Contractor forgot vapor barrier 2

[jimtheengineer10]

I received an email from a client saying the contractor forgot to put the vapor barrier under the concrete slab in their crawl space. They wanted to know if they should seal the concrete floor with something. Any thoughts on this?

 

[Archie264]

Take a quick look at this:

http://buildingscience.com/documents/insights/bsi-003-concrete-floor-problems

That said, if it's just a crawl space I think I'd just leave it alone and make sure the area is properly ventilated.

 

[Teguci]

"Yes, but California is different." and "Real structural engineers are now involved in slab design – the adults are now in the game—..."

so much fun! Great link Archie264

 

[BUGGAR]

The link lost credibility when I learned they were selling services. Sorry.

 

[Archie264]

Then by all means, ignore his free advice: put sand between your vapor barrier and slab. That’ll show him.

I’m not affiliated with him and have never used his services but he usually makes a rather compelling case for his positions.

I learned of him from this site and didn’t give him a fair shake at first because I misunderstood what he was saying and I thought his style was too abrasive. I gave him a second chance, though, and wound up learning a lot.

 

[jimtheengineer10]

Do others agree with Archie264 that because it is a crawl space just make sure it is well ventilated?

 

[XR250]

I agree. The concrete will provide some vapor retardation anyway. It could be coated with something possibly to enhance the effect.

 

[TehMightyEngineer]

I'd say leave it as is with good ventilation and only attack it if problems arise.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[canwesteng]

What floor covering in the crawl space is the moisture going to wreck?

 

[msquared48]

If this is just a mud slab, so what? (adequate venting assumed)

Mike McCann, PE, SE (WA)

 

[BAretired]

I agree that vapor barrier is not essential in this case. I have come across one or two contractors who refuse to use vapor barrier even when specified under a grade slab.

ACI had an article a few years ago suggesting that vapor barrier is not required under grade slabs, so I tried it once and had little pop-outs throughout the vinyl tile over the entire floor. I didn't try that again. Live and learn.

BA

 

[dhengr]

BUGGAR:
I’d read Joe Lstiburek’s article twice, and give it some serious thought while doing the reading, so you got the full meaning and thinking behind it. Joe’s a pretty damn smart fellow on these types of matters. He’s spent most of his life thinking about them, and doing some publishing and consulting on them too. The fact that he’s a Consulting Engineer on building envelope matters should not detract from (but rather add to) the value of his thinking on these issues. We need more real thinking engineers like him, and we would have a lot less water and moisture problems with our bldgs.

Canwesteng:
Excess moister in the crawl space sure may raise hell on the building itself and on the floor sheathing and joists in particular. As long as you have no objection to mold and mushrooms in the crawl space, all is well.

Many times, venting alone is not a very good solution, since you have moist outside air moving through a slightly cooler space than the outside temps and you can end up with a higher relative humidity in the crawl space and with condensation on surfaces down there. Sometimes venting is enough, if you don’t have a high perched water table or other water issues. The latest thinking is drain tile, outside and around the footings; water proof, not just damp proof the found. walls; vapor barrier under the bsmt./crawl spa. slab; all tied together to keep water and moisture out. And finally, to condition the crawl spa./bsmt. with some air from the house moving through and out, taking any moisture with it. These can all be adjusted depending upon the local soils, temps, RH, and rainfall, but don’t just blow them off.

Jimtheengineer10:
Since the contractor is the one who shot the bull, let him fix it, it was shown on the plans he was working from right or guarantee that there will never be a problem if he leaves it as is. Have him condition the crawl spa. and pay for some years of the system operation. That may be less costly for him than tearing it out and doing it right, if other things indicate that this may be a reasonable solution. Treatment from the inside may not be very effective. Moisture and vapor pressures can be pretty high and just push through any inner treatment.

 

[boo1]

Could you place a paint on the barrier on top of the slab?

 

[JAE]

The crawlspace must be vented by code.

Any vapor in there should be kept from the inside of the building by the building's own vapor barrier system which should be placed on the bottom of the floor structure above the crawlspace.



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[msquared48]

Interesting thought, but the presence of a vapor barrier, which a concrete slab still is in a limited sense, could also limit the presence of radon gas as well as humidity.

Mike McCann, PE, SE (WA)

 

[JAE]

With a vented crawlspace and a sealed structure underside the radon isn't a problem then is it?

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[msquared48]

No more than the moisture...

Mike McCann, PE, SE (WA)

 

[dhengr]

Boo1:
If the floor slab has any vapor or moisture drive to the inside, it will more than likely just cause pealing and blistering of the paint on the top (inside surface) of the slab. As mentioned above the vapor pressure is quite high in this process, and it just tends to push the paint of the concrete. Slabs on grade, on well draining sand, with a well drained foundation and building site, usually don’t suffer many problems. But, if you dig a found. for a crawl spa. or a bsmt. and it acts something akin to a bath tub, in holding water for a period of time, becuase of poor draining soils, lack of drain tile, lack of found. water proofing, poor site grading and drainage, you can pretty well expect some moisture problems in that lower spa.

 

[BAretired]

I don't think a vapor barrier can be relied upon to keep out serious amounts of water. If the water table rises above the level of the slab, water will find its way in through joints around the edge of the slab and if the water pressure gets large enough, the slab will fail in bending.

BA

 

[darthsoilsguy2]

a crawl space slab sounds like a nice add. i spent so much time in my own crawl space in my last house dealing with ants, hvac, and insulation issues... and would have liked that slab a lot. if i had someone pour a slab, i would have been pretty upset if a vapor barrier hadn't been set. i would be stomping my feet and pushing the contractor to give me a barrier with a 2" topping slab. i might not get it, but i wouldn't be thinking about all that ventilation. just because the wind has humidity too doesn't make it all right. Slab moisture is just extra moisture with no benefits like the ventilation does for potential soil gas. maybe a good application of crystalline waterproofing (i.e. Xypex) would help cut some of the moisture transmission rate down.... but there is nobody or no product that i know that guarantees negative-side waterproofing approaches. it is hard to find manufacturer literature for the slab with no vapor barrier because they don't need to stick their neck out there... people will go to there products under ideal manufacturer recommended scenarios or in desperate attempt to do something that helps without destroying everything.