insulation advice

[paul_v]

we have a dormer bungerlow and the upstairs bedrooms are cold so need to upgrade the insulation.

The dormer cheeks and roof are built using 3x2 so this limits how much insulation i can get in. At the moment there is only a thin layer of fiberglass.

My question is if i use celotex will i need to leave an air gap? There doesn't seem to be soffit vents.

i've looked at rockwool flexi but im unsure how much this would compress so unsure what thickness i could get in or would i need an air gap with rockwool as well.

Any help will be greatly appreciated as im getting more and more confused about all the different types of insulation.

 

[r896neo]

if there is a breather membrane (or no membrane) on the outside then you can full fill with whatever you prefer.

If they are old and there is some kind of impervious felt then you can't. You would in that case need to leave a 25mm airgap and ventilate it somehow.

If you have a breather, which seems likely if the existing insulation was fully filling the space, then 75mm of kingspan and overboarding the face of the studs with another 25mm and then plasterboard would be a massive difference.

 

[^woody^]

Full fill, and fill every single gap with expanding foam, and then fix a vapour barrier (polythene or foil backed plasterboard) across the inside face.

Better still to fit a continuous later of insulation across the inner face to prevent cold bridging at the timber studs, before plasterboarding

 

[paul_v]

thanks for the quick reply,

the cheeks are done with felt and then tiled so i will have to leave an air gap.

The roof is a flat roof with ply and felt on top. Would i need to leave an air gap here as well?

What would be the best way to ventilate the soffits?

 

[^woody^]

You don't need an air gap if you don't have any air voids and stop moist air getting in in the first place

 

[paul_v]

thanks for all who replied, very much appreciated