Radon and French drains
rhl_
| Posted in General Questions on
Suppose you install a French drain in your basement, and it drains to a sump outside your house, underground but in something accessible from grade. Since air can enter the French drain, is this enough to give an exit for radon?
I can easily throw a tee on the pipe and have air exit the drain, I’m just wondering if that is necessary.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Ryan,
Radon is a gas, and it can go wherever other gases can go. That said, it may be hard to predict which way it will go. In most houses, the chimney effect keeps the basement slightly depressurized compared to outdoors. This means that any radon in the soil will be collected by the French drains and vented to the interior, driven by outdoor air pressure.
This is why most active radon systems include a fan that depressurizes the French drains to below the pressure of the basement, so the natural airflow is from the basement and soil towards the drain/vent system.
The only way to know for sure is to run a radon test once everything is finished and see the result. If you've already got French drains and you get a high radon test level, it is relatively simple to install an active vent system to draw the radon out of the drains so that it doesn't evolve into the basement.
Seal the slab as much as you can first, making it as gas-tight as possible.
To move air through the French drain requires depressurizing it on the far end of where it daylights or has free access to outdoor air. A simple 4" PVC stack rising 10' above grade can do a lot, but higher is better. A wind operated venturi type vent cap such as the Empire SV04A can enhance that.
Thanks. To be clear, radon testing when i bought the house did not indicate there was really any radon (i will look up the results of this more precisely), im not sure how "insane" we have to be here. I also may have an old fireplace flue that is lined which I can use to vent radon. its in the right place, farthest from daylight, but, new mechanicals are in the way.
If i can't make that work ill probably just leave the thing unvented, rather than puncture a 4" whole in the side of the house.