Insulation on single skin around window

[Jamie2106]

I've had a window replaced with a smaller one and the gaps to the side bricked up. To save money I told the builder I would finish off the inside so it has been left as in the photo with just the outside skin. Rather than bricks/blocks on the inside, I was thinking of insulating the outside skin with 50mm cellotex, then building a timber frame to line up with the existing inside wall, finishing with foil backed plasterboard. I've read up on how to insulate a single skin wall, which is in effect what I would be doing, but can't find any consistent advice. Can I stick the cellotex straight to the outer wall, should it be battened, with or without an air gap? Should there be a gap between the cellotex and the plasterboard or should I aim to fill the whole space? And do I need to put any kind of damp proofing on the outside skin first? Any help gratefully received

 

[Doggit]

Fix 3 vertical battens of 25x5o to the wall, with a plastic DPC membrane behind them. The fix the cellotex to the battens, and then the plasterboard to the battens with 3" plasterboard screws. You may want to juggle the cellotex depending on the depth of the cavity.

 

[Jamie2106]

Thanks Doggit, just so I understand, there would be a gap between the wall and the cellotex (ie the thickness of the battens)? And the cellotex should fill the whole cavity from battens up to the plasterboard? The distance from the outside skin to the inside edge of the wall is 18cm

 

[Doggit]

Yes, you've got the idea. At 25mm for the battens, and 15mm for the plasterboard and skim, then you either want 140mm of cellotex, or a deeper batten.

 

[Jamie2106]

Nice one, thanks again

 

[Jamie2106]

Sorry to be a pain, but I wondered if this is the only way to do it? Just thinking of the difficulties of fixing the plasterboard through 140mm of cellotex, I haven’t seen any drywall screws long enough.


Some sites I’ve read talk about having a service void between the insulation and the plasterboard, would this mean I could do something like my original idea of having the plasterboard fixed to a timber frame inside the window reveal? I’d completely seal up the cellotex so no moisture would get through that way, with the foil skin being the vapour layer, and the plasterboard has it’s own foil skin on the back preventing moisture going through from the room.


So the order would be:


Outer wall

DPM behind battens

Battens – air gap between outer wall and cellotex as you said above

Cellotex (100mm?) on top of battens

Air gap

Foil backed plasterboard


Or if the airgap has to be filled, can I do as above but stuff the gap with rockwool to ensure there are no empty corners around the timber frame?

 

[noseall]

You will need to fix a timber against the jamb of the door in order to have a solid reveal to fix to.

So let's say for arguments sake you had 170mm to play with. I'd fix a 100mm x 50mm timber on edge up against the door jamb (forming the reveal), one across the header and one up against the exiting jamb. I'd fill this with 100mm Celotex.
Id then fix a 25mm x 50mm batten on edge on top of the reveal timber then fix another 50mm layer of Celotex across the studs. fix 12.5mm boards.

Crapdrawing

• noseall
• 5 Jan 2018

 

[Doggit]

It's totally up to you how big you make the air gap/service void, and whether you fix battens to the wall, or to the surround. At the end of the day, it's just a case of getting some insulation in, and then plastering the old hole over. Your method is fine, and you can have a big air gap where the cavit was without any problems.

 

[noseall]

Obviously you would fix a plasterboard up the reveal also.

 

[noseall]

No need for any air gaps. Fill it solid and use foam and foil tape to seal edges, joints and gaps.

 

[Jamie2106]

Thanks guys, that's a great help. I'm probably overthinking it, it's only a short section to make good, I'm just a bit nervous about creating cold spots next to the window and mould forming

 

[motorbiking]

Celotex PL range can be dot n dabbed directly on to the wall and backed up by a few frame fixings. Given the size it wont cost too much and avoids the need for battens which reduce the thermal benefit. No air gap is required.
https://www.celotex.co.uk/products/pl4000

see

 

[noseall]

Celotex PL range can be dot n dabbed

I wouldn't advise fixing 150mm+ thickness of Celotex using this method. Plus the dabs will be in direct contact with the external leaf.