I'm looking (without success) for a thin, impermeable sheet-like product for use in a below-grade exterior application. What I want is a material like fiber cement, which is very thin, fire-resistant is good, and hard, and hopefully impermeable. I'd like to apply it below grade like a very thin impermeable sheathing on a wood-framed below grade curtain wall. I don't want pressure treated plywood due to its environmental issues. Fiber cement seems perfect, but I just spoke with a Tech rep at James Hardie and he completely seems against the idea. The product is made, he says, of type 2 cement, silicon, and wood fibre, and absorbs water. But so? Concrete absorbs water too - and we just damp-proof it with an asphaltic spray and apply that dimpled HDPE membrane below grade. - Or not. Why not do the same with Fiber-cement? I can resort to sheet aluminum, perhaps 1/8" thick. This is certainly impermeable - but coefficient of thermal expansion is high - still maybe OK.
Next option - Fibreglass sheet - like those circuit board materials, only thicker. Concrete sheet is thin, of course, that is an option, but costly as well. Plastic Sheet - PVC is eww. But perhaps HDPE, or PEX sheet?