I'm planning to install a box-type (but smaller due to limited space) than Pauls in an older house with a very low foundation, with little crawl space. Estimating the weight of the thermal mass is hard in a 4' x 6' x 2' area, (still undecided as to cob or pea gravel) but I believe I can reinforce my floor joists with a couple of shimed railroad ties, & then overlay the floor with a 3/4" plywood sheet, which should help distribute the weight.
