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Bonzoso 12-10-2012 08:02 AM

Sealing XPS foam board around pellet stove wall thimbal
Hi again,

Another basement insulation question. I have a pellet stove in the basement that vents into a tile masonary chimney and was wondering if any special attention to sealing up the foam board around the wall thimbal is needed. Is great stuff o.k., does great stuff fire block add any better high temp. performance , or do I need a high temp. caulk. Pellet stoves, as you probably know, have much lower exhaust temps. than regular wood stoves. Thanks for your thoughts.

Windows on Wash 12-10-2012 08:09 AM

What type of flue vent it it?

I would never run foam directly to it regardless.

Always better to ere on the side of caution and if it were my home, give yourself 6" clearance and seal the connection with an intumescent caulking.

Bonzoso 12-10-2012 10:15 AM

It is a 3" double wall flue vent which is insulated with of about 3 inches of high temp fiber insulation around the circumference of the pipe with a thin guage steel case(the construction of the wall thimble) which is meant to pass through a typical wall. I'm sure there are some experts out there that have come across this issue for basements. Even exterior sheathing foam board installations should encounter this issue. Just want to finds out the best solution. Are there specific high temp foams to use. Thanks.

ineclispe 12-10-2012 06:05 PM

High temp foam may not be the answer. I called great stuff about their fire blocking foam last year and they told me that it was effective til something only like 230 degrees. a surprise to me, too.

Ultra High temp RTV silicon sealant is commercially available in caulking tubes like home depot and lowes, also at every one of your wood shed places. Its good to something like 600 degrees. If your still worried, you could go overkill with the 1200 degree copper top stuff found at your auto parts store.
But honestly, that stove pipe is code to 2" clearances. And that wall thimble multiplies that by something like 10 times. Regular old high temp RTV will do you great, but that system is designed to be bullet proof.

I'm a fire-fobe, and never saw a working wood stove system before I installed ours. We took every precaution, and I still slept with one eye open our whole first year. Since then, I have seen the most backwards, poorly planned, crazy chimney set ups from all over. Pipe running the wrong way, rusted and patched with fiberglass insulation stuffed and wrapped with HVAC aluminum tape. Horizontal runs through a wall and out past an eve that totals maybe 5'....
not that it makes a difference in your situation, but it has sure brought me comfort knowing that our properly installed double wall chimney, and thoroughly seasoned wood stack will keep us warm without worry.
Good luck, sounds like a great porject!

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