TL;DR: Looking for a material that won't off gas at high temps. Temps you would find 2" from the top of a barrel stove. I would be breathing the air, don't want to die or get sick!
I'm building an outdoor wood fired forced hot air furnace. Picture a woodstove in a doghouse. The woodstove heats the air in the doghouse. There are 6" insulated inlet and outlet ducts that go to a basement window that will be fashioned with some type of flange. There's a 6" duct fan inside the basement that pushes air from the basement through the duct into the bottom of the doghouse, moves up around the woodstove, and returns to the house via the 6" duct at the top of the dog house.
I'm building the doghouse out of metal studs and using rockwool insulation to keep the heat in. It's basically a 3'x3'x3' cube. The woodstove is a barrel stove. It protrudes out the front of the doghouse just a tad to ensure there's no mixing of conditioned air and combustion gasses when I start/load the stove.
I planned on using some old duct stack as the inner facing of the metal stud walls. Then I realized that the duct work was galvanized. My research says galvanized over 400deg F will off gas zinc fumes.
I basically need 5 sheets of material that are 30"x38" for the sides and the roof. I had one piece of sheet metal that I used for the floor already. I have at least 4" of clearance from the stove to the inside of the doghouse around the bottom and sides. I accidentally measure my uprights studs a bit short and only have about 2" of clearance at the roof.
I'm in Connecticut, zone 6a.
My question to you all is what material can I use to construct this doghouse?
I'm looking for something cheap and readily available that won't off gas at high temps. Things I've thought of are:
-Aluminum flashing. Is it coated? Can I burn/wash off the coating?
-Durock covered with aluminum foil or flashing?
-Can I used the galvanized duct? I've heard of soaking it in vinegar to remove the zinc but the pieces would be too big to soak; I don't have a container large enough.
-Just leave the rockwool bare? no walls? maybe filter the heated air coming back into the house for tiny rockwool fibers that might fly off?
Any input is much appreciated! Feel free to ask any questions!
I'm working on a similar concept -- a movable wood furnace in an insulated shack, so I can heat outbuildings or my basement in a pinch to prevent pipes freezing.
Personally, I wonder if your "doghouse" is way too small, and that's why the offgassing issue becomes a problem. My plans were for a 6x6x6 so I would have a nice big reservoir of heated air, with the potential of adding bricks as thermal mass once it's moved into place.
It's a project for this winter, but I was thinking metal studs and rockwool just like yours. I want it to be non-flammable in case it needs to sit beside the house. Same idea, stove door outside to avoid smoke getting inside. I think I will try it with rockwool only at first; I don't think it sheds as much fiber as fiberglass batting (horrible stuff).