You want the final underfloor foil based poly-iso to have the foil side facing down, not up.
Heat flows from hot to cold. Your crawl space is never going to be warmer than your main floor. In winter the main floor will be much warmer.
The effective R value for a downward facing radiant barrier towards an open space in such an application is R16 by itself (excludes the poly insulation value). You have to tape the seams with foil, and shore up the ends.
The reason they want the air seal, besides the obvious air leakage, is for moisture issues. Same reason the local wants vapor permeability, as they assume you can't fully seal every single nook so there always needs to be a way for the structure to dry, and most flooring and sub flooring combined is not vapor permeable.
I'm going to violate that rule too by using a foil. The only way to make it vapor permeable cost effectively is by poking small holes in it, which also makes it open to air. There might be a product out there that laminates something to a foil sheet that is vapor permeable, haven't seen it and would likely cost a fortune.
I'll have to pay attention and watch for any leaks, and if I do have them, not just assume all will be ok, but to dig in the floor and make sure.
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