A little background on what I am trying to do. I have a hunting camp that I am converting to livable space. The building is built on posts. It is a large building 30'x60' so 1800 square feet but if we say it averages about 3 feet of height underneath (this is generous most of it is lower) and the average ceiling is 8' then the air volume underneath is only equivalent to about a 675 square foot building. The issue is the floors are freezing cold in there on cool or cold days.
I plan to skirt the building in 2" rigid foam with expanding foam to the sill and posts and tape on all seams I will try to bury it 2-3" on the bottom edge to keep that edge air tight. Then roofing metal over that to keep the chickens from pecking it all apart. This will help some I am sure but I still need some heat under there. The building faces South South West and gets great solar exposure. I am planning to build a solar hot air collector something in the realm of 60 square feet of collector area. At 50% efficiency I think I can expect about 9000BTU from this unit for say 5 hours a day. I will pump this directly under the building. Rather then just pump it under I was thinking of pumping it through a rock bin. I would build a box maybe 24 wide by 18" high not sure on length maybe 8' or so. this would be insulated on the bottom and walls and then filled with largish rocks say 6"-10" round. Then I would skim coat the top with cement or cob. It would have a 6" inlet on one end and a 6" outlet on the other end. This is where I would pump the air in I think there would be enough air space around the stones for the air to flow through while heating the stones. I was thinking doing it this way would allow the heat to be more even and last longer then if I just pipe in the air for the 5-6 hours a day of useable sun mid winter.
Anyone have any thoughts on if this will work as I expect it to. Or any input on sizing the rock bin, I don't know how to calculate how many BTU this bin could theoretically store or release. There is also the possibility I could use this same bin as a thermal mass for a rocket stove and run a stove pipe through it as well this would take some more engineering so perhaps Id just do a separate secondary bench for this. I could conceivably have the stove part outside and the bench under the building. Maybe build a small outdoor kitchen outside with the barrel as the stove top. I would like to see how much heat I get from just the solar first as it requires much less user input.
The skirt you propose doesn't sound very durable, but as long as you are okay with that, you can always beef it up if it starts to fail. Metal to protect it from weather and chickens would certainly help.
As long as you have reliable sun in winter, the solar collector, possibly feeding a rock bed, seems like it would help. If you can get the bottom edge of the collector lower than the crawlspace floor, I would favor a design that relied solely on convection and skip the rock bed to start with.
But the first thing to do in my opinion is to make sure you have as much insulation in the floor as possible. Spending effort and money on keeping heat from leaking down trumps spending effort and money on supplementing heat. If you have a semi-heated crawlspace with a soft enclosure, I can see animals finding it and making themselves at home.