Actual fire safety of polyiso vs. plywood
I’m purchasing a 1920 house with kneewall attics and I’m making plans to insulate them better. In my city, retrofitting insulation does not require a permit or code compliance, so I am free to do whatever suits my needs.
I’m not yet sure whether I want to add to the (decades old, barely adequate) kneewall and attic floor insulation, or start over with insulating the underside of the sloped roof, but either way, I’m looking to save money and mostly use fiberglass, furring out the framing, and doing my best to air seal on all 6 sides, maybe using an inch of foil-faced polyiso as the interior air barrier.
My concern is that the building code normally requires a thermal or ignition barrier over rigid foam, and even though I can ignore code, I want my home to be safe, so the code requirements are influential to my decision making.
So I’ve been looking into the actual fire safety characteristics of polyiso, and I’m confused. All of the subjective descriptions (and videos) I can find about polyiso show that it does not ignite easily and develops a layer of char that limits further burning. The data says that it has a low flame spread and developed smoke ratings. Lower than plywood. But the code says to cover it with plywood for fire safety?
How does this make any sense? Is the code perhaps simplified to lump all foams together? Is polyiso’s fire safety underrated?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Polyiso is better than eps or xps, in part because polyiso tends to char in place while the other two tend to melt and drip burning blobs of themselves onto stuff below them.
If the attic space you’re insulating is not accessible, you probably don’t need a fire/ignition barrier there. I would put the polyiso on the attic side of the wall, so that it will double as a rear-side air barrier for the fiberglass batts you plan to use in the walls. I’d consider blown cellulose on the attic floors, which is even cheaper than fiberglass and also performs better.
Regarding the codes, I tend to agree with you about polyiso, but you still need to follow the building codes. There are polyiso variants that have been tested and have a faced that is rated to be left exposed (Dow thermax is one example), but they tend to be pretty expensive compared to “regular” polyiso.
Bill
When I read kneewall and 1920s my guess is this is a story and a half building and likely very drafty. Before you insulate you need to air seal the building! If my guess is correct air sealing will be next to impossible. I hate spray foam and conditioned attics but an old half story building is a desperately bad situation and desperate measures are the only real option.
Consider filling the rafter spaces with closed cell spray foam. The spray foam will air seal the building while packing the most R value in the very limited space you have. Note this will bring the space behind the kneewalls into the conditioned space you should remove any old insulation so those spaces stay at about the same temp and humidity as the rest of your home.
Walta
You refer to thermal barriers and ignition barriers together and the function is different. I don't know much about these things, but ignition barriers are to keep surfaces from burning while thermal barriers isolate concealed spaces from habitable spaces.
If you are concerned about fire safety then you should verify fire-blocking of all the horizontal to vertical concealed spaces. those knee-walls are really tedious to fire-block and original construction methods probably don't provide blocking to modern standards. If you are putting up continuous insulation, then the blocking becomes more critical since you are creating more concealed areas.
I think I may have found the answer. Regardless of how easily polyiso burns, it still releases hydrogen cyanide gas when it does burn, which is one of the most dangerous substances in smoke. It can quickly incapacitate people, even in small quantities.
Plywood and drywall aren't sounding so bad now.
Pretty much all plastic-y materials release nasty stuff when they burn. The biggest difference with polyiso is that it is much less likely to SPREAD the fire, because it tends not to release melty blobs of burning material during a fire, it chars in place instead. Polyiso is also harder to get started burning. The code doesn't make any distinction though -- they ALL need protection if they aren't rated to be left exposed.
Note that it won't hurt to put up one of the approved barriers (1/2" drywall, plywood, or 1/4" hardboard, even steel sheet of a minimum gauge that I can't remember because I've never done it that way) though, regardless of if it's needed or not. Sometimes you're allowed to use an intumescent paint or even flam retardant primer (check with your local building department people).
Bill
Thermax brand polyiso is the only foam I'm aware of that is tested to be safe to leave exposed in non-habitable spaces.
If you're putting any effort into new insulation I strongly recommend insulating the slopes instead of the kneewalls. Kneewall/kneespace floor insulation is nearly impossible to get right and stay right long-term.
The site is being wonky yet again. I posted the response above.
I believe hunter xci panels have a code report allowing exposed in non habitable spaces as well. I can't seem to find it right now but I'm pretty sure it has a NFPA 286 cert
You are correct: https://www.hunterpanels.com/wp-content/uploads/2023/05/286-TDS-Web-File-Version-5.8.24.pdf. I keep forgetting about them. I take issue with their claim of "essentially zero GWP" but I take issue with pretty much every foam manufacturer when it comes to their environmental claims.
When did we start having anonymous postings that are unnumbered????
Not that I disagree with the advice.
Walta
Walta,
Tech-support is the one real weakness of GBA. I have been waiting for about six weeks now for my Expert Commenter tag to re-appear.
That's been happening sometimes when I post. When I posted, it actually said the site does not exist, and I couldn't get back in until now.
For a while I was reporting spam and glitches to Kiley, who fixes what she can, but it's clear that AIM has little interest in keeping the site functioning properly, so I'm not bothering anymore.
Edit: another frequent glitch: I was responding to Walta but it ended up here. When I try to delete it and add it again as a response in the correct place, it says it's a duplicate post.
Edit #2: I just noticed that my "mystery" post time stamp is 2.5 hours from now, or 3+ hours from when I posted. Maybe the site just doesn't like posts from the future?
I have relatively frequently found issues with the site loading slowly, if at all. I think it might be hacking attempts or DDoS attacks in those cases, but those are guesses on my part. I haven't had issues myself with posting since back when there were problems getting images to post some time ago.
Bill
I think it has more to do with the site adding an absurd amount of cookies. Whenever I clear my cache it is substantially faster. I have started accessing via browsers that have easy cash clearance buttons (duck duck go) or else being diligent about clearing it myself. its a bit annoying as I have to constantly reselect preferences and login, but its alot faster