So I have been reading up on insulating beneath concrete foundations and radiant heating floors lately, and the general concepts are straight forward enough. In Vermont it seems to me that I'd want R-10 under the entire house with the perimeter bumping up to R-20 or so. I've got some ideas about using straw bale above grade to insulate the back burial of the house, but I think the best under grade insulation to use is Foamular250. I would be open to suggestions of alternatives that could stand the weight of tire walls however.
My question after all this is.... Would it be a "better" insulating value to use a single layer of 2" foam taped at the seams, or two 1" layers of foam both taped and the seams offset to the opposite layer?
I'm torn myself because I think the 2" would withstand weight compression "better", but there would be a more complete heatflow barrier with the two 1" layers (unless they compress so much their R-value shrinks)
I say go with two layers overlapped to seal up the seams. I don't think you need to worry much about compression with Foamular. It's really made for applications like that and can hold up fine under pressure. I definitely wouldn't put it under a tire wall though, unless it's part of a specially engineered concrete footer or something.
As for insulating the slab, that's a hotly debated topic. The folks in Taos recommended against it for my build, rightly pointing out that it would cut off a huge part of the thermal battery. I live in a very wet location though, so I'm considering using this stuff under the sub-floor: http://www.thebarrier.com
How high is the water table at your build site? As long as you're more than 5 feet above year round, it may be more effective to spend that money on extra drainage to keep the thermal wrap dry (french drain around the whole perimeter). Definitely stick with at least R-20 XPS for the thermal wrap.
I can't say anythingn about R-values or materials, but I can say with certainty that warm floors are amazingly awesome! It's the one thing I truly miss about the home my xh built for our family. Insulating the area around your home will obviously be a good thing, but I don't see why you can't have both. Our floors were above a 4-foot "crawl" space (we never could understand why anyone building a house would make a space under their house where you literally had to crawl!) and not in a poured concrete floor, but I think I'd want it all the more because of that.
Insulating under radiant floors is practically mandatory for slabs or earthen floors on grade. I cant believe Iam making two closed cell spray foam recommendations back to back but I guess its destiny. I just think its the best way to insulate in these locations from my experience in my area. Its pricey for sure but so is properly detailed rigid foam and you still need to seal the edges to the walls. This is where spray foam really shines and it could be more cost effective to use taped foam in the field and spray foam the edges. Speaking of edges, always insulate floor edges as they tend to bleed more energy than below.
"If you want to save the environment, build a city worth living in." - Wendell Berry