Insulating Radiant Heat Tubing in a Crawl Space

I haven’t been happy with the performance of my radiant heat system over my crawl space, so I’ve been spending some time re-doing the insulation job underneath.

The company I bought the radiant tubing system from recommended that I would only need to insulate my crawl space joists by simply stapling up sheets of thin reflective foil-bubble-foil insulation to the bottom of the joists, and then tape the seams between each sheet with some foil seam tape.

I did just that, but after a cold winter this year, I was not satisfied. My joists are 2x12s, and that left a lot of airspace between the tubing stapled to the subfloor, and the foil insulation stapled along the bottom of the joists. When I would go check the crawl space, the foil felt warm to the touch, but the crawl space was frigid, and it just seemed to me that a lot of heat was lost trying to warm up that airspace and the foil.

So now I’m going back and redoing each joist bay, and individually insulating it with both the foil insulation, and then regular fiberglass underneath.

My first task was to go back and re-do some of the stapling. My first time through I left the staples a little too far apart, and the tubing was starting to sag in between, so I went back and moved them closer (about 12 inches instead of 18). That made the tubing flush with the subfloor everywhere.


After that, I cut several 20″x48″ strips of the foil (the roll comes in a 48″ width), and began stapling it up about 1.5-2 inches below the tubing, in an upside down “U” shape. I got the 20″ measurement by using the tape measure to figure the approximate amount I needed for the U shape between the joists. I could have cut a couple of really long strips that spanned the entire length of the joist bay, but at 20″ I could have only gotten two strips, and would have had a wasted 8″ strip leftover.

I stapled one side first, and then pulled the other side taut to keep it flatter, with a lower arc, and away from directly touching the tubing.


I used an electric stapler to make things go quickly. Of course, the chincy PowerShot stapler I used had to get a jammed staple, which cracked the plastic track on the bottom, ruining the unit. After a trip to Lowes, 45 minutes later I was back in business with a really good stapler made by Arrow. I bought the Arrow model because the whole chamber and track where the staples go is all metal, and not subject to cracking.


After finishing the first piece, I started stapling up the next piece, slightly overlapping the original piece by 1/2-3/4 inches. I continued down the joist bay like this – it took about five sheets per bay.


When I was done with the foil, I went back and used a foil tape (like the type used on ductwork) to tape the seams.


Finally, I used rolls of R25 insulation (not batts) underneath, and hung it up with insulation hanger wires (much easier than stapling everything).

NOTE: If you are using Kraft (paper) faced rolls of insulation, make sure the paper side is facing up. In a crawl space, it would be a fire hazard to have the paper side exposed. I prefer the ComfortTherm rolls, because they are wrapped in plastic and less messy to work with.

Fun times. Right :)