The attached section is taken from Building Science's site, and shows details for a conditioned crawlspace for a house built in a very cold climate. Please provide tips and techniques, material recommendations, etc., for the vapor barrier on the gravel floor fill, its taped joints, and the fastening of the turned-up edge to the foam insulation that is applied to the inside of walls. This looks less expensive than a rat slab, and we want to get it right.

Fire rated rigid board that is also vapor semi-permeable (a necessity for the below grade installations in the lower 48, but not in perma-frost territory) is a bit hard to find. Some R20 fiber-reinforced closed cell iso used in roofing might meet the fire rating specs though. Otherwise, count on installing a thermal-barrier on the interior side of the wall insulation.

Under no circumstances should you use fiber insulation for the band joist/sill in a cold/very-cold climate. Closed cell spray foam would be best. (In lless-cold climes fiber insulation can be used there, provided it has a good air-barrier on the interior side. Without the air barrier the R-value is severely compromised, and the convective infiltration through the fiber will condense on the band joist all winter and create mold/rot problems. Rigid foam board cut to fit and foam sealed to prevent air movement works there too.