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