Dormer Window Insulation

[lc155]

Hi,

While the roof part of the dormer is ongoing (https://www.diynot.com/diy/posts/5859047/), I didn't manage to get the slates finished before 6+ inches of snow got dumped on everything. I hope the breather membranes can take a little bit of snow and ice before installation completes!

Aside from that, I'm thinking about the actual dormer walls as well, and how to insulate them. The studs making up the walls are simple 50mm posts, so as far as I know, I can't even put anything in between them as that would leave zero airgap to the outside and definitely lead to condensation and rot, right? It feels like the only feasible option is to cover both dormer walls with 25mm PIR (can't go any more than that as too much loss of space plus it would make the window look weird as it is a bay window, not a flat one). The PIR is probably necessary anyway because the bottom half of the wall has the posts sitting on two large rafters, so can't insulate between those!

Any advice on this?

 

[lc155]

I looked up on this more and they are specifically called dormer 'cheeks'

People doin't seem bothered about stuffing them to the brim with insulation like elsewhere where they'd leave a ventilation gap. I don't really understand it!

 

[freddiemercurystwin]

What are you cladding the dormer cheeks with?

 

[lc155]

What are you cladding the dormer cheeks with?

I plan to take off the slates on the sides (in order to replace old flashing that is failing at the valley), install a breather membrane over the existing sarking (cromar vent3 Pro) and then put the slates back on.

 

[freddiemercurystwin]

So you would fully fill the studs with 50mm insulation, then fit a board eg 12mm plywood, then your breather membrane then vertical counter battens to achieve your ventilation gap) then your horizontal slate battens. The slates should be vented top and bottom. Then internally fit your continuous insulation, then either a vapour barrier+plasterboard or foil backed plasterboard instead.

 

[lc155]

So you would fully fill the studs with 50mm insulation, then fit a board eg 12mm plywood, then breather membrane then vertical counter battens then your horizontal slate battens.

Are those counter battens required? We don't really use them up in Scotland due to just nailing slates and the membrane direct onto sarking. Never seen it done any other way, even on new builds. Are all these in between steps supposed to provide enough of a gap or something?

 

[freddiemercurystwin]

The vertical battens create a ventilated void behind the horizontal battens that is vented top and bottom, it's the right way to do it in the whole of the UK.

 

[lc155]

The vertical battens create a ventilated void behind the horizontal battens that is vented top and bottom, it's the right way to do it in the whole of the UK.

The problem with extending the depth of the cheeks like this is that it's going to clash with the bay window. Not sure how to work with that.