Pros and cons - spray foam insulation

paddylonglegs

hi folks,

The search option is terrible as usual, so apologies if there is a lot of discussion on this.

We need to insulate a new roof, the plan being to insulate between the rafters as the first insulation step. The option is 100mm rigid foam boards between the joists, or spray foam insulation between the joists.

Spray foam has gotten some bad press over the last few years, but I believe it was due to applying without any ventilation between the tiles and the insulation. My understating this is just the method of application, and a vent card at the back prevents issues occurring, such as joist rot etc.

Has anyone any deeper knowledge on issues that are caused? If the foam is applied correctly and enough space is left for ventilation, is there any other issues to Be aware of?

thanks.

whizbang

Its very difficult to airtight fit the rigid boards, even at build time without perfectly pre cut boards, and manicured timbers.

10-10-20

https://www.boards.ie/discussion/comment/123804893#Comment_123804893

Agreed, its a waste of time and money push-fitting them in without lapping them in tape and an airtight barrier, etc.

But I think that the jury is still out on spray foam, especially with how it went in the UK... its definitely a more complete job in terms of creating a consistent and air resistant layer, but even with vent-cards the ventilation is the crux, especially at the ridges where ridge vents might not be present…

User567363

Why insulate the attic space, do you plan to heat it?

Lumen

https://www.boards.ie/discussion/comment/123804914#Comment_123804914

Yeah, but "air resistant" is a funny expression. If it isn't continuously and permanently airtight (shrinkage!) then you need a continuous and permanent air tight layer provided by other products, and then what's the point of the foam?

I think it's popular in retrofit applications because it's fast and cheap. The industry has been dodging the issue of non certification for years.

10-10-20

https://www.boards.ie/discussion/comment/123805196#Comment_123805196

"Air-tighter" layer might have been a better choice of non-specific words. It was midnight. 😂

It's a gamble really is what I'm saying. I have a friend here who had it done (he was having a 'long blink' when his builder was explaining his build-plan to him) and the foam got installed without him seeing the installers in about 2hrs on a standard estate semi-d. When I found out and took a trip over to see, it amused me as the house now had this neatly installed air-tight (but vapour-permeable open-cell) roof insulation, but the rest of the house was in no way air-tight, so the risk of building-up excessive humidity and causing rot was probably nill; so there's that installation type. 🤣

paddylonglegs

https://www.boards.ie/discussion/comment/123805156#Comment_123805156

it’s a dormer roof and the first floor will be occupied

Sleeper12

In the UK most insurance company's won't cover it. It's not uncommon for spray foam insulation having to be ripped out, at massive expense to get mortgage approval.

This doesn't mean it's all bad. It does mean serious research is needed & any work is fully documented, photos during the process etc

fits

I wouldn’t go near it.

Lumen

Several of these products now have NSAI certs. Just read them carefully and make sure they're installed precisely according to the details in the cert.

https://www.google.com/search?q=open+cell+foam+nsai

enricoh

Banks in the UK insist on ripping it out before they'll lend on a house. I had mine n ma's house done years ago, good job for retaining heat. Wouldn't go near it nowadays

Tefral

Lumens answer is correct here. Go only with the ones given an NSAI cert and only ones that leave the 50mm air gap.

Do not use anyone that says they are covered to spray directly to the roof. There's a particularly popular one well funded by ads called Icynene here that in my opinion, I would steer well clear of for that reason.

Lumen

https://www.boards.ie/discussion/comment/123811922#Comment_123811922

I hate to disagree with someone agreeing with me, but it's a bit more complicated than that. If you look, for instance, at section 2.4.2 of the NSAI cert for Purcell, there are applications where it can be applied without a 50mm ventilation gap.

https://www.nsai.ie/images/uploads/certification-agrement/IAB160389.pdf

2.4.2.4 New roof – LR Underlay

When installing POC 500 or PCC 2000 spray foam

insulations into a new build pitched roof with a taut

breathable LR underlay, the product can be directly

applied to the underside of the LR underlay

provided the natural drape of the underlay is

retained. Adequate ventilation must be provided

above the LR underlay through the provision of a

batten and counter batten or air permeable roof

finishes.

