Building an internal stud wall on an external wall

[d000hg]

I want to insulate an external wall, using either 75 or 100mm PIR board in a timber stud. I can't/don't want to simply dot'n'dab insulated plasterboard.

As I understand it you'd often leave a small gap for running wires/pipes, presumably on the inside of the PIR?
This is starting to get the stud depth quite thick like >100mm, I don't really want to build a stud from 6x2" and wondered if there are any tricks/tips like offsetting the stud from the external wall?

How should the stud actually be affixed to the wall if you're using say 4x2" or similar? Is it simply a case of using really long screws all the way through the studs? e.g. 150-200mm?

Thanks.

 

[Notch7]

There is an option:

1 cover the wall fully with 70mm celetex (use a few screws, wedges to hold it in place or adhesive tape).
2. foil tape the joints
3, screw on 50 x 25 or 75 x 25 battens
4 run services in between battens
4. fit plasterboard to battens -use foil backed plasterboard if you want belt n braces

-advantages: no thermal bridging (except at screws), no studwork, service void on inside

-disadvantages -no cavity against wall, slight risk of damp, no weight loading studwork to fix to, aljthough if thebattens go to the floor it will support plenty of weight -much better than dot n dab

I did find some testing of both ways somewhere -and the batten option is acceptable for internal wall insulation.

 

[d000hg]

So basically the same approach as a warm roof, interesting. One reason I don't want to use dot'n'dab is load-bearing, which you mention. We will have a kitchen fitted against (part of) this wall. But perhaps we can treat that specially, if I recall wall-mounted cupboards essentially hang from the top so we need a single strong horizontal element there.

What is the more traditional way of doing this then?

 

[Notch7]

So basically the same approach as a warm roof, interesting. One reason I don't want to use dot'n'dab is load-bearing, which you mention. We will have a kitchen fitted against (part of) this wall. But perhaps we can treat that specially, if I recall wall-mounted cupboards essentially hang from the top so we need a single strong horizontal element there.

What is the more traditional way of doing this then?

Ideally use wall cabinets which can be fitted with a long rail rather than individual wall clips -most do
https://www.locksonline.com/Cabinet-Hanger-Wall-Rail-2032mm-Long-6492.html

in which case you could just use the same method -but use 3 x 1 battens and closer spacing -it will easily hold the cabinets. Dont forget most homes are now built with thermalite blocks which are like fixing to polystrene.

 

[John D v2.0]

It might not even be essential to fix to the wall. For our ensuite shower we were going to do 75mm celotex then 3 by 2 studwork slightly in front, only attached to the ceiling and floor. In the end i couldn't justify the space so i did the same but insulation between the studs but still not touching the wall with the studs.
We went belt and braces and taped all the joints and used a poly sheet, but with it being a shower it was a good idea.
For the bay window we did almost exactly the same thing, but used wool in places as it was too wonky to justify chopping up celotex to match everywhere. It the studwork again is free standing as we didn't want any sound coming in from the A road outside

 

[d000hg]

It might not even be essential to fix to the wall. For our ensuite shower we were going to do 75mm celotex then 3 by 2 studwork slightly in front, only attached to the ceiling and floor. In the end i couldn't justify the space so i did the same but insulation between the studs but still not touching the wall with the studs.
We went belt and braces and taped all the joints and used a poly sheet, but with it being a shower it was a good idea.
For the bay window we did almost exactly the same thing, but used wool in places as it was too wonky to justify chopping up celotex to match everywhere. It the studwork again is free standing as we didn't want any sound coming in from the A road outside

I was wondering about this but the loading made me a bit worried, and the fact a ceiling attachment might be fiddly - it's single-storey open roof. I think we're going to be doing a long L shape so there will be some bracing, in an ideal world it would be 3 or even all 4 walls and a standalone 'cage'.
It'd definitely seem to make things a bit less work if we can figure it out.

I was even wondering if I could put stud uprights attached to the joists below floor level, rather than just building the stud on top of the floor... this would give a lot more rigidity. But it's a little unusual, probably for a good reason