New answers tagged materials
There is a product called Thermo-Lite which is probably what you want. it's available in a sheet/board form and it contains a closed cell foam with fiberglass reinforcement. It's incredibly nasty to work with (fiberglass dust) and it isn't terribly strong, but it's leagues ahead of any type of foam insulation board that has been suggested. I work in the ...
Practically speaking, SIPs are about it. Sounds like you might be thinking of some of the hopeful but largely unrealized press releases for autoclaved concrete or foamcrete (two different products, both with VERY limited practical availability despite nice websites and articles) though the "dimensional lumber sizing" aspect does not fit with that.
Landscape fabric because even if you was to put wood which would rot. Once sand was put in the weight would keep you from being able to move. Landscape fabric always easy to get and replace. Here is a great design to keep critters out and let the rain drain thru. Jae
Lots of things are better insulators than wood. Most of them aren't strong enough to be structural, or not better enough at insulating to bother, since generally people insulate with insulation materials that are designed to be good at that. If the thing you're describing existed, people would almost certainly build houses out of it instead of wood, unless ...
Use a tarp with lots of tiny holes poked through it. I used a meat tenderizer to poke the holes through. I simply cut it to size and stapled it to the bottom. Works great :)
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