Insulating stud wall to eaves

[WillKen]

I’m currently renovating my 70’s built chalet bungalow style house. The first floor rooms all have dormer windows/walls which are studs with plasterboard on room side, but open to the eaves on the other. Therefore there’s no wall insulation on the external walls. I need to upgrade that.

 

I plan to remove the plasterboard and insert PIR insulation, then replace plasterboard in front of the studs. My heat loss calculations for our heat pump install are based off 150mm of PIR insulation.

 

Given the space on the cold side of the wall I can either:

 

1. Install 150mm of PIR between studs, with approx 50mm sticking out behind the 100mm studs; or

 

2. Install 100mm PIR between studs (and flush at the back) with 50/12.5mm insulated plasterboard in front.

 

Which is better thermally? Are there any problems with 2? I think 2 would be cheaper and faster and of course I won’t lose 50mm in each room (no big deal though…).

 

Thanks!

Edited September 13, 2023 by WillKen

[WillKen]

In my haste I of course meant:

 

”any problems with 1? It would be cheaper and easier”

[FuerteStu]

2 would eliminate the thermal bridge from the stud walls. 

 

If you did 1, you'd have to baton the studs to have something to fix plasterboard to. 

[WillKen]

Thanks

 

How significant would that thermal bridge be?

 

Sorry if I wasn’t clear, but for 1, the 50mm excess would be on the backside of the wall, so plasterboard would still be flush to the front of the studs.