Insulating a Dormer

[n30_mkii]

So we are doing some refurbishment to our property which was built in 1969, some of that is to fit EDM rubber to the flat roof on our dormer. At the same time, I've asked the roofer to insulate the dormer as a warm roof. I also need to sort out if possible insulation for the dormer walls and cheeks.

My current thoughts are to take off a small portion of plasterboard internally to see what depth there is for the frame. I'm assuming there would still need to be a 50mm air gap, so suspect that it won't be possible to get any major depth.

For the cheeks would it need insulation just on the triangular cheeks that extend above the roof, or also on the underside? Also would it be worth opening up the section below the window which is hollow before it gets to the brick layer?

Would love to know if anyone else has done this previously and the best approach to go about it. If I can fit something like celotex I'd look to fill any small gaps with expanding foam and then aluminum tape over the joins before it then gets boarded again

 

[n30_mkii]

Warm roof would negate the need for an airgap as I understand, you'd want to insulate the whole wall not just the triangle bits as the rest will just be leaking heat into your roof/attic space to the sides, if you can access them you could put pir boards on the outsides in between the timber which i'm assuming is the structure there, and same for the hollow if you can get in it, every bit of insulation you can put in there whilst being mindful of ventiliation to avoid damp issues will be of benefit.

When you say the whole wall, you mean the undersides of the tiles (under the cheek) and under the window? Or something else? I'm going to chat with the roofer when he comes to see what options there are for accessing the void from the outside to cut in some PIR whilst the scaffolding is there if that will be easier.

 

[n30_mkii]

I'm going to start opening this up at the weekend. Any further advice is appreciate, my initial inspection hole seems to indicate that there should be enough room for 25mm of celotex if it's put up against the membrane material in the cheeks which I assume sits behind the hung tiles on the outside. I've seen different guidance around air gap for the cheeks and wall with some people stating no gap is required, and others suggesting 50mm gap which wouldn't be possible as the timber isn't deep enough