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Basement: vapor barriers and moisture question

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roam

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Jun 1, 2023
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Hey, all!
Been wondering about positing here, since this is a Garage forum, but folks seem to know a whole lot about floors here.

I am trying to figure out the logic of vapor barriers and how they interact with moisture. I am thinking of doing LVP for the floor, but first putting down a vapor barrier for some thermal break and moisture control. I did a sodium chloride test and came up with roughly ~4lbs of moisture per 1000 sq ft per 24 hours. There's one problem area with ~15, but I think I am going to leave that uncovered by the vapor barrier, as it's outside the main area I actually care about. This issue I will have to address some day in the future.
To the main point. So I have this moisture transmission, right? If I put a vapor barrier down, what actually happens to the water? It's still going to wick through the slab, and it will still condense underneath the vapor barrier, right? Is it going to reach some kind of equilibrium? I am having a hard time wrapping my head around the moisture that is now blocked from simply evaporating.

Side-question: would epoxy be OK with this level of moisture?

Cheers and thank you!
 
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billconner

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What climate are you in (mostly heating or mostly cooling, and humid or dry? Is the space conditioned now? Will it be conditioned after flooring? What were you thinking of using for a vapor barrier?
 
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R

roam

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Good questions! North-East, split half-n-half cooling / heating. The space is not air-conditioned. The house is basically on a swamp: there are partially wetlands out back. I have a dehumidifier down there as humidity does go up pretty high without it. It stays a pretty much consistent 65-70F though throughout the year, as it's a good 6+ feet below grade.

The space will not be air-conditioned after due to the aforementioned stable temp. Ultimate goal for it is to serve as a playroom, gym, storage, and a pantry. It's about a 1000 sq ft in all, so plenty of room to fill it in.

For vapor barrier, I was thinking of using this stuff: https://www.flooringunderlayment.com/product/natural-lvt-foam-underlayment/
I contacted the manufacturer and they warranty it up to 8 lbs / 1000 sq ft / 24 hours, which would work well enough for the non-trouble areas (most of it). The trouble area I could put some black 20 mil down, or I might just leave it exposed altogether.

I did grind the entire basement when we first bought the house, thinking of epoxying. I then got worried, reading how epoxy would not stand up to slab moisture in the basement. That option is still definitely on the table.
 
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ConCretin

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There are really two issues here. The first is moisture entering finished spaces in general and the second is whether you can apply an adhered floor system such as epoxy.

You could block moisture intrusion into your finished space with a membrane on top of the slab and cover it with a floating floor system. This might create condensation issues underneath but that's probably better than the alternative. The second option would be a topically applied vapor barrier to the slab surface, which would block the moisture within the slab. Either would help protect your interior spaces and finishes.

The second issue is an adhered flooring system such as epoxy. This is pretty risky If your slab doesn't have a vapor barrier underneath and you have moisture in your soil. The aforementioned chemical vapor barrier might be an option but if moisture finds to way to the surface, it could ruin the bond and cause your floor to come up. I'd be very reluctant to apply an adhered floor system without a series of moisture tests over a long period of varying climatic conditions.
 
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R

roam

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LLWillysfan said:
There are really two issues here. The first is moisture entering finished spaces in general and the second is whether you can apply an adhered floor system such as epoxy.

You could block moisture intrusion into your finished space with a membrane on top of the slab and cover it with a floating floor system. This might create condensation issues underneath but that's probably better than the alternative. The second option would be a topically applied vapor barrier to the slab surface, which would block the moisture within the slab. Either would help protect your interior spaces and finishes.

The second issue is an adhered flooring system such as epoxy. This is pretty risky If your slab doesn't have a vapor barrier underneath and you have moisture in your soil. The aforementioned chemical vapor barrier might be an option but if moisture finds to way to the surface, it could ruin the bond and cause your floor to come up. I'd be very reluctant to apply an adhered floor system without a series of moisture tests over a long period of varying climatic conditions.
Click to expand...

> This is pretty risky If your slab doesn't have a vapor barrier underneath and you have moisture in your soil.

So from what I can tell, based on house plans, there's 20 mil underneath the slab. But maybe it's fallen apart or has otherwise failed, I do not know. I am not sure why the moisture would be as high as it is, if the 20 mil was intact. Wouldn't it be about zero with a thick sheet of plastic underneath? Though, clearly, moisture can be much much higher - as I mentioned, it's 4x higher in one, visibly damp, area. The ground is heavy clay and drainage is very poor. There's drain tile at the footer under the slab that funnels to a sump pump, and there's basically always some water running.

What defines a properly applied chemical vapor barrier? Layer thickness and cure time? I have the same question in general though about this kind of vapor barrier: where does the moisture go that would have normally come up?
 
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ConCretin

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A 20 mil vapor barrier would be a pretty high end product. Most residential slabs are lucky to get a 6 or 10 mil poly membrane, which are usually poked full of holes during construction. Commercial projects often get a specialized 15 mil product such as Stegowrap. If your slab really has a 20 mil product, you should have no issues whatsoever.

I'm not an expert on chemical vapor barriers but they are basically a surface applied product that blocks water vapor from penetrating the slab surface keeping the moisture within the slab where it won't do any harm.
 
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R

roam

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billconner said:
Well, I trust LLW, but would think your proposed underlayment would work.

No personal experience but could you consider Dricore?
Click to expand...
Stuff looks pretty rad! Pricey though. This did lead me down the rabbit hole, finally arriving at my favorite Canadian: RenoVisionDIY, where he goes over some underlayment ideas.

Looks like he argues that you only need dimples or channels for water management, not necessarily vapor management, where (according to him) a basic foam board that is also a vapor barrier does great for the price. He did mention this stuff that seems to be the cheapest of the "channeled" systems: DMX 1-step. It's 77c/sq ft, so roughly double of the grey stuff I was considering. Though again, I do not seem to need water mgmt, as much as vapor. Still, all this dimpled noise keeps talking about how you'll get mold if there isn't air-flow. Seems like I am going in a circle a bit.

EDIT: Changed to a link
 
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A

Armorpoxy

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Hi, here is a product we have recommend for many, many years for people to use where there may be moisture under our www.supratile.com PVC tiles which are also impervious like VCT. It works great, is inexpensive, and ZERO issues in about 20 years. Spray this down and you should be just fine in our opinion. You can buy this product online direclty from them for $49.99/gal.

 
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