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Slab edge detail: how to effectively seal for radon around 2″ XPS

ohioandy | Posted in Green Building Techniques on

About to break ground on a house in northern Ohio. Ground Zero for radon. Architect specifies basement slab run clear to foundation wall, leaving only 1/2″ expansion gap presumably inserting bitumen sealer material of some sort. He says I can’t insulate the slab edge because that would allow radon to seep up.

But I’d like to have my 2″ XPS sub-slab insulation turn the corner and continue up the wall as part of my foundation wall insulation. I don’t want to have that gap in the XPS that he proposes; it seems like too much heat bridging.

I figure a 2×4 of XPS can be placed around the slab perimeter, serving also as a screed for leveling the floor.

The question is: how do I detail the slab edge, so that the radon cannot penetrate upwards? Is it as easy as extending the 4-mil vapor barrier up and over that 2×4 XPS screed, then securing it to the wall behind the XPS installed later? Advice appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    Whether you choose to leave a 1/2-inch gap (filled with asphaltic board) or a 2-inch gap (filled with vertical XPS) at the perimeter of your basement slab, the crack still has to be sealed. Usually the crack is sealed with silicone caulk. If you end of finishing your basement wall on the interior, you can establish an air barrier at the interior drywall (or cement board, if your prefer to avoid the use of drywall in your basement), and you can caulk the crack between the concrete slab and the drywall.

  2. emd360 | | #2

    I know this question is a bit old but I have the same issue. The XPS on the slab perimeter is extended all the way to the top of the slab on our remodel slab. This was a recommendation that I thought made sense instead of cutting the XPS at a 45 degree angle and breaking the insulation barrier at the edges of the slab. Some people will frame out the area for a double wall and seal at the perimeter of that wall, as in the answer above, but that is unfeasible in my remodel project. There are some products I am investigating to seal over the gap. The most promising right now is a liquid rubber material that is supposed to seal out radon emissions as well as waterproof the area. It is made by Sani Tred.
    If someone else finds this issue, perhaps there will be more information about options for covering the perimeter XPS if it is allowed to extend to the top of the slab. In my case, the underslab vapor barrier does extend up over the edge of the XPS as the original poster suggested. So perhaps a caulk adhesive that glued that to the wall would be a possible solution too. Then use laminate or tile for flooring over the edge? I'm still trying to figure out the best solution for this edge. I have a radon sub slab mitigation system installed but the edges of the slab should be sealed too.

  3. cdgatti | | #3

    Resurrecting this old thread, as I am facing the same detail as Ellen described several years ago. I have 2" xps at the slab perimeter, cut off level with the top of the slab. It is part of my underslab and perimeter slab insulation. Building inspector was concerned about Radon entry at this perimeter. The Sani-tred product mentioned here looks to be a good solution to seal over the gap, spanning from the slab - over the 2" xps edge - to the foundation wall. Has anyone done this and what were your results? I should mention that a 4" sub slab pipe was installed for venting the soil gases though the roof.

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