Insulating a window reveal

[thomp1983]

Hi all,

I'm internally insulating a North facing wall in my house.

I've removed all the existing plaster and now I've measured up I think I'm going to have an issue with the reveals.

My original plan was 25mm celotex, 9.5mm plaster board and allowing for 3mm skim that comes to 37.5mm which on the right side reveal will come out over the beading for the glass.

The max I have available is 35mm, can I use a 6mm tile backer board and will it take board finish plaster ok? I've seen you can get 6mm plasterboard but it's stupidly expensive for a single sheet at about £70.

I could go down to 20mm celotex but I'd rather keep the extra insulation and lose the thickness in the finishing board, and I've already got the 25mm celotex.

Has anyone got any other ideas, the only other idea I've got is to stihl 10mm off the blockwork but that seems extreme and may cause issues with the lintel bearing.

Thanks
Chris

 

[jono_h]

I tile backer boarded my reveals and plastered straight over the top. Took ages to dry, but Ok a few years later.

You could get away with something thicker than 6mm then.

 

[^woody^]

Don't skim and decorate straight on the board, saving 3mm.

Or use foam adhesive and rely on that and air, and you won't have any practical or demonstratable difference

Or shave the insulation board or fit a thinner board, likewise you won't notice any difference.

Introducing a cold cement board to reveals introduces increased risk of local condensation.

In any case, properly seal those gaps to the frame and cill first

 

[noseall]

Hi all,

I'm internally insulating a North facing wall in my house.

View attachment 399180

I've removed all the existing plaster and now I've measured up I think I'm going to have an issue with the reveals.

My original plan was 25mm celotex, 9.5mm plaster board and allowing for 3mm skim that comes to 37.5mm which on the right side reveal will come out over the beading for the glass.

The max I have available is 35mm, can I use a 6mm tile backer board and will it take board finish plaster ok? I've seen you can get 6mm plasterboard but it's stupidly expensive for a single sheet at about £70.

I could go down to 20mm celotex but I'd rather keep the extra insulation and lose the thickness in the finishing board, and I've already got the 25mm celotex.

Has anyone got any other ideas, the only other idea I've got is to stihl 10mm off the blockwork but that seems extreme and may cause issues with the lintel bearing.

Thanks
Chris

Because you have so little room to work with, your best bet is to use low-ex foam adhesive and 12.5mm plasterboard. Make sure you have some props (off the other reveal) ready to prevent the board from creeping out after you have stuck it. You can remove the props after an hour.

 

[stuart45]

Deleted

 

[Deluks]

Introducing a cold cement board to reveals introduces increased risk of local condensation.

Surely, by tilebacker board, they mean the foam sandwich type. Marmox, Wedi, Jackoboard.
Not Hardie board.

That would be my answer, it's available in 10,12, 20mm. Glue it straight on and then skim over it.

 

[thomp1983]

So with all the plaster off and out the way it turns out there were loads of gaps around the window frame, some big enough I could reach in and touch the cladding outside.

I've folded a piece of rockwool into the top of the cavity to stop the foam dropping through and now the whole frame is foamed properly and the first bit of 25mm insulation is fitted at the bottom.

I think I'm going to go with trimming the 25mm insulation enough to allow me to fit a 9.5mm plasterboard and skim to keep it all uniform.

Thanks
Chris

 

[noseall]

View attachment 399305

So with all the plaster off and out the way it turns out there were loads of gaps around the window frame, some big enough I could reach in and touch the cladding outside.

View attachment 399306

I've folded a piece of rockwool into the top of the cavity to stop the foam dropping through and now the whole frame is foamed properly and the first bit of 25mm insulation is fitted at the bottom.

I think I'm going to go with trimming the 25mm insulation enough to allow me to fit a 9.5mm plasterboard and skim to keep it all uniform.

Thanks
Chris

We make our own cav' closers using Celotex, DPC and fixed with foam. I hate proprietary cav' closers because they are pants.

woodfill2.jpg

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[^woody^]

Surely, by tilebacker board, they mean the foam sandwich type. Marmox, Wedi, Jackoboard.
Not Hardie board.

That would be my answer, it's available in 10,12, 20mm. Glue it straight on and then skim over it.

Hmmm, maybe.

I think it's best to bear in mind the principles of using some insulation, not necessarily the same thickness as the other walls, and minimise cold surfaces and bridging, and do what's easiest at least cost.

 

[jono_h]

Surely, by tilebacker board, they mean the foam sandwich type. Marmox, Wedi, Jackoboard.
Not Hardie board.

That would be my answer, it's available in 10,12, 20mm. Glue it straight on and then skim over it.

Yep, thats what I meant. A nice bit of 20mm.

 

[stuart45]

Hmmm, maybe.

I think it's best to bear in mind the principles of using some insulation, not necessarily the same thickness as the other walls, and minimise cold surfaces and bridging, and do what's easiest at least cost.

^^^^^^^°°°°
This
The idea of insulating the inner reveals is more about getting enough insulation to stop condensation forming and mould growth.

 

[thomp1983]

So after wondering how I could cut the 25mm celotex down to 20mm I had the idea to use 2 old offcuts of 18mm mdf.

That's got the celotex down to somewhere between 18 and 20mm, with it fitted in place I've now got a minimum of 18mm between the bead edge and insulation so with a bit of care I can fit and skim a 9.5mm board and still have 3mm to the bead edge.

Thanks
Chris

 

[thomp1983]

Just to tie this up for anyone looking in the future,

All the old sand/cement plaster chipped off.

50mm celotex stuck with low expanding foam.

Stud wall built from 3x2 laid so the 2" formed the depth, 50mm celotex infill with all gaps foamed.

I then added 15mm thick lats to the studs to bring them flush with the celotex.

And boarded with 12.5mm board and 9.5mm in the reveals.

Should get plastered by a mate next week and the rest of the room finished for Xmas.