I have a bay window in my front room and it has a cavity above it, which is connected to the cavity of the cavity walls either side. Unfortunately it is also completely open to the cavity between the ceiling and the floor of the room above and I am wondering how I might seal it.

The cavity above the window has some sort of membrane on the external side. On the other side of that is pebble dash render. It is supported on a timber frame and it is tied into the external skin of the cavity wall either side and on top of this frame is the bay window of the room above. It is completely open to the void under the floor of the room above, but there is a block wall built on top of the floorboards in the room above, which is tied into the inner skin of the cavity wall either side and supports the window ledge for the room above.

I am thinking of cutting out squares of plasterboard the right size and foaming them into position, then foaming round all the gaps to close the first floor cavity. There are also a couple of joists that go into the brickwork of the outer layer of bricks, so I was thinking of sealing these too, maybe with the same foam. Is this a good approach?

I am thinking of using one of these two foams:

https://www.toolstation.com/soudal-flexible-expanding-foam/p69089

https://www.toolstation.com/soudal-flexible-expanding-foam/p69089

I will attach 2 or 3 photos so you can see what I am talking about. Thanks.

One of the joists going into the wall.

Thanks, it's useful to know that the foam will stick plasterboard and PIR boards. I mistakenly linked twice to the flexible foam. The other one I was looking at was this since it is low expansion and designed for gluing:

https://www.toolstation.com/soudal-soudabond-easy-adhesive-foam-gun-grade/p37023

I was thinking to seal the gaps in the void between floors with plasterboard since I think that is a bit breathable and I am worried how any moisture in that cavity might escape. I will then fill the void with some breathable insulation before replacing the plasterboard ceiling. For the reveals above the window which at the moment just have plasterboard, I was thinking to glue PIR backed plasterboard to the existing boards. Here I want to minimise any moisture passing from inside the room, so PIR might be better here.

I actually renovated the room above a while ago, but didn't think about this area. I put 40 mm IWI against the block wall above before plasterboarding.

Later I might try and do a drawing to explain better what I mean.