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  1. #1
    Dick Guest

    Default Poly in Crawl Space

    In situations where there is mold or mildew in dwellings, and it appears that it is coming from damp soil under the dwelling, some recommend laying poly on the soil. Wouldn't that exasperate the problem by trapping the moisture under the poly, causing mold to develop between the earth and the poly? If there appears to be plenty of cross ventilation, and the moist soil does dry out every year, but you still have the problem, wouldn't it be better to attach the poly to the underside of the joists? Or is this going to create even more problems because air can't circulate through the joist area, and the foundation vents are now in the joists, not allowing any air to circulate between the poly and the damp earth? Would it be better to install fiberglass bats between the joists with the vapor barrier down, and not install any poly at all?

  2. #2
    Glenn A. Davis, P.E. Guest

    Default Re: Poly in Crawl Space

    Put the poly on the ground. Poly attached to the floor joists is going to be a disaster. Floor will rot out for sure. (YOU know, vapor barrier on warm side.)

    Better to attempt to install subdrains to drain water away if you have the proper conditions for it, rather than leave out the poly and attempt to evaporate it away with ventilaton. That amount of moisture is hell on a floor system, especially with a low crawl space.

  3. #3
    Mike O'Handley Guest

    Default Re: Poly in Crawl Space

    Hi Dick,

    What you have to understand is that crawlspace soil never really dries out. Even after being uncovered for weeks on end, it will always remain moist. I don't remember where I learned this, but when I first got into home inspection I was informed that a crawlspace of 1000 sq. ft. without a poly barrier evaporates at least 5 gallons of water every 24 hours in DRY soil.

    If you nail poly to the underside of the joists moisture is still going to get in there through various plumbing and support penetrations as well as through migration from the home. When that moisture comes up against cooler poly, it'll condense and create a big petrie dish under the joists. Not a good situation.

    Think of the crawlspace as an oil lamp. The soil is the wick and ambient ground moisture is the oil. Leave the cap off of an oil lamp wick and it constantly wicks and evaporates oil until it is all dried out. Cap the wick and the wick becomes fully saturated and reaches equilibrium. The same thing happens to the soil under the poly barrier. Tightly applying a barrier will cause the soil under the barrier to be eternally damp, yet this damp soil is in effect a saturated wick which won't draw any more moisture. Remove the poly, and the evaporation will continue, but never be able to dry the area out.

    Keep the crawlspace well ventilated with good cross-flow and vents within 10 feet of each corner, and the crawl above the barrier is able to reach equilibrium with the air outside and generally remains dry. I say generally, because in some states where humidity is very high in summer (N.C. is one I can think of), humidity can condense on the topside of the cold vapor barrier and cause severe problems if insufficient cross-flow or other means are present to relieve it.

    Barriers are best applied as high as possible on the foundation walls with battens, but should not cover the sill or band joist. Seams should be taped with waterproof tape and overlapped no less than a foot. Barrier material should be at least 6 mil and black, so that the absence of light prevents formation of some micro-organisms and plants under the barrier. Care should be taken to ensure that the barrier is carefully trimmed to fit tightly around the base of any piers, where it can be weighed in place with gravel ballast or taped to the pier if possible. All wood support posts should be clear of barrier material so they can breath and remain dry. Barrier should be lain neatly, but fit loosely enough that it can float up on occasional flooding and then return to it's original position without allowing moisture to escape or water to pond on the top.

    Think this is impossible? Come on out here to Seattle and I'll take you to a 31 year old home on a flood plain near Everett, where you can crawl around on a barrier that feels like a water bed with about 3 inches of water underneath. The underside of the home - insulation, joists, posts, girders, sills looks like the day it was delivered.

    Evidently the fellow who installed that barrier felt that if something was worth doing it was worth doing correctly.


    Mike O'Handley

  4. #4
    Jim Chalkley Guest

    Default Re: Poly in Crawl Space


    I would look to see if there is an adequate amount of vents for the square footage of the structure, there is a formula for calculating this.

