RHM- Drywall, clearance and insulation

Hello, first of all, I love this site!!! I've been reading just about everything about rocket mass heaters but there a few questions that I have that are more specific to my place.

1. I will be building the RMH in a pole barn that has been finished off on the inside. About 600 square feet. Is 6" pipe good this size of living quarters? Also I am having trouble finding used chimney pipe for the RMH. I am quite surprised that we can't go to junk yards anymore and find treasures! I've searched Craigslist, junkyards (no one sells to the public), restores, habitat for humanity, etc but having no luck. Should the pipe throughout the whole system be the black stove pipe? The size of the bench is 11' by 5' 1/2". If my calculations are correct, I'll need at 33 feet of pipe for the bench which doesn't even include the vertical exhaust pipe (the ceiling is 10' and the attic is 5' tall). Am I correct in thinking that this will cost several hundred dollars?

2. I will be building this bench and RHM along an exterior wall which is finished with drywall but my question is, do I need to put anything between the drywall and the bench? I've been told that I could put up concrete board and fill in with vermiculite between the board and the drywall but concrete wicks moisture which is bad for cob.

3. The clearance between the RMH and the wall should be at least 4" correct?

4. I want to use insulate firebrick, do I need to add any further insulation for the heat riser?

I think that is it for now.

Thanks in advance!!

Diane

Hi Diane;   Lets see,  #1) 6" or 8"  I think 6" would work fine, I would build it 8".  You will only need one piece of black stove pipe leaving your transition area, then you can switch to cheap thin wall HVAC pipe sold in 5' lengths (the thin silver pipe) use one more piece of black pipe when you pop up out of your mass (for abrasion)and more hvac up to the roof jack. Depending on your location / roof / owner / renter you will need a proper roof jack and maybe one piece of metalbestos pipe to go thru your roof. Do NOT skimp on this spot!  Another option is using plain clay bricks as your horizontal tunnel or even length wise split barrels laid end to end work as a burn tunnel.  Brick bells may be built in line with or in place of your horizontal piping.  Lots of choices not all cost huge bags of money.  #2)   Yes use concrete board against your cob , (its only 1/2" thick) then 4" gap to the drywall. Filling with perlite would be optional. If you don't, it will just collect dust and debris.  #3) 4" to an outside wall , cob could touch sheet rock away from the core...your barrel however should be more.  #4) an insulated firebrick, clay mortared riser should not need any further insulation.  Oh and if you don't already have it...buying the builders guide is highly recommended.

It is by code not legal to reuse chimney piping (not sure this is nationwide), so you may not find a business that will resell it. I have seen plenty on craigslist, though.