That's good in theory, a few inches will give you R-18 but doesn't really solve it.
Breaking the thermal bridging is important and for good reason is why code now requires rigid under cantilevers. 2-3 inches of R5/inch rigid board under an R-31/R40 batt is a better choice.
On new construction the best method is a warm airspace between the insulation and the subfloor.
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