Lumen

Actually, I'm not even sure I understand that example, because "natural drape" and "taut" seem to contradict each other, but I'm not a roofer.

Tefral

Lumen That leaves the 50mm airgap too. If you read the cert in all other instances they say 50mm gap also.

Op here's a good thread. https://www.boards.ie/discussion/2058368235/spray-foam-insulation-a-good-idea-or-not/p1

Lumen

https://www.boards.ie/discussion/comment/123811937#Comment_123811937

OK, I'm not debating this to be difficult, but to demonstrate that the certs can be misinterpreted, since one of us must be wrong.

From the cert:

Adequate ventilation must be provided above the LR underlay through the provision of a batten and counter batten or air permeable roof finishes.

The "or" is important, and further up

Air permeable roof coverings, as defined in BS5250:2011+A1:2016, typically consist of natural slates, clay and concrete tiles whereas man made slates would be considered as an impermeable roof covering.

I interpret this to mean than if you have a taut breathable underlay below a natural slate/clay/concrete tile roof, you can spray an open-cell foam directly on to the underlay and not have a 50mm air gap above it.

Tefral

And I dont read it that way. You could do it that way if the airgap was above this.

paddylonglegs

thanks for the debate and the links lads,
looking further into it, my understanding is the controversy in the uk occurred when companies were using both closed cell and open cell insulations in domestic attic spaces.

Open cell is more suitable for domestic attics once there is air flow (the points debated above show air flow channel isn’t needed in some instances, but why risk it?)

A mix of bad installation by not using a vent card to help with air flow, plus using the unsuitable closed cell product, led to the issue of rotting timbers. However the Uk mortgage companies would not mortgage house no matter what product was used. There is a drive to change the policies to allow open cell to be an acceptable form of insulation in the right setting, once installed correctly.

I think this is a case of figuring out what method is suitable for your case, the best practices for it, and ensuring the company follows these. Even though we have a breathable membrane under our roof slates, I will still insist on a vent card with 50mm airflow gap - it’s what would have been done if we were putting in rigid kingspan.

We will be putting a 50mm insulated plasterboard infront of the rafters then, so that’s the next check to make sure there isn’t any issue with this.

GreeBo

https://www.boards.ie/discussion/comment/123804914#Comment_123804914

Isn't that what Gapotape was invented to resolve though? Our man Robin swears by it and it does seem like it will solve that issue, though it does rely on your timbers being installed somewhat correctly/square.

10-10-20

https://www.boards.ie/discussion/comment/123824243#Comment_123824243

It is, but it's only a patch-job (paper-thin pun) for poor product design by the rigid PIR board manufacturers. Clevett annoys me with his ease of precision and chipper can-do attitude. I barely have a single square rafter in my roof hence my bias. 😂

Lumen

https://www.boards.ie/discussion/comment/123824243#Comment_123824243

Robin would swear by chlamydia if it sponsored his content. Gapotape is insanely expensive.

GreeBo

https://www.boards.ie/discussion/comment/123824894#Comment_123824894

I think he benefits from working on his own rafters, so if they are not square then he only has himself to blame!

I dont know what other option the PIR manufactures have though?

https://www.boards.ie/discussion/comment/123825449#Comment_123825449

It is very expensive, but it certainly seems to work!

Lumen

https://www.boards.ie/discussion/comment/123827511#Comment_123827511

I dont know what other option the PIR manufactures have though?

Foam around the edges instead of squishy tape.

GreeBo

https://www.boards.ie/discussion/comment/123827523#Comment_123827523

Ah yeah, but I wouldnt say thats the design of the PIR board, more the process for installation. I don't know what the PIR manufacturers can do about wonky timbers. Its a perfect product in an imperfect world. The Gapo tape just add tolerance.