Michelle, you might also consider stapling sauna foil to the area as well. Its craft paper backed and way cheaper than the aluminum coated poly vapour barrier material.

A huge amount of heat loss is radiant and neither fibreglass nor rigid foam insulation has any effect on radiant heat loss and gain. However, I wouldn't recommend it, if you cant' place it directly against the subfloor, as it will also act as a vapour barrier as well, which you don't want on the wrong side, ie. exterior of your insulation.