65×more resistant to radon migration than a 6 mil polyethylene membrane, at only 1.25 inches of closed-cell foam.

Radon is a colorless, odorless, radioactive gas that comes out of the ground and into a building through whatever gaps it can find: cracks in the slab, the joint between the slab and the foundation wall, sump openings, and every pipe or drain that passes through the floor. Western Pennsylvania sits in one of the higher-radon regions of the country, so the sub-slab and foundation interface is worth getting right the first time.

What the testing showed

Independent laboratory testing of Huntsman Heatlok HFO Pro and Heatlok HFO High Lift found that a 1.25 inch layer of the closed-cell foam is 65 times more resistant to radon migration than a 6 mil polyethylene sheet, the material most builders lay under a slab. The reason is not mysterious. Poly is a thin film with seams, tape and punctures. Closed-cell foam is a continuous, bonded, rigid membrane that seals itself to every surface it touches.

Slab–wall jointCracksSumpPenetrationsSlab–wall joint closed-cell foamcrushed stonesoil (radon source)
Where radon enters a basement, and where a continuous layer of closed-cell foam closes the path: under the slab on crushed stone, and up the foundation wall.

Why foam works where film does not

  • One application seals seams, penetrations and the wall-to-floor joint. There is no tape, no sealant, no cutting around pipes. The foam flows into the joint and cures there.
  • It can be sprayed directly on crushed stone. Heatlok's compressive strength lets it carry a slab; the same layer is the radon barrier and the under-slab insulation.
  • Continuous insulation with no joints. Every seam in a film is a future leak. A sprayed layer has none.
  • It exceeds code for under-slab insulation at approximately R-7.4 per inch (manufacturer initial value), and the assembly is ICC compliant.

Where it goes

LocationWhat we doWhy
Under a new slabSpray closed-cell on the compacted stone before the pourRadon barrier, vapor retarder and insulation in one layer; warm floors
Existing slab, new floorSpray the old slab, pour a new floor on the foamRetrofit option when head height allows
Foundation walls and rim joistClosed-cell from footing to sillSeals the slab–wall joint and the rim, the two leakiest lines in a basement
CrawlspaceFoam the walls to create an unvented, conditioned crawlStops ground moisture and soil gas from rising into the house
A barrier, not a mitigation system. Spray foam dramatically reduces the paths radon can take, but if your home already tests high, the fix is an active sub-slab depressurization system installed by a licensed radon mitigator. Foam makes that system work better by closing the leaks it would otherwise fight. Test first, then seal.

Closed-cell foam on basement walls is also listed by FEMA as an acceptable flood damage-resistant material and resists mold growth, which is why we treat the basement as one continuous envelope rather than a slab problem and a wall problem.