What is the best way to seal a French drain board due to high levels of radon?

Question

The Radon level in my crawl space is above the threshold (~5.5) and the level in my basement is around 4. I'd like to install a radon mitigation fan that sucks air from the sump pump of a French drain system.

The problem is that the French drain has a black plastic drain board at the edge between the new concrete and the cinder block wall. It's a few inches tall. I was told I should seal that because the radon mitigation system's fan would basically just suck air through those gaps and wouldn't do much sucking from the soil. Furthermore, a powerful fan would create some backdraft and I may get a CO problem.

It's a pity to have to seal that thing, but looks like I'd have to do it. What is the best thing to do?

I have some ideas but they are probably not great:

1. duck tape the drain board
2. spray foam
3. polyurethane spray
4. cement

By the way, I'm finishing the basement and putting some rigid foam insulation (1 or 2 inch) on the wall so I'd have to take that into consideration too.

Answer 1

If the French drain is between your basement slab and the exterior walls, it may be the source of most of your radon. Sounds like a small suction from the drain sump to the outside is a good plan. A small fan with its own exhaust pipe shouldn't be a problem for the water heater or furnace but a CO detector is never bad for closed rooms with gas appliances.

Answer 2

If you seal the french drain, it will cease to drain. Whatever issue was the trigger for its existence may return to haunt you again.

Instead, if you just seal your sump and install an external radon mitigation fan on the opposite corner of your house, you may not need to seal the french drain after all.

All that being said, I'd still consult with a radon mitigation professional and get their advice before doing anything permanent.