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!

[ST12AT]

https://www.insulationsuperstore.co.uk/browse/insu...

I wouldn’t be laying PIR on top of pipes and cables and then permanently securing it with expanding foam!

[elanfan]

Pretty impressive fire load you may have added to your house there. Do you know what the fire rating of the boards have? Using expanding foam will be adding more combustibles.. you’d have been better off with Rockeool.

[MJNewton]

It sounds like you may have been given poor advice by your friend. Rockwool would've been a far better choice for this application.

[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.

[paulrockliffe]

elanfan said:

Pretty impressive fire load you may have added to your house there. Do you know what the fire rating of the boards have? Using expanding foam will be adding more combustibles.. you’d have been better off with Rockeool.

Building Control insisted that I put 80mm of that between all my joists and 60mm over the top. There are pipes and cables too.

Can I sue them when it kills me?

[anonymous-user]

2Btoo said:

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.

I would be going back to the supplier for the fire rating of the material, alternatively take a small offcut outside somewhere safe, hold it in place with a brick or two and test it.

[elanfan]

paulrockliffe said:

elanfan said:

Pretty impressive fire load you may have added to your house there. Do you know what the fire rating of the boards have? Using expanding foam will be adding more combustibles.. you’d have been better off with Rockeool.

Building Control insisted that I put 80mm of that between all my joists and 60mm over the top. There are pipes and cables too.

Can I sue them when it kills me?

Bit of a flaw there.

[elanfan]

gottans said:

2Btoo said:

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.

I would be going back to the supplier for the fire rating of the material, alternatively take a small offcut outside somewhere safe, hold it in place with a brick or two and test it.

The odds on finding out the fire rating now is very unlikely. There are many different types of PIR insulation. Many will be combustible, a few may have some degree of fire rating but some may be restricted in performance. The way some are tested might be be a bit dubious. Try to set them on fire isn’t reliable, you have no way of knowing the temperature of the heat you are applying. The PIR might even give off cyanide and other nasties so not a good idea. It’s likely the smoke would kill you well before the fire got anywhere near you.

The OP isn’t going to replace it now, a potentially very unwise move. If he doesn’t care about himself he ought to consider his family.

[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.

[elanfan]

Final thought. If you’ve got a small pice you could try sending it to Kingspan and ask if they can identjfy it. Whatever it turns out to be it won’t be as good as rockwool which is non combustible.

[MJNewton]

If expect it to say on it what it is? I'm sure there's usually info like batch number/date, product code etc on the foil face with the branding.

[paulrockliffe]

elanfan said:

Final thought. If you’ve got a small pice you could try sending it to Kingspan and ask if they can identjfy it. Whatever it turns out to be it won’t be as good as rockwool which is non combustible.

Maybe I was too subtle earlier.

PIR is used extensively in every element of house building. It's under floors, inside walls, in lofts, between joists, over joists. It's used in warm roofs and externally. It's the standard solution for many projects that go through Building Control without problem. There aren't houses burning down because PIR has been used.

Why are you telling the OP that he's going to kill his family, when the advice he needs is on avoiding condensation and making sure he knows not to put too much weight on the ceiling joists?

[archie456]

I asked a race-mechanic friend if the expanding foam seat I was making for a car needed to be fire-resistant etc. He said if the fire had got that far it wouldn't really matter.

I'm no expert but in a similar vein I suspect that if the fire has got into the roof then it may be a little late already for anyone still in the house.

[Andeh1]

The risk is if there is any issues with all the cabling, downlighters etc that could cause the fire in the first place.

... but IANAL, and have no idea on the fire risks of the PIR. My parents had some installed in an extension to pass building regs due to the loft needing boarding & there not being enough space for sufficient rockwool.

Edited by Andeh1 on Saturday 28th November 15:45

[Aluminati]

PIR has a smoke and flame resistance. PUR does not perform as well. The boards you have purchased are non foil, and could be PUR.

However, your fire barrier is your ceiling, so, fireproof down lights etc and an intumescent strip around the fire rated access hatch etc...

Back in the real world....probably be fine...

Oh, and according to the Grenfell Enquiry, Kingspan and Celotex fake their fire ratings anyway...

Edited by Aluminati on Saturday 28th November 17:18

[elanfan]

I recall one manufacturer of expanding foam claimed a very high fire resistance time. Their test comprised drilling a small hole in a concrete block filling it with foam and setting a blow torch or similar on it. They claimed say an hours resistance which had nothing to do with the concrete block at all oh no.

I’m sure the industry could do with standardised fire tests (in fact I’m sure they do it’s just many manufacturers know they won’t pass so conduct their own more favourable tests). The law needs changing before more people die unnecessarily .

[guindilias]

He's not cladding a tower block FFS, he's insulating his ceiling. My house has wooden floorboards - the horror, they catch fire really quickly!

[elanfan]

Bet they don’t!, relatively speaking and whilst they give off smoke it’s nothing like what foam produces. Bet you’ve still got polystyrene tiles on your kitchen ceiling too!

[guindilias]

Nope, my kitchen ceiling is part plasterboard and half gaping holes. And some minor fire damage (I'm an excellent cook).
But I have plenty of non fire rated foam furniture to kill me, PIR insulation above the bedroom ceiling isn't a concern whatsoever.
People just don't die from it in domestic house fires. Ever. It isn't an issue.