    Another problem could be the lay of the land, you might have adequate ventilation, but if water is coming in every time it rains, this might contribute to the amount of moisture present.

    I specify on my drawings that all construction debris be removed and a minimum of 6-mil poly to be applied to the crawlspace floor before decking.

    Just my two cents..

  5. #5
    Mike O'Handley Guest

    Default Re: Poly in Crawl Space

    Hi Jim,

    The rule of thumb formula you refer to is one square foot of net free ventilation for every 150 sq. ft. of crawlspace area. That is a minimum standard. One must also factor in, as you say, topography, local climate, etc., and add additional vents as appropriate. It's the "as appropriate" part that is tricky. That is generally born out of experience with the local climate. The local AHJ can usually provide some insight into this, even if he/she only enforces to code.


    Mike O'Handley

  6. #6
    Eric Sellers Guest

    Default Re: Poly in Crawl Space

    Mike O'Handley... yours is the best explanation of vapor barriers that I've seen to date. I've been told by several termite inspectors that the edges of the vapor barrier should be folded back a few inches from the foundation wall. They have reasoned that this provides less of a hiding place for termites. In central VA., the CABO map shows us in the moderate to heavy probability of infestation zone. I'd love to know your take on the issue.

  7. #7
    Mike O'Handley Guest

    Default Re: Poly in Crawl Space

    Hi Eric,

    In Virginia your primary foe will be subterranean termites. These fellows will nest anywhere from 6 to 10 feet below ground in colonies numbering in the many thousands and can be pretty destructive. Out here, I have to contend primarily with dampwood termites. These guys are about three times the size of a normal sub and live in wet wood above ground. I once had a small terrarium in which I kept a nest of about 5,000 dampwoods for nearly a year. They were pretty interesting to study, but one day I came home to find them all gone and my pet crow was sitting nearby dozing with a cheshire cat look on his beak.

    Okay, I digress. Back to the subject. Subs will move along the surface under a barrier on their way to their next meal, but they need to cross the open expanse of a foundation wall by building mud tubes, or by transiting through cracks in the foundation wall, or up through the cores of block foundations where they can get through a crack at the footing or a void in the mortar. They do this to protect themselves from other insects, but also to prevent exposure to air and light, which will dry them out and kill them.

    My take is that, since they have to cross open space, regular inspections by PCO to spot tubes and proper barrier treatments should be more than adequate to handle them.

    Want to know where you can find a really good pub on how to deal with subs? One that you helped pay for? Go to your nearest US Government Printing office and pick up a copy of Subterranean Termites - Their Prevention and Control in Buildings(Home and Garden Bulletin #64). It was co-authored by the USDA and US Forest Service. It provides good guidance about construction practices that will help to frustrate termites and discusses barrier treatment techniques. This book is like a mini-entomology class and will make you really smart, really fast about these little buggers(pun intended). It is worth every bit of the $1.00 I paid for it a few years ago.


    Mike O'Handley

  8. #8
    Kenneth D. Wiggers Guest

    Default Re: Poly in Crawl Space

    It is my reading of articles written by people of the building science community that crawl spaces are best treated as a conditioned space, i.e., cover the soil (6 mil poly or concrete), no vents, and insulate the perimeter walls. Mold under poly would seem to be a non-issue. Mold spores emanating from uncovered soil may be an issue (to be concerned about).

    The floor above crawl spaces (in Iowa) are often cold. The uninsulated metal duct work in crawl spaces is typically leaky, loses heat or cold by conduction, and often has great amounts of condensation on the surface during weather (90 to 100 degrees with a higher heat index)like we are having now -- very conducive to mold growth. We cover the crawl space soil with 6 mil poly, "tack" (foam is sprayed over the perimeter poly and up the wall into the rim joist space)the poly to the wall with Icynene. We also foam the duct work. Icynene will stick to ducts covered with dripping condensate. The crawl space becomes a warm dry space. The floor above is immediately warmer. The conditioned air in the duct work is delivered to the intended place.

