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Contractor forgot vapor barrier
2

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

"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

RE: Contractor forgot vapor barrier

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

RE: Contractor forgot vapor barrier

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.

RE: Contractor forgot vapor barrier

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

RE: Contractor forgot vapor barrier

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

RE: Contractor forgot vapor barrier

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
www.americanconcrete.com

RE: Contractor forgot vapor barrier

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

RE: Contractor forgot vapor barrier

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

Mike McCann, PE, SE (WA)


RE: Contractor forgot vapor barrier

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

RE: Contractor forgot vapor barrier

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.

RE: Contractor forgot vapor barrier

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

RE: Contractor forgot vapor barrier

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.

Check out Eng-Tips Forum's Policies here:
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RE: Contractor forgot vapor barrier

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)


RE: Contractor forgot vapor barrier

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

Check out Eng-Tips Forum's Policies here:
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RE: Contractor forgot vapor barrier

No more than the moisture...

Mike McCann, PE, SE (WA)


RE: Contractor forgot vapor barrier

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.

RE: Contractor forgot vapor barrier

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

RE: Contractor forgot vapor barrier

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.

RE: Contractor forgot vapor barrier

I do not necessarily disagree with this article. I will, however, put it to the test in any way I can. That is my responsibility as an Engineer.

RE: Contractor forgot vapor barrier

on a side topic here.... everybody out there sells "services" or sells "product". i usually put more grains of salt on advice from people selling product. i can't think of any professor, instructor, and/or member of any committee like ACI/ASTM/etc... who doesn't do work for the private industry.... except people who are phasing themselves into retirement.

RE: Contractor forgot vapor barrier

If you absolutely gotta' have a vapor barrier, you could try injecting a bentonite slurry under the slab. Be careful not to overdo this, and accidentally jack the slab up so it cracks.
Dave

Thaidavid

RE: Contractor forgot vapor barrier

what about coating the slab with a densifier?

RE: Contractor forgot vapor barrier

Interesting digression in this thread in attacking Lstiburek for being both an expert and a consultant... Whether or not you accept his wisdom there is no question that Lstiburek is THE building science expert in conventionally built building envelopes.

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

I can't believe we still have people that don't believe what Lstiburek says in his article. He's right. There is an awful lot of research out there that supports his position.

RE: Contractor forgot vapor barrier

Lstiburek's style may not suit everyone, I find it wearing some times, but I had copies of papers published by him on building envelope issues back in the '80s and they have done a world of service to me and my clients since then. I'm in a very different climate than your crawlspace so I can't comment on it, but I'll tell you to believe Dr. Joe on envelope issues.

RE: Contractor forgot vapor barrier

O.K. I believe.

RE: Contractor forgot vapor barrier

Hey, I’m with you and I should have been clearer about that. I actually turned off the first lecture of his that I saw because I incorrectly thought he was selling LEED services with an attitude. Somehow I gave him a second chance and found that I did like green eggs with ham. I should have remembered that his style can be off-putting at first, though I’ve grown to enjoy it.

RE: Contractor forgot vapor barrier

(OP)
I was told this was a crawl space but when I stopped by to take a look I found out it is a full basement. Does this change anyone's mind about things?

Are there any remedies other than tearing the slab out and redoing it?

RE: Contractor forgot vapor barrier

what about coating the slab with a densifier? (repeated from before)

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