Installing insulation/insulated plasterboard in a concrete bay window

birdboy89

We've very recently had new windows and I'm now looking to DIY some internal insulation on to the insides of our living rooms' 1950s/60s solid wall concrete bay type window and I'm after some advice please!

I've started removing the old plaster inside back to the concrete (still need to get the plasterboard off the top) so will need to sort the internal surface anyway to smooth it off (stabiliser and filler presumably?) which is all fine and the window frame has an extender on it allowing for ~45mm insulation on the sides and top plus plasterboard (or insulated plasterboard if that's easier). However I'm confused about actually installing the insulation/insulated plasterboard as the online advice seems contradictory/not suitable for my case! As its straight onto concrete do I need a vapour membrane and more generally how do I attach it to the concrete?

There was some minor evidence of mould in the bottom corners hence concerned about damp but this may be due to our rubbish old windows and I'm also going to repaint the external surfaces so it should be more waterproof in general.

Thanks.

grumbler

I don't see any point in vapour membrane. And I'd use insulated plasterboard, long screws and wall plugs.

FreeBear

grumbler said: I don't see any point in vapour membrane.

Use multiple layers of foil faced PIR (Celotex/Kingspan) type boards. Tape joints with alumnium tape, and the foil will act as an effective vapour control membrane. What I do on walls, is to fix vertical battens at 600mm spacing, fill the gaps with Celotex. Foam & tape the joints. Cover the whole lot with anther layer of Celotex, again, foam & tape the joints. Horizontal battens are then fixed to the top with screws going through the second layer in to the vertical battens. One more layer of Celotex between the battens (more foam & tape on the joints) before covering the whole lot with plasterboard. Around the window reveal, just one layer of 25mm Celotex finished off with 12mm ply screwed through into the brickwork.

I've done one wall where I put a thicker layer of Celotex up and used hammer fixings to secure a single set of battens to the wall. Bit of a crap shoot hitting solid brick rather than weak mortar joints, and the battens needed counterboring to lose the heads of the fixings.

With the OPs window, I'd probably go for a single layer of Celotex with a sheet of ply screwed through in to the concrete. The ply could either be painted or have a thin skim of plaster over the top.

justwhat

As above put vapour barrier on. Its not expensive and could save time and money in the long run.

birdboy89

FreeBear said:

With the OPs window, I'd probably go for a single layer of Celotex with a sheet of ply screwed through in to the concrete. The ply could either be painted or have a thin skim of plaster over the top.

That's helpful thank you @FreeBear and arguably simpler than what I was thinking.

Any advice on the bottom where the tiles are? I was planning on leaving them as we quite like them and there's much less depth to play with since there's no extender on the window frames there. But I assume if I insulate the sides and top, the bottom will still form a thermal bridge with associated condensation risks etc? Could/should I consider a thin layer of Celotex there too like you suggest above for window surrounds since that's better than nothing?

FreeBear

birdboy89 said: Any advice on the bottom where the tiles are? I was planning on leaving them as we quite like them and there's much less depth to play with since there's no extender on the window frames there. But I assume if I insulate the sides and top, the bottom will still form a thermal bridge with associated condensation risks etc? Could/should I consider a thin layer of Celotex there too like you suggest above for window surrounds since that's better than nothing?

The tiles will always be cold, so condensation will form on them. Removing the tiles will give you a little more room to play with, but probably not enough for 25mm of Celotex. What you could try is a couple of layers of woodfibre boards (as used for laminate underlay) topped off with 12mm ply. Finish the edge with a half round (bull nose) molding say 18-20mm thick. This will give the appearance of a solid wooden window board, yet still be thin enough to have insulation under it. The combination of woodfibre & ply won't give the same level of insulation, but it should be enough to stop condensation forming.

birdboy89

birdboy89 said:

Any advice on the bottom where the tiles are? I was planning on leaving them as we quite like them and there's much less depth to play with since there's no extender on the window frames there. But I assume if I insulate the sides and top, the bottom will still form a thermal bridge with associated condensation risks etc? Could/should I consider a thin layer of Celotex there too like you suggest above for window surrounds since that's better than nothing?

Alternatively if we decide to insulate but then retile the bottom, would something like this insulated tile backer work? My OH likes to keep house plants there so there's always a slight risk of water spillage etc.