Contractor forgot vapor barrier
Contractor forgot vapor barrier
(OP)
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?






RE: Contractor forgot vapor barrier
http://buildingscience.com/documents/insights/bsi-...
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.
RE: Contractor forgot vapor barrier
so much fun! Great link Archie264
RE: Contractor forgot vapor barrier
RE: Contractor forgot vapor barrier
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.
RE: Contractor forgot vapor barrier
RE: Contractor forgot vapor barrier
RE: Contractor forgot vapor barrier
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Contractor forgot vapor barrier
RE: Contractor forgot vapor barrier
Mike McCann, PE, SE (WA)
RE: Contractor forgot vapor barrier
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
RE: Contractor forgot vapor barrier
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.
RE: Contractor forgot vapor barrier
RE: Contractor forgot vapor barrier
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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RE: Contractor forgot vapor barrier
Mike McCann, PE, SE (WA)
RE: Contractor forgot vapor barrier
Check out Eng-Tips Forum's Policies here:
FAQ731-376: Eng-Tips.com Forum Policies
RE: Contractor forgot vapor barrier
Mike McCann, PE, SE (WA)
RE: Contractor forgot vapor barrier
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.
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BA
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RE: Contractor forgot vapor barrier
RE: Contractor forgot vapor barrier
Dave
Thaidavid
RE: Contractor forgot vapor barrier
RE: Contractor forgot vapor barrier
If you are going to vent a crawlspace and let in moist air the vapor barrier should go on the bottom face of the floor framing envelope so all the condensation will form on top of your mudslab instead of the floor framing. If you don't want any condensation on your mudslab you should have insulated under that slab, though rigid board insulation on top of that slab may work for that?? As far as vapor barriers atop slab-on-grade:
Recent eng-tips thread on coating the top surface of S-O-G
See these links for good discussions on general crawlspace approaches, and on condensation in vented crawlspaces. Be warned -- after reading these you'll probably want to steer clear of crawlspaces altogether in the future!
New Light in Crawlspaces
Vented vs Unvented Crawlspaces
RE: Contractor forgot vapor barrier
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Are there any remedies other than tearing the slab out and redoing it?
RE: Contractor forgot vapor barrier