Insulation, Radiant and Vapor Barriers - Insulating below a radiant heated floor

Have recently had radient heating installed under an existing living room floor. It is of the type where heat is supplied by water lines attached to metal plates tacked to the underside of the floor. Between the joists they used R19 fiberglass insulation with the paper vapor barrier facing up. Below is an unfinished unheated concrete basement with 3 exterior walls - it gets very cold in the basement during the winter. I want to ensure that the heating is all going up into the floor and not down into the basement which must be happenning now. I have thought that putting something below the insulation that would minimize any air movement would help. It could be attached to the bottom of the floor joists. There are no other pipes or electrical that must be accessed.

What would you suggest I use? Plastic sheeting? Foam panels? Sheetrock? Another layer of fiberglass insulation held up by stapling the paper to the joists (ther is room)? Any other ideas on how to minimize heat loss into the basement? Of course low cost and easy install is best.

You may not have as big a problem as you think.

If your basement is cold, that means you are not losing much heat to the basement. Keep in mind that in the winter, the soil (and basement) are warmer than the outside air.

You can add more, but any fiberglass will have the same air movement problem you now are concerned about.

If you put up plastic, slit it so you do not have two vapor barriers.

If you go to extremes and are in a northern climate you could end up with frost problems (heaving) since the footings are not below the frost line - it happens on unheated structures frequebtly.

Dick