I need to insulate the wall under a bay window. I have removed the plasterboard and there is a 75mm airspace then roofing felt and the back of the external render. There are wooden uprights spaced about 30cm apart.

I was thinking of fitting 75mm celotex all round – I know the installation won’t be by the book, but probably as good as I’ll get I figure. Otherwise I can put 50mm board in to get the air gap or add timber to get more depth on the upright timbers.

Do I need an air gap between the external wall and the insulation? It’s an upstairs bay so not sure what’s underneath ATM. Do I need a vapour barrier between plasterboard and insulation? Will I be better off putting rigid rock wool in instead?

I’m currently insulating and then dry lining my outside timber office. I too only have 75mm within the timber frame so looked around for an insulation at 50mm to give an air gap between the board and the timber.

Arleflex Platinum

I intend to bind this to the plaster board with the vapour barrier to the outside, then fit the plaster boards to the timber frame. It has an excellent U-Value, even at 50mm.

*bond^

Why would you bond it yourself? Just buy insulated plasterboard in the first place from kingspan, celotex etc.

I would use kingspan and then finish with foil back plasterboard as a vapour barrier

dooosuk – Member
Why would you bond it yourself? Just buy insulated plasterboard in the first place from kingspan, celotex etc.

In my case, the insulation is within the depth of the timber frame and the plaster board is fixed to the face (flies over) the timber frame. And this method also allowed me to ensure no bridging to the external, 22mm thick timber cladding creating a nominal 25mm air gap.

best way is to fill gap with celotex, then use celotex backed plasterboard to finish