Our experience in CO with a 10' × 14' room we added, 2×4 frame on a deck set on piers. STD paperbacked fiberglass insulation in walls, 1,5 inch foam board and 1/2 inch chip board under floor, pad and rug inside. STD roof insulation. Only air leak e as a light fixture which was not caulked. Even doing all that, nothing could keep it warm until we closed in the "crawl space". Used left over galvenized roofing bermed with dirt. Mainly heat migrates from the main house. But, a coil of black 4 inch irrigation piping on a south exposure exhausts into that space underneath. We screened the ends.
Seems heating the underside helps the most.