Detailing Wood Stove Chimney to Minimize Air Leakage
I’m currently building a house to passive house standards.
My (hopeful) plan is to have a wood stove, that is its own, self contained air system. Sealed front, sealed air supply from the outside.
The benefit of a wood stove is it would be a backup heat/cooking source in the winter, if the grid went offline and the solar couldn’t keep up. Plus, nice to have a cosmetic fire sometimes.
However, I’m having trouble with the chimney– it seems you have to have a 2″ air gap around the chimney pipe, which seems like a MASSIVE air leak– so much so as to be a deal breaker for the entire wood stove concept.
Any tips as to how to have a wood stove metal chimney, without a huge air penetration?
Thanks!
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Replies
Simple--it's a code requirement to install sheet metal to cover the 2" gap, so just make the sheet metal airtight using sealants, on both the interior and exterior of the thermal control layer. You'll still lose heat through conduction and radiation but not by convection, which has a bigger effect. You may still have trouble getting Passive House certification but real-world performance will be very good.
They make red silicone “fire caulk” (note that this is NOT the same as the red intumescent “fire stop” caulk) for this kind of thing. Put in the metal thimble as Michael mentioned, then seal it with the red silicone caulk made for high temperature applications and you should be good.
Bill
I have zero expertise in this area, but are there any approved insulations that can be used to insulate the attic side of the thimble (i.e. rock wool)?
Mineral wool should be acceptable, as it's noncombustible and accepted for use as a fire blocking material when packed into gaps. You can also get high temperature fiberglass wraps that are usually used for exhaust systems on large engines that will work, but neither material is going to give you an air seal since both are air permeable. Those two materials would give you thermal insulation though, but I'm not sure it would help much with the thermally conductive metal flue passing through the thimble.
Bill
Mineral wool is essentially fireproof but it also heats up itself, and will eventually pass the chimney's heat to the framing. Insulation slows heat flow, it doesn't stop it. No insulation should touch a chimney at any point.
This is one of many reasons that indoor combustion is not easily compatible with high-performance construction.