Problem of rigid foam on attic floor
dwilliams123
| Posted in General Questions on
Putting some lengths of rigid polyiso between some attic joists, to insulate the ceiling of the rooms below, would work well in a few places in my attic, but there seems to be little written about it that I’ve seen. Is that because it would need an ignition barrier, one that was part of an assembly? You couldn’t just seal 1/4 ply over each piece of polyiso, right?
I have some polyiso panels on hand rated for exposed use on walls/ceilings, and the manufacturer tells me they think this application would be fine, but I don’t really see how. Am I missing something?
Thanks.
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Replies
I would try to make blown-in cellulose work myself if at all possible. Lots less work.
You can use an ignition barrier (I think you need 1/4” hardboard though, I’m not sure 1/4” plywood will meet code) if the attic space is not used for storage and doesn’t “communicate with” (have connected air passages) to areas that are used for storage. Check with your local inspectors if you’re unsure. Storage areas need a thermal barrier which is generally drywall. You could also use Dow Thermax which is rated to be left exposed.
I don’t know why you would have problem with the polyiso in terms of it being your insulation though. All the usual “use enough to meet code” and “airseal everything” would apply, and I’d expect air sealing would probably end up being more work with rigid board if cut to fit between rafters.
Now if you’re intent is to use the polyiso panels AS the ceiling, that might be a problem. The polyiso, even if it’s rated to be exposed, won’t be listed as a fire barrier to seperate spaces in a building. Drywall is. Polyiso isn’t considered a structural element in any way either. The fire barrier and structural strength issues are the only issues I can think of that you may have.
Bill
Dave,
It sounds like you want to use the cut-and-cobble approach for an attic floor. You can do that, I suppose, but as Bill explained, it's a lot more work than cellulose.
As long as there is a drywall ceiling or a plaster ceiling below the rigid foam, and a vented attic above the foam, there is no need for an ignition barrier or a thermal barrier in the attic. (The drywall ceiling is the thermal barrier.)
For more on the cut-and-cobble approach, see "Cut-and-Cobble Insulation."
Thanks for the replies. I guess I was extrapolating from the need for spray foam in an attic to have an ignition barrier. I'm still confused why the same need for an ignition barrier would not apply to the floor. Or would it apply only if the polyiso was not rated for exposure?
The ceiling below is plasterboard.
Dave,
If you turn the attic into conditioned space -- in other words, a potential bedroom or home office -- you need a thermal barrier between the foam insulation and the conditioned attic. (That's the situation with spray foam on the underside of the roof sheathing.) If it's a vented unconditioned attic, however, the thermal barrier goes on the side of the foam facing the conditioned space -- so your drywall ceiling is the thermal barrier in that case (cut-and-cobble between the floor joists).
I'm still confused though. I thought the need for ignition barrier extended to even places like accessible crawlspaces, which presumably would be unconditioned. For example, this link
https://www.energyvanguard.com/blog/72240/Does-Your-Spray-Foam-Insulation-Need-a-Thermal-or-Ignition-Barrier
Seems to say that an unconditioned attic still needs an ignition barrier. Is there some reason I'm missing why this would not apply to the floor of the attic? Thanks.
Dave,
1. Code requirements for a thermal barrier differ from code requirements for an ignition barrier.
2. If you suspect that (a) the code is confusingly written, and (b) every code enforcement official interprets the code idiosyncratically and inconsistently, you would be correct.
3. If you want to learn more about code requirements, see this article: "Thermal Barriers and Ignition Barriers for Spray Foam."
But to quote from that article:
"If your attic or crawl space meets the requirements mentioned above — if it’s only accessed for repairs or maintenance; there is no easy access to the space; it isn’t used for storage; it has no floor — then you can get away with installing an ignition barrier instead of a thermal barrier."
It still seems to me that cut-and-cobble foam panels need an ignition barrier when placed on an attic floor. Am I misreading?
If the attic is inaccessible you don’t need a barrier. Inaccessible means you need a saw to get in, there is no access hatch or door.
My read of the code would be you’d need an ignition barrier.
Martin is entirely correct with his comment #2. Code is interpreted differently by different inspectors and in different areas. Sometimes there are more stringent local codes too (the classic example is seismic bracing codes in California). As a consulting engineer, I get all the excitement of arguing code details with inspectors frequently. Your best option is to ask the local building department people in YOUR city, ideally the actual inspector that will inspect YOUR project, to find out what they want in your specific area. That’s the safest option.
Bill
I live in a semi-rural area, and the AHJ here is very hands-off. No permit or inspection is required for this, and when I've called the inspector in the past about nuanced issues like this, he pretty much tells me not to worry about it.
I'm just trying to figure out if there's a violation here that would affect safety, or have insurance/resale repercussions.
I guess I am missing something.
From an economic point of view, foam insulation is the most expensive form of insulation R/$=.
Foam is a good choice where you need to get a lot of R value in a small space R/inches=.
Generally on the floor of an attic we have lots of room for thick insulation, so something cheap and fluffy goes there.
From a green point of view, recycled paper is a much greener product than any foam.
If there is something different about your house that makes foam the best choice for you, please help us understand.
Walta
Dave,
Q. "It still seems to me that cut-and-cobble foam panels need an ignition barrier when placed on an attic floor. Am I misreading?"
A. If there is an access hatch that allows access to your attic, I would agree, although most code inspectors probably wouldn't care. In any case, 1.5 inch of mineral fiber insulation (fiberglass or mineral wool) is an ignition barrier -- relatively cheap and easy.
But this whole mental exercise is a little odd, as Walter correctly notes. Most people just blow cellulose or fiberglass on the attic floor, which is fast and relatively cheap. Why cut-and-cobble?
According to ROCKWOOL® :
"ROCKWOOL COMFORTBATT® at 5½” [140 mm] are approved and certified for use as thermal barriers over foamed plastic insulation in Canada meeting the requirements of CAN/ULC - S101 and CAN/ULC - S124. "
https://cdn01.rockwool.com/siteassets/o2-rockwool/documentation/technical-bulletins/residential/thermal-barrier-technical-bulletin.pdf?f=20180618130807