Help with Insulating Dormer

[mattwaddy]

So I've asked a roofer who is about to DPM the flat roof of our dormer to also add insulation to form a warm roof as this was called out as an issue in the survey so whilst rennovating a little it would be good to tackle. That then leaves the cheeks and roof underside as per the following images. What I'm keen to understand is that if I remove the current plasterboard I'm hoping there is enough depth to fill under the windows and the dormer cheek (triangles) with celotex and then foil tape all the joins. i don't think I can get away with an additional layer over the battens due to room size etc. The bit I'm not sure about is what to do with the underside of the roof in the corners, should I also pull off the plasterboard there on each side of dormer to fit celotex, is so would that need an airgap? I'm assuming no airgap for the cheeks and under the window area?

So to summarise thinking warm roof with celotex over the top of the roof as part of the DPM fitting then celotex between stud timbers, but currently unknown depth until I open this up. Then I'm not sure on the roof underside next to the cheeks if someone has done this before, or could advice?

First post here, but looks like a great community, so invaluable for a bit of a novice like me, but willing to learn and try to DIY as much as I can

 

[mattwaddy]

Anyone, I guess I'm not asking a sensible question>?

 

[harrythefurrysquid]

add insulation to form a warm roof

I had this done, and was pretty happy with the result. The roofer added 150mm of insulation on top of the existing felt (which acts as the VCL), fastened down, and then felted on top. Obviously added additional height but looked fine with the drip edge brought down to the original level. Edit: Also insulation behind the fascia all the way around to seal the original air gap.

I asked about adding insulation to the cheek walls and was told that it's best done from the inside. If you did it from the outside, the whole structure including the roof would need to be wider. Inside, you would probably need to sacrifice a bit of room space and replace the existing plasterboard with insulated plasterboard, as there is unlikely to be much space behind the existing plasterboard. You could cut away a bit of plasterboard to see what you're working it, as it'll be easy enough to patch it again if it's not worth improving. I ended up leaving it be but it's quite visibly cold on the thermal camera so definitely a weak point.

You'll likely have more room to add insulation to the existing structure underneath the window - I'd expect that to be a deeper structure. Hack a bit of the plasterboard off to see what you're working with, but likely there's little or nothing between the studs at the moment so some celotex, foam, and tape would be a significant improvement.

 

[Mr Chibs]

Standard practise seems to be infill between the stud work on the cheeks, and then another 25mm (or more) on the room side covering the infill, foam and tape all layers wall to make gap/draught free etc.

I've been walking past a house renewing their dormer, they are only putting 50mm of PIR in the cheeks, which I don't think will satisfy regs, never mind it will still be cold, previous roof had no insulation! You need to see what timber sizes you are working with ,as harry mentioned hack of some plasterboard to have an inspection.

It would be possible to external insulation, but you would be adding a lot to job.