Insulating a bay window wall

cwhaley1

Hello all!

My partner and I have just bought our very first house together. We're 25 and committed to really getting to work on some jobs. Not lots to do but the biggest problem we currently have is condensation and mould...

Fortunately, it's confined to a single place - directly underneath a bay window. As I understand it, the rest of the house is 9" brick (proven by no condensation or mould on these walls), but the bay window is 4" brick (lots of condensation and mould). Water is beading on the wall and dripping down on to the floor - it's very cold to the touch and no a nice room to sleep in. There are trickle vents above the window which do promote ventilation, but I think the real issue is the pure coldness of the wall forming a "dewing point" for the moisture in the air. Checking the skirting board last night resulted in chunks of rotting wood.

My plan is to take the plaster off and take to bare brick. Once there, screw wooden studs to the wall to form a frame. Put 50mm Celox in between the studs (cut to size), fill the joining gaps with filler/tape. I'd then fix plasterboard to the studs, tidy and decorate.

I can go ahead with this, but I'm just worried about trapping moisture behind the Celotex on the brick wall. This may not be an issue, but it's the one thing I don't know how to overcome if it is.

Would really appreciate any help or simple advice you could give.

Callum

FreeBear

Have a look at "warm battens" - In essence, one layer of Celotex fixed to the wall, then battens (25x50mm) screwed through to the bricks. The gaps are then filled with smaller sheets of Celotex. The advantage of doing it this way means that there is no timber in contact with (possibly) damp bricks, so it won't rot, and also avoids any cold bridging by the battens.

I'd also suggest going for 75mm in overall thickness of insulation - Using a sheet of 50mm and then 25mm between the battens works out quite nicely.

If the bricks on the outside are bare (i.e. haven't been rendered with cement or painted with a waterproof coating), then any damp on the inside face should be able to migrate out and evaporate. The layer(s) of Celotex will act as a vapour control layer and should prevent moisture from the inside entering the brickwork.