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.

Thanks, Tony. Then I will add a layer of plasterboard / insulation on top of the masonry after I seal the joists at either side. The tricky bit is going to be blocking up in the bay window because I want to leave an air gap around the stud work. I will post photos once it is done in case anyone is interested.

And a crude diagram of the construction of the bay window first floor void. I plan to fill the space between the joists in the void with insulation before closing the ceiling again and sealing the remaining gap above the ground floor window probably with PIR backed plasterboard.

It is indeed a block wall resting on top of floorboards. It is connected to the inner leaf of the cavity wall either side. They are dark grey blocks that fall apart very easily when you try to fix into them.

I was a bit surprised too - initially I had wanted to get all the floorboards up in the room upstairs but I decided against it!

is there a reason you didn't just fill the void with a high density wool insulation?

Posted By: mattpWhen I put the internal insulation on the block wall upstairs I was advised not to fill the cavity with anything. Although the rest of the house has cavity wall insulation it didn't seem to be installed into this area.

Who gave you the advice? I would guess the issue is possible dampness and/or condensation in that void, together with the presence of wood that might rot. Presumably the rest of the house is masonry?

Again, I would guess the external render is impermeable, so you'd need to be careful about sources of moisture in that space.

Posted By: mattpDo you think I'm making a mistake?

I don't know, because I don't know enough about your house. What you have to consider is (a) the presence of material that can rot (the timber), and (b) the possibility/likelihood of condensation on that material. Insulation will make whatever is outside it colder and so increase the likelihood of condensation. I suspect that is what they were thinking about. But there may be ways around the issue.

Posted By: mattpI would happily pay someone to come and look at it and tell me whether what I have done is ok or not, but I'm not sure where to go for this sort of advice.

Depending on where you are in the country, there should be a regional 'retrofit hub' that can point you towards a professional offering this service. There is also a national hub who might be able to assist.

As ever the issue is one of trustworthiness and quality control. There is now a recognised standard (PAS 2035) which should be met by anyone claiming to offer services in this sector. I'd guess that you probably want a 'Retrofit Designer' rather than a 'Retrofit Assessor' given the scope required.

https://nationalretrofithub.org.uk/

To get started, I'd look at someone like Greengauge (warning, they are probably way too busy) who can offer the kinds of analysis required: https://ggbec.co.uk/what-we-do/moisture-risk/

The AECB also have a members directory, which is searchable & another good place to start:

https://aecb.net/directory-members/

Thanks Thomas, I am in the Southampton area. From the Greengauge link it looks like what a professional would do is put the construction into a 1D model to estimate moisture risk, is that right?

Tony, how would filling the void with EPS beads alter the risk of condensation?

Posted By: GreenPaddymattp, your section sketch shows a timber frame wall with the render on a carrier board of some type outwith that frame. The frame would NOT get filled with insulation, but everything else would.

I think you're misreading the diagram. There's a timber frame and render on a render board, but nothing backing the frame on the other side.

But even if there were, the presence of insulation in the void would mean the whole timber frame structure would be cold and susceptible to condensation.

@djh - you are right. The timber frame is attached above and below to the two PVC window frames. It is presumably also attached to the outer skin of bricks on either side. My 70 mm estimate of the void is a bit off. The studs are about 40 or 50 mm then there is about another 50 mm gap between the studs and the block wall giving a maximum space of 90 or 100 mm where there are no studs. I think the 70 mm was maybe a measurement between the membrane and the edge of one of the floorboards which juts out.

Whatever I do those timbers are going to be cold because they are touching the back of the render. They are currently dry and in good condition and have been there who knows how many years, but obviously there has been a gaping hole at the bottom which I am set on closing and this will be a significant change.

@GreenPaddy - so you suggest sliding sheets of mineral wool into the gap but not leaving the spaces between the studs with air? I would probably have to do this from above so it won't happen immediately because I have "finished" that room. You spotted the mistake I made in the room above of not insulating under the window board.

@mattp - slide the wool up to the cill from below, along the inside face of the timber studs. That would leave the gap between studs NOT filled with insulation, and therefore the wool would NOT touch the back of the render, incase there is any ingress of water.
If it's tricky to slide the wool in, because it catches on the timber studs, slide a piece of say cardboard (or any semi rigid thin board) up the face of the timber studs, slide the wool over that cardboard till it hits the cill, then remove the cardboard.
Or take the cill off and work from above :) - (might be better now you've sealed the area off with plasterboard pieces??

Re prevention of condensation in the void, filling with fire retardant eps beads will make it warmer, any condensation will form on the very outside of the insulation ot building elements beyond it. - I like a vapour barrier on the warm side of the insulation and then the eps beads are breathable and moisture moves away from the (warm) house