Foil barrier on underside of joists

I have seen some recommendations to put reflective foil barriers on the underside of floor joists to retain heat in the room above.
i could do this in my crawl space but I thought radiant heat barriers were only for the sun? How much radiant heat is there in a room heated by electric baseboard heaters? I thought it was mostly conductive and convective?

 

Hi qwerty, Radiant heat transfer is a much overlooked path that can indeed be used at room temperatures. In a crawlspace the gap eliminates the conduction path and any warm air will be pushed back up to the bottom of the floor above so no convection. The primary heat loss down there is radiant transfer. A foil covering at the bottom of the floor joists can be very effective with one addition, excellent air sealing in all of those joist cavities. Add good insulation and sealing to the end cavities and the effective r-value is impressive. In fact it boils down to the insulation value of the air which is pretty good. I like to recommend using a layer of foil faced rigid instead of a sheet of flexible foil plastic as being rigid helps reduce air movement as well as adding a bit of insulation value. Foil tape the seams also.

Bud