighbour wasn't interested in paying for a repair which he didn't need.

I'll take down the plasterboard under the deck and see what we have got to play with, and how to get some ventilation in.

Cheers

Dave