    A 20x20x3 foot crawl space would end up having about 38 pounds (5 inch depth) of cured foam with "zero" fuel contribution and perhaps 25 pounds (?) of 6 mil poly which will probably support combustion. Some local codes may prohibit the application of uncovered foam in crawl spaces.

  9. #9
    Mike O'Handley Guest

    Default Re: Poly in Crawl Space

    Thanks for your input Kenneth,

    Your's is a good example of how there are regional differences in approaches to building science. No two homes are alike and it can sometimes be difficult to dicipher which method works best for one's own area.

    I think that is the real value of this forum. In years gone by, folks would read an article in a construction mag and it was suddenly the best thing since sliced bread. They would change their way of doing things and down the road pay for it in spades via callbacks. With this medium, these issues can be thoroughly discussed and thought through before being placed into implementation.

    I think Dick is getting some great input here on both sides of the vapor barrier issue. One of the reasons I sign off with.....


    Mike O'Handley

  10. #10
    Jim Katen Guest

    Default Re: Poly in Crawl Space


    Sorry if this seems like a dumb question, but Im really becoming intrigued by this foam thing.

    Have you ever skipped the poly, and simply applied a thin coat of foam over the entire crawlspace floor? Is it prohibitively expensive?

    - Jim Katen

  11. #11
    Kenneth D. Wiggers Guest

    Default Re: Poly in Crawl Space


    No. The expense would be one issue. Another issue would be "surface" water (flooding). It would be a difficult application (spraying the floor). Six mil poly works fine -- tack (with foam) the poly to the sidewalls and pillars. A "piece of cake" as crawl spaces go -- not generally much fun, but it works. I looked at a crawl space this week, fiberglass batts hanging down all over from the floor joists. What a mess! We send a lot of fiberglass to the land fill. The upside is that fiberglass batts are cheap, the downside is that they don't work very well unless you make a lot of effort to make them work (I would not want that task). We see fiberglass batts as a disposable item based on the problem calls we get.

    I also looked at the roof underside (condensation problems) of a 1986 built 15,000 ft2 commercial building -- has those vinyl faced fiberglass batts draped over the red iron and held in place between the purlins with metal strapping. All the vinyl faced fiberglass will have to come down. The trapped condensate will eventually rust out the metal roof from the bottom side. The building owner is very unhappy with the builder. The same insulation techniques are being used on a metal roof underside of a building under construction just south of Ankeny, Iowa -- doomed to failure!

    We finished spraying the roof underside this week where that very same thing (urethane was sprayed 17 years ago on the metal roof underside -- hog confinement, metal moves, urethane does not move, urethane delaminates and cracks -- condensation)happened. The metal building looked like an advanced stage of "flesh eating disease," there are still holes all over the steel siding -- we sprayed over the holes to provide a temporary seal for the coming winter. The siding will be replaced at a later date. The adage "build it right the first time" comes to mind over and over as I look at buildings in respone to calls.

  12. #12
    Jim Cerone Guest

    Default Re: Poly in Crawl Space

    I am presently construction a studio with a crawlspace. I used 2" dow board from footings to top of foundation wall on inside of foundation. Because we have to cover foam in Massachusetts, I installed osb over the dow board. I used 6mil plastic on the floor but nothing on the walls. I am hoping to create a "conditioned space." Do I need to install vapor barrier on wall?

  13. #13
    Kenneth D. Wiggers Guest

    Default Re: Poly in Crawl Space

    No. The Dow blueboard will serve as a vapor retarder. The junctures of the blueboard should be sealed with a caulk such as Vulkem 116 so that warm moist air does not reach the cold concrete and cause condensation.

  14. #14
    rich Guest

    Default Re: Poly in Crawl Space

    i am told that a builder is only required to have 1 square foot of ventalation per 150 square feet of crawl space and this number drops to 1 per 1500 if poly is present. Could this be true. Where could i find these codes for my area. (georgia).

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