Vapor barrier for concrete shop floor

[Ned Otter]

Hi folks,

Wondering if anyone has any recommendations for a vapor barrier to be used on top of the concrete floor in my shop.

Thanks in advance --

Ned

[Jamie Buxton]

Are you thinking you’ll walk directly on this vapor barrier? Or are you planning on putting it underneath some additional floor - for instance wood?

[Ned Otter]

Hi Jamie - thanks for your reply --

I'll be putitng the vapor barrier underneath additional flooring (wood).

[Greg Funk]

My recollection is we used 6mil poly on top of concrete followed by 3/8 & 5/8 plywood and then 3/4" hardwood. No problems in 20 yrs.

[Paul F Franklin]

Usual recommendation is 6 mil (at least) poly, with seams overlapped at least 6 inches, and seams taped with 3m construction tape or equal. But if you're installing commercial flooring, they will specify a method.

[roger wiegand]

Do you already have a vapor barrier under the slab? Modern energy-efficient construction will incorporate that as part of or along with a foam insulation layer under the concrete. Creating sandwiches that might trap moisture is not generally a good idea, though not so bad in concrete as in other materials.

Do you currently have moisture coming up from the floor? If you tape down plastic over an area you can check for condensation.

[Steve Rozmiarek]

Is the slab above or below grade?

[Ned Otter]

Roger - "under the slab" is a bunch of rocks, so no, there isn't a vapor barrier underneath. The slab is quit thin, less than 1". This construction looks to be late 19th century, circa 1870. I will verify if moisture is coming up with plastic, but I'd understood that it's a good idea to put down a vapor barrier if you are planning on putting wooden flooring above it, i.e. 2x4 laid flat, with plywood on top. This structure would not be tethered to the concrete, it would simply float.

[Ned Otter]

Steve - the slab is above grade - a very thin layer of concrete over stones. It's a late-19th century shed.

[Steve Rozmiarek]

Steve - the slab is above grade - a very thin layer of concrete over stones. It's a late-19th century shed.

A vapor barrier over a concrete slab under flooring will serve no purpose. It may actually make things worse if water can get in from above the floor. Plastic or paper underlayment under a wood floor quickly gets poked full of holes by the flooring moving around, especially over something as abrasive as concrete. It then ceases to serve as a vapor barrier. The standard reason to put a paper or plastic vapor barrier under wood plank flooring is for squeaks. Under plywood, if the concrete is not emitting vapor as Roger asked, you can skip it. It is cheap though, so for peace of mind, do what you want.

When I come up on a situation like this, I just put 1" foam sheets on the concrete and 3/4" TG subfloor glued down with PL300. Stagger the seams. No sleepers needed, and the foam board will conform to the little bumps in the concrete.

[Ned Otter]

Thanks very much, Steve - what you propose would be a lot less work than what I had intended. The only thing I'd have to make sure of is being able to get foam that can handle the desired compression strength.

Some of my equipment will weigh 1,500 lbs

[roger wiegand]

Steve's suggestion is a good one. Standard big box pink foam is good for either 100 or 200 pounds/sq in, you'd be hard pressed to exceed that with subfloor and flooring above to distribute the point sources of weight. You can get foam thats rated up to 1000, but that is way overkill.

[Eric Arnsdorff]

I put 1” foam board then 3/4” tongue and groove osb (the kind treated for some moisture exposure) on my basement concrete floor then carpeted it. I used the 2” foam board on the poured concrete walls with sleeper strips to screw the drywall to. We did have a good drainage system put in when we built the house as well. It’s been over 20 years and it hasn’t had any issue at all as well as being a very comfortable flooring.
However, my shop is part of the same basement which is day lighted. I have the raw concrete floor for my shop floor. I have no moisture issues in this portion of the basement either. I suspect the good drainage with the sprayed outer rubberized coating on the walls is likely the key to the low moisture. But the foam board with osb is definitely a good option for a concrete floor.

[Ned Otter]

Roger, thanks for your reply -- from my research into foam board, you have to subtract one digit/divide by 10 to get the compression per square inch. For example Foamular 1000 supports 100psi, 600 supports 60psi, etc.

The highest rating I've seen at the big box stores is 250, which would be 25psi, which seems kind of low. I've searched far and wide, and near me (lower Ulster County in NY) there is no supply of the higher psi product. Perhaps one could special order it, which will be my next research.

And to be clear, my floor will only have 3/4" ply on top, no additional flooring.

[Charlie Velasquez]

Roger, thanks for your reply -- from my research into foam board, you have to subtract one digit/divide by 100 to get the compression per square inch. For example Foamular 1000 supports 100psi, 600 supports 60psi, etc.

The highest rating I've seen at the big box stores is 250, which would be 25psi, which seems kind of low. I've searched far and wide, and near me (lower Ulster County in NY) there is no supply of the higher psi product. Perhaps one could special order it, which will be my next research.

And to be clear, my floor will only have 3/4" ply on top, no additional flooring.

Our Menards can special order the Foamular 600, ask your local big box. But even the 25psi stuff is pretty stout. Your house with the refrigerator or freezer on the kitchen floor is only speced at a live load of 40 pounds per square FOOT, so 25 psi is about 100 times more support than a house. You can spread the force by going with a stiffer sheet, like advantech osb, and a thicker osb sheet- say 1" or 1.25" if you have concerns.

PS. With that weight I would not do plywood, too much chance of an interior void and fir ply is too soft.
I would use osb. The various makers each have their own special waterproof type osb, but Advantech is the only product I would use over concrete.