Radiant heat loss in crawl space

Tom Hopkins

Does anyone have any good info on how much radiant heat transfer there is from the subfloor bottom side to the ground in a crawl space? I'm getting ready to build a new ICF wall house with radiant floor heat. I'll be using Roth radiant panels on top of the subfloor so will have 1" XPS between the tubes and subfloor.

I just did a test that showed the bottom side temperature of the subfloor will be on the order of 72F for a floor temp of 85F. The crawl space will be sealed and perimeter insulated (ICF) and vented into the living space. Conduction and convection heat loss from the bottom side of the floor will not be an issue since the heat goes back into the living space. But I'm wondering about the radiant heat transfer to the ground.

In my test I found that a radiant receptor, a dark plastic tray, was about 5 degrees warmer when the subfloor bottom side was bare. Foil on the bottom side of the subfloor yielded a cooler garage floor and radiant receptor. According to my test results, and the radiation transfer formula, foil will reduce the radiant transfer on the order of 95%. But, am I reducing 95% of a 15,000 Btuh heat loss, or of 2000 Btuh heat loss? Realize that in this installation we're dealing with radiation of heat from the foil, not reflecting heat back into the floor, although some of that does take place.

The radiation formula calculates a heat transfer on the order of 7 Btuh/sq ft from floor to ground, but we all know real life doesn't always match calculations!! Hence my question at the beginning of this message.

The HVAC program I used to calculate the house heat and cooling load indicates the crawl space heat loss will be on the order of 2000 Btuh. I derived this by comparing the total heat load against a calculation with zero heat going into the crawl space. The program has an option for including an infinite resistance in walls, floors, roofs, etc. The program doesn't indicate what sort of heating system is used, which actually doesn't matter except for the floor to ground radiation I'm wondering about.