PIR wall insulation between studs AND on top of studs internally

[DanHW]

Hi folks,

 

I'm building a timber-framed garden office and have an insulation question. I'm cutting PIR to fit between the studs as you'd expect. The studs are 5x2, with 120mm Celotex boards that fully fill the cavity. OSB sarking and a breathable membrane are on the outside later, which will then be battened and clad.

 

But I also have some spare 25mm or 30mm PIR boards, so was planning to fit these boards whole, on the inside face of the walls (across the studs), to try to minimise thermal bridging through the timbers. Vapour barrier and plasterboard will then go on top of that. Pic attached for clarity

 

I'm also thinking the same approach with the roof, except that it has a 50mm vented gap at the top of the rafters as it's a cold roof.

 

I don't think there are any increased risks of condensation inside the stud walls by adding this extra 25/30mm layer across the studs internally, but am doubting myself. Anyone able to advise?

[Dreadnaught]

It's unlikely to be a problem I would guess for that thickness. Many insulation companies will do an interstitial condensation assessment free-of-charge.

 

I had one done for the roof build up when I decided to add another layer of insulation underneath. I added an extra 100mm of woodfibre insulation beneath a warm roof of 200mm of PIR. It wasn't a problem.

 

Good airtightness will help of course as the main source of interstitial condensation is warm moist air migrating in to the fabric from within the building.