I'm a builder, and I generally build on sealed, conditioned crawl spaces. The ideal solution is professionally installed spray urethane, sealed with a sprayed fire rated coating. It's expensive, and the only local guy who was remotely competititve appears to have gone under recently. I now use 1-1/2" Thermax attached to the walls with Tapcons and big fender washers. The idiot code nazis in this township will not let me use it on the band joist, since in their tiny little minds, the band is part of the wall assembly and needs to be done to the same standards, which means R-21batts. Thermax in now locally available at some lumber yards. 1-1/2" R10 lists for $48 a sheet and I pay about $40. Other builders in the area use 2'x4' Dow XPS board, then slop a coat of fire rated paint on it. The job looks like shit, and absolutely fails to meet the coating manufacturer's specs. but it's all the code idiots need to see to satisfy their tiny little brains, so it's now SOP in this area. Thermax can be detailed, on edges and seams, with the shiny silver metal duct tape, and it makes a really professional looking job. I just spoke to an outside sales guy from my local lumberyard. He is now stocking a fire rated thermax with a white pebble finish interior skin. It's a slick way to do a basement, and it isn't any more expensive than the foil faced stuff.