Flammability concerns of rigid foam for interior insulation
We’re in the middle of a kitchen/dining/living room renovation for our 80s ranch home in Massachusetts and are a bit stuck on insulation. We are avoiding closed cell spray foam because we have children with significant environmental allergies.
The inspector has told us in order to meet the energy code requirements we can either use ccSPF in the 2×4 walls, or we can put an R-15 mineral wool batt in there then cover it on the interior with 1/2″ of rigid foam board and put the drywall over that.
Originally we settled on EPS both for the least environmental impact and no offgassing, but our insulation contractor is having a hard time sourcing 1/2″ EPS.
We are now looking at the foamular NGX XPS foam because it is readily available in 1/2″ thickness. I originally wanted to stay away from XPS because of potential offgassing and the large environmental impact, but it appears foamular NGX is greenguard gold certified for low/no VOCs and uses a new blowing agent that has far less environmental impact.
My only concern with XPS is the apparent flammability of the material. I know this will be covered by drywall which will provide the 15 minute thermal barrier required, but we are still a bit hesitant. Are we overly worried about the flammability aspect or should we consider other options?
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Replies
jpkad,
It depends a bit on what you are worried about. Fully involved structural fires are harder to fight and more likely to result in the total loss of the house when it includes significant amounts of foam insulation, but I haven't seen any data that suggests that the occupants face a greater risk of injury or death in those houses.
I guess what I'm worried about is greater risk of fire in general due to having something that is more combustible than the other materials in the wall. It seems that's only a concern if a fire somehow started in the wall? Starting in the home and getting into the walls would require it burns through the drywall which I could compensate for by using the type X like Andy suggested below.
For example, the oven will be on an outside wall that will require rigid foam. In a scenario like this if the oven overheated or caught fire, it would have to burn through the drywall in order to ignite the foam.
jpkad,
The first thing that jumps out at me when I look at fire related statistics is how safe newly built houses are. The threats are overwhelmingly related to occupant behavior inside the dwelling, not the structure itself.
This is a useful link where you can explore the various causes and chances of fire by type. https://www.nfpa.org/News-and-Research/Data-research-and-tools/US-Fire-Problem/Home-Cooking-Fires
Once a fire is inside the walls the level of damage is massive no matter what materials are there. If you're worried about the fire starting in the room and moving through the drywall, you could use 5/8 type X which has a higher fire rating which is why that's required in attached garages.
I tend to agree that if the fire is already *inside* your wall, you're going to have a major fire regardless of the flamability of individual materials used within the assembly. Mineral wool is going to do a lot to limit the spread of fire here though. The issues you sometimes hear about with XPS/EPS and big fires is when the foamboard is right against an air gap that can allow for flame spread. With the foamboard tightly sandwiched between drywall and mineral wool, both pretty fire safe materials, your potential for fire spread is very much reduced.
I would go with 5/8" type X drywall here, which gives you a better fire barrier, but also makes for a much more solid feeling (and quiet!) wall assembly. I only rarely use 1/2" drywall for anything. I would also consider 1/2" polyiso instead of EPS or XPS. Polyiso will give you a bit more R per inch, and if you use fiber faced polyiso, you don't have a vapor barrier to deal with which may make for a safer assembly depending on what you have on the exterior. Polyiso is also the least fire risk of any of the three commonly available types of rigid foam.
ccSPF in a 2x4 wall is only going to get around R18 at best in the cavities, and that's with a perfect installation. In reality, you're unlikely to get much more than the R15 the mineral wool batts would provide, so I would stay away from ccSPF in the wall for that reason. I'm surprised your inspector is pushing this though on a less than whole-home renovation, I would think you'd be grandfathered. You will get better thermal performance with the layer of rigid foam, so the inspector's request/requirement is not without merit, I'm just surprised they'd be forcing the issue in this case.
Bill
+1 on polyiso. I would go for foil faced instead of fiber, in colder climate the vapor barrier behind drywall is fine and it is much easier to tape for a solid air barrier. Polyiso is also higher density than most stock foams and less squishy.
In areas where you are looking to hang cabinets, you might want to put a horizontal strip of 1/2" ply. This can also be taped to iso for air barrier continuity.
Thanks Bill, that is reassuring.
As for the assembly of the wall, we've got hardie siding affixed to OSB, then are planning on mineral wool in the wall cavity and the 1/2 of foam as required by the inspector.
We had ruled out polyiso because of potential offgassing since we have children who have some sensitive environmental allergies. We've been trying to choose materials for the renovation that are as inert as possible. This is why we were looking at the greenguard gold certified XPS.
My understanding is that polyiso has cured and mostly offgassed by the time it gets to your home, but there is still some offgassing that happens for a while after it is installed. I know there is the ability to source polyiso that has been taken out of other buildings which has already offgassed, however, my wife is a little concerned about reusing something and not knowing where it comes from (was it a factory, a place where someone smoked heavily, etc).
The inspector is being a bit of a stickler, requiring R-60 in the attic or roof which I believe the code just changed over to this January and there is no concurrency period. Because we have made part of the ceiling a cathedral, we can only get R-38 or so with 2x8 rafters and dense blown in mineral wool so he is saying we need to put the layer of foam on the 2x4 walls to make up for the lack of R-60 in some places in the attic/roof. With the 1/2" of foam it passes the rescheck for the renovated part of the house.
Just about everything will offgas a little, that's why you get that "new smell" from new stuff :-) Polyiso is pretty inert as things go, with only EPS likely to be a bit better. In my own sniff tests (very scientific ;-), new EPS has more of a smell to it than new polyiso does.
Reclaimed polyiso is the fancy name for the used stuff. It usually comes from roof assemblies of commerical buildings where it was sandwiched between a steel roof deck and the protective surface of the roof itself (usually some kind of membrane). This means the polyiso itself is actually pretty well protected from anything that might have been going on in the building below, such as heavy smoking or some kind of industrial process. I've never found the used stuff to really have much of a smell to it, although I usually use factory seconds and not reclaimed stuff (just because of what is available to me). Factory seconds are like "slightly irregular" and sold more cheaply, but there is usually nothing visibly wrong with the panels at all.
You could possibly challenge that inspector about this. Usually you are only required to bring the entire structure up to current code if you are changing more than a certain percentage of the overall structure, or modifying the foundation. This all depends on the muncipality though. Note that "challenging the inspector" can open a big can of worms though, so if you can accomdate the inspector's requests without signifcant cost or redesign, then that is likely to be your easiest option here.
Bill
Once again, super helpful, thank you. I get the feeling that if we challenge the inspector we are going to open a big can of worms.
If we do foam that is not polyiso with the facer, I am guessing we will need some sort of vapor membrane? It's been suggested we need something like Certainteed Membrain, but I'm not sure if that's supposed to go between the drywall and the foam, or between the foam and the studs.