Vapor & Radon Barrier for a 10×16 Floating Slab – What’s necessary?
Hi all,
I had a slab poured in my backyard for a shed that I’m finishing. I did not have have insight to have the concrete contractor put in a vapor barrier.
As I’ve been working on the inside over the past few months, I’ve notice that humidity is significantly higher than outside, so I’m thinking some type of vapor barrier prior to LVP flooring is a good idea for a number of reasons.
The 10×16 slab is ‘semi-floating’: the outside 12″ perimeter has a 12″ deep footer, and the remainder of the slab is 6″ deep. Since it’s installed on a slope, one side has 2″ exposed at the top, and the other 8″ exposed. There is no chance for actual water entering from the sides: the problem appears to be limited to vapor.
What do you folks believe is necessary to control humidity? I would imagine there is little to no hydrostatic pressure. And is radon a concern?
The options I’ve been weighing are Roll-Cote, RadonSeal, and simply 6mm poly (but I’m concerned about trapping moisture).
Thanks,
Steve Williams
GBA Detail Library
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Replies
You can find the EPA's radon maps at https://www.epa.gov/radon/epa-maps-radon-zones-and-supporting-documents-state. Zones 1 and 2 are the areas with greatest radon potential. Mail-in test kits will confirm if you have radon or not, and whether whatever radon mitigation approach you take is effective. How concerned you should be depends on how much time anyone will spend inside the shed. Will it just be used for storage, or as a workshop or a kids playhouse?
If you aren't going to be insulating and heating your shed, then keeping it well-vented will help a lot. Water vapor is lighter than air so it rises, while radon is heavier and tends to settle near the floor. Having some vent openings high up and some low down will allow both to dissipate.
You can check to see if moisture is coming through the slab by taping a roughly 2' square of clear polyethylene plastic down on the concrete, sealing it at the entire perimeter, and leaving it in place for a few days. If can see water droplets on the bottom side of the plastic or if the concrete appears darker then there is moisture coming up from below.
You don't have to worry about poly trapping moisture. That's what it's there for, and neither the concrete nor the poly will be damaged by water.
Hi Frank,
Looks like I'm in zone 2 (Southern NJ). The shed is going to be used daily as a music rehearsal space, so exposure is a concern. There will be extensive insulation and a mini-split, along with an ERV to ventilate.
I'll try the clear poly method to see if I get condensation, but I think the slab would be the only source since the shed is already airtight from extensive sealing / soundproofing.
So no concern about trapping moisture? What about the sill - just tape it well at the edges?
Thanks