Under-Slab Vapor Barriers

[kbs2244]

A nice 1 page article with a full page PIC
By a builder spec writer

https://www.probuilder.com/quality-matters-under-slab-vapor-barriers-done-right

 

[schurtjl]

Good link. Reinforced that I made the right call on mine. Had a well known local concrete guy give me a bid where they would put the vapor barrier down first, then put rock over the top of that, then concrete. He didn’t want to do it the other way. I ended up doing it myself by placing 12” of compacted rock, then a 10 mil vapor barrier, taped at all seams, rebar on 18” centers criss-crossed, then 6” of concrete (concrete poured and finished by a different contractor). Turned out great, and no moisture issues.

 

[matt_i]

I agree although I feel certain the cost gets very high if a contractor is paid to install the vpaor barrier plus reinforcing steel on chairs and wire-tie it.

But, I'll represent those steps as easy labor anyone can do...just hire the contractor to place + finish + saw cut the surface.

 

[ConCretin]

Some good basic information there. Thanks for posting. One thing I'd add is that polyethylene sheeting is usually described as a vapor retarder as it will let some water vapor through. For a true vapor barrier, you'd need to step up to a product such as Stego.

Since this can get expensive, it's fair to ask if you really need a vapor barrier. The primary purpose of a vapor barrier is to block moisture that could interfere with the adhesion of floor coverings such as epoxy. If you aren't going apply a floor covering, you probably don't need a high end vapor barrier.

Another consideration is damage during placement. A finishing crew can be hard on a vapor barrier and it won't work as well if it's full of holes. A 15 mil product like Stego will hold up much better under the abuse.

Finally, if you live in a very dry or arid area, you might not need a vapor barrier at all. If your soils contains a lot of moisture, you might want to consider a layer of stone to act as a capillary break against liquid water as well as a vapor barrier.

Is there a downside to a vapor barrier other than cost? Well yes there is. By blocking excess mix water in the concrete from migrating downward into the soil, it goes up in the form of bleed water, which can present a range of surface problems. If you install a vapor barrier, it's vital to control mix water. Vapor barriers can also contribute to curling although this usually isn't a problem in modestly sized garage slabs.

 

[Bretny]

Home depot plastic or tar paper is not concrete vapor barrier. Get the real stuff.

I have a basement that had a dirt floor and had a river running through it. I put in drainage, stone then vapor barrier and poured a floor. Huge huge difference in humidity.

 

[PNWguy]

I'm surprised they left out insulation placement.

I'm putting 2" of XPS under my slab, and was assuming it would go on top of the vapor barrier. If the barrier is in direct contact with the slab, then the foam can't be.

What's the correct way to do it?

 

[ConCretin]

I assume the statement in the article about the vapor barrier being 'in direct contact with the slab' refers to placing a blotter course of sand over the vapor barrier to protect it. This was the recommendation at one time but since then the thinking has changed to placing it on top of the base i.e. in direct contact with the slab. It's a little misleading and doesn't take insulation into account.

A vapor barrier does not have to be in contact with the slab to work properly. I prefer to put the vapor barrier down first followed by the insulation. This protects the vapor barrier and if you are doing radiant heat, it allows you to staple the pex to the insulation without poking a thousand holes in it.

 

[PNWguy]

I assume the statement in the article about the vapor barrier being 'in direct contact with the slab' refers to placing a blotter course of sand over the vapor barrier to protect it. This was the recommendation at one time but since then the thinking has changed to placing it on top of the base i.e. in direct contact with the slab. It's a little misleading and doesn't take insulation into account.

A vapor barrier does not have to be in contact with the slab to work properly. I prefer to put the vapor barrier down first followed by the insulation. This protects the vapor barrier and if you are doing radiant heat, it allows you to staple the pex to the insulation without poking a thousand holes in it.

Once again, thank you for your input.

Can you recommend a product or method for stapling the pex? I was thinking of a 12" mesh and 6,000 zip ties.

 

[ConCretin]

Can you recommend a product or method for stapling the pex? I was thinking of a 12" mesh and 6,000 zip ties.

I had a heating company install my pex. They used a device that drove plastic staples into the insulation to secure the pex. It worked great but I don't know how practical it would by for a diy job.

The challenge with zip tying the pex to wire mesh - other than the time involved - is positioning the wire. Too low and the wire is less effective at crack restraint, too high and you expose the pex to potential damage from anchors, saw cuts, etc.

I suspect a search of the heating sub forum would yield a lot of good ideas. Good luck with your project.

 

[like2wheel]

I just went thru this (pour was tuesday) & the zip tie thing wasn't that bad. My concrete guy put lightweight 6x6 mesh over my foam, my son & I zip tied about 1300' (260ish x 5) of pex to the wire. Didn't really take that long, went faster than I planned. Then he brought in the real mesh & set it up over the pex.

I paid a package price, so I don't know how much the bottom wire added to the cost, but i dont think it was substantial. But i got the added benefit of 2 layers of steel (even though one is at the bottom, in my mind it has some benefit), & got my pex lifted slightly higher into the slab. More likely to be surrounded by concrete when it's not fastened directly to the foam.

I think the cost might be comparable to staples & renting the stapler.

 

[tricountytrail]

X 2

 

[joes169]

I had a heating company install my pex. They used a device that drove plastic staples into the insulation to secure the pex. It worked great but I don't know how practical it would by for a diy job.

The challenge with zip tying the pex to wire mesh - other than the time involved - is positioning the wire. Too low and the wire is less effective at crack restraint, too high and you expose the pex to potential damage from anchors, saw cuts, etc.

I suspect a search of the heating sub forum would yield a lot of good ideas. Good luck with your project.

I agree with you sentiments on stapled pex vs. attached to WWM.

I'd say 98% of concrete over pex that we've poured in the last decade or so is stapled to foam, which is a far superior approach IMPO. Mesh always has some memory, pex is safest being low while mesh is best being suspended in the middle, and installing 2 layers of WWM is completely wasteful IMO. I can't think of one scenario where cable tied pex to mesh beats staple-to-foam pex, unless you like wasting money and resources, or perhaps you enjoy playing "Russian Roulette" when sawing control joints......

 

[joes169]

I think the cost might be comparable to staples & renting the stapler.

It's not even remotely close, even if you would buy the stapler and throw it away after the job. Currently, wire mesh costs more than it ever has before, plastic staples are cheap. The labor to staple vs. cable ties is probably half, as well. It's extremely rare to see anyone cable tie pex here in the last 20 years......