PIR board over joists

[2Btoo]

Chaps,

I have bought an old house with chuff all loft insulation. Ceilings are lath and plaster, the joists are uneven (probably hand-cut), the loftspace is full of pipes and cables and there is a little bit of old rockwool fluff between the joists. At the suggestion of a friend I have bought some large slabs of PIR board and simply laid them over the top of the joists, cutting them to fit as neatly as possible in the loft area. However due to the fact that the boards are rigid and the joists not entirely even then they don't lie flat and move around a little. There is consequently a gap around the edges of them and the whole thing is far from airtight.

I am thinking of simply buying some cans of aerosol expanding foam and squirting this into the gaps to seal them. Is this a bad idea? My concerns are around moisture control as I am aware that damp can build up in lofts and cause problems. I am also aware that expanding foam in between the PIR boards will make the installation very permanent and would take care to allow some of it to be removed if necessary.

(I also boarded over the joists in the loft over a modern extension on the side of the house and these fitted perfectly; everything lined up perfectly, the PIR boards were able to be lightly jammed in so there are no gaps and everything is very solid indeed. It is comparing this with the same insulation in the older part of the house that makes me wonder how I can improve things.)

All advice welcomed - thanks!

[2Btoo]

Chaps,

Thanks for your answers.

ST12 - good point. I'd have to ensure that things could still be accessed, which would take a bit of thinking about.

Elanfan - fire load? I guess that's combustable material. The stuff in question was from Seconds and Co, like this:

https://www.secondsandco.co.uk/product-page/130-14...

I don't know what the fire rating of it is.

MJNewton - thanks. Oh well, it's done now and I'm not taking it up. Given that it's not entirely even then what's the best way forward? I guess I could just put some wide gaffer tape over the gaps which would seal it quite well.

Thanks again for your help.

[2Btoo]

Thanks for the answers. Gottans, I had wondered about doing something similar.

Elanfan, thanks for your input. The stuff is in there now, having cost me nearly £800 and five days graft to get it into place. Given that finding the fire rating now is very unlikely and that trying to set a piece on fire is not reliable then I'm genuinely not sure what you are suggesting. I appreciate from your posts on here that you are involved in Building Control and know a whole load more about this sort of stuff than I do but I'm not going to rush to take it out without a good reason to do so. A "maybe", "dubious" and "isn't reliable" is not that persuasive. Thanks for your input all the same.