Air sealing attic: materials and fire safety
I’m a homeowner thinking to do some DIY air-sealing in the attic. I was preparing to use caulk on narrow gaps (top plates), and spray foam on larger gaps (penetrations for cables, plumbing vent stack, etc). Not being a pro, I was going to just stay away from anything thermally hot, like the furnace vent.
Then I noticed that many spray foams meant for the homeowner have surprisingly low ignition temperatures – generally at 240 deg F. Even the orange fireblock stuff is down at 240 F, for many brands. This ignition temperature is lower than many other materials in the attic, which means the foam may be the first thing to ignite up there. And seeing some videos online, many common DIY foams can propagate a flame pretty well, too.
So I started second-guessing my plan. I got a quote from a pro for the job, and they use a foam with an ignition temperature around 450 F, so that seems better at least. On the other hand, the estimator said they’d use this foam around the furnace vent, too. Is that allowed?
I’d like to err on the side of fire safety, so any advice on what materials to use? Should I use caulk where possible, instead of foam? Getting trickier, per residential codes (this is a townhome with adjacent neighbors) do I need thermal or ignition barriers around the foam? If I did, that would seem to negate the convenience of using spray foam in the first place. There will be fiberglass insulation over these areas.
To be clear, I am not talking about fires that occur during or shortly after application due to incorrect application techniques.
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The maximum air temperature at the hot air plenum of a furnace is about 170°F. If you have a high-efficiency furnace, the temperature will be considerably lower. By the time this air gets to a remote register, it will be lower still.
Spray foam installed against a register boot in your attic is not going to start a fire.
That said, your question raises a few points. The most obvious point is that you don't want ducts in your attic unless you have a sealed, conditioned attic -- in which case there is no need to seal air leaks in the attic floor.
For more information on ducts in the attic, see Keeping Ducts Indoors.
If you are intrigued by the idea of transforming your unconditioned attic into a conditioned attic, you may want to read Creating a Conditioned Attic.
If you can't afford the cost to create a conditioned attic, you may want to proceed with your air sealing plan. If you do, you may want to read this article: Air Sealing an Attic.
Once you've completed your air sealing work, you may want to seal duct seams or install duct insulation. When all that work is done, pile on the cellulose. If you have a layer of cellulose on top of any exposed spray foam, the cellulose will act as an ignition barrier, reducing any fire danger.
I took "...furnace vent...." to mean the exhaust venting of the burner, not the ducts. The type of exhaust vent and it's ratings make a great deal of difference as to whether it can be in contact with combustible foam (or even fiber insulation.
What type of vent are we talking about here?
Good point. Chimneys and flues are air sealed with metal flashing; the seams in the metal flashing (and the seams between the flashing and the flue or the chimney) need to be sealed with high-temperature silicone caulk.
Spray foam insulation should never come in contact with a chimney or flue.
It's probably be fine to air-seal around CPVC vents on most condensing furnaces with a flextible "windows & doors" foam (to deal with thermal expansion/contraction) but not metal exhaust venting.
Some polypropylene vent piping is rated for 0" clearance to combustibles even with exhaust temps as high as 230F, but if your stack temps are that high it might be pushing your luck to use can-foam.
Silicone caulk & sheet metal is still the better choice, even with low temp condensing equipment.
Thanks for the responses - it really helps me. Sorry I was ambiguous - I did mean the line for the exhaust/flue gases from the gas burner. If I decide to use a contractor, I will request metal flashing and high temp caulk around there, and not spray foam.
For other, narrow, gaps, like around top plates - is there any particular type of caulk I should use - silicone, latex, the red firecaulk - does it much matter? I'm comfortable using any of these.
Also, the article at https://www.greenbuildingadvisor.com/blogs/dept/musings/air-sealing-attic mentions using caulk around the knockouts and penetrations of electrical boxes. I'm not sure I could do that well. Instead of caulk, can I wrap a firestop putty pad around the electrical boxes in the attic instead? Anybody have experience with that, or objections to it?
Q. "For other, narrow, gaps, like around top plates - is there any particular type of caulk I should use - silicone, latex, the red firecaulk - does it much matter?"
A. It doesn't really matter too much. Either silicone or polyurethane caulk is likely to be more durable than cheap latex painter's caulk.
Q. "Instead of caulk, can I wrap a firestop putty pad around the electrical boxes in the attic instead?"
A. I've never done that, but it sounds like it would work fine. Once you understand the basic principle -- seal the air leaks in a manner that is durable and fire-safe -- you can implement the principle in a variety of ways.