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Community and Q&A

Vapor barrier / retarder for plywood

Chris Dirmann | Posted in General Questions on

I’m putting down a laminate floor over a 3/4″ plywood subfloor that is over a unconditioned crawlspace in North Florida. The manufacturer of the flooring suggests a 6 mil plastic underlayment for the flooring to block moisture, but my concern is the potential for damage to the plywood by locking any moisture in. Would 15# tar paper be a viable alternative to use? Or would I be okay with the 6 mil poly?

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  1. GBA Editor
    Martin Holladay | | #1

    What you want to do is keep the plywood dry. You don't need 6-mil poly above the plywood; what you need is closed-cell spray polyurethane foam on the underside of the plywood to provide a vapor retarder and insulation layer to separate the plywood from the humid crawlspace below. Another option: install a continuous layer of foil-faced polyisocyanurate foam insulation on the underside of your floor joists. Be sure to seal the seams between the foam sheets. You also need air sealing at the perimeter of the foam (where the foam meets the rim joists).

  2. Ken Levenson | | #2

    Chris, If you happen to be looking for a foam alternative, we would suggest filling cavities with batt insulation and then installing an air-tight, vapor retarding membrane on the underside of the joists - air sealing all penetrations and at the perimeter. We offer such a membrane, called "DA"and can be found on our website This membrane is meant as an exterior grade vapor retarder (1.43 perms) for assemblies that are otherwise vapor open - allowing drying principally to the inside - a very good approach to hot and humid climates generally when looking for foam alternatives. We also provide tapes and gaskets to help ensure a complete and robust and long lasting air tightness. Feel free to give us a call.

  3. GBA Editor
    Martin Holladay | | #3

    Watch out! Chris's suggested floor assembly is not going to dry to the inside -- not if the flooring is laminate.

    I think your suggestion of installing a vapor retarder under the joists, along with air-permeable insulation, would be very risky.

    Once Chris cranks up the air conditioner, that plywood subfloor is going to be cold. All of the air between the fibers of the air-permeable insulation will be loaded with moisture, of course. What happens next?

  4. Ken Levenson | | #4

    Martin (and Chris), If the floor is going to be a vapor barrier - fair enough. But then you may ultimately have problems even with the spray foam - as you are creating a vapor barrier sandwich and the foam will leak air. You don't want a vapor barrier on the inside - it's creating a problem that can never be 100% fixed. Instead, put down a vapor open floor and put the vapor retarder/barrier only on the outside.

  5. GBA Editor
    Martin Holladay | | #5

    Good luck convincing Americans that they don't want vapor-barrier flooring (for example, vinyl tile, sheet vinyl, or laminate). That's an uphill battle.

    The fact of the matter is that tens of thousands of hot climate homes have vapor-barrier flooring over crawl spaces. The solution is to insulate the underside of the floor sheathing with closed-cell spray polyurethane foam, or to install a continuous horizontal layer of foil-faced polyiso (with taped seams) under the floor joists. These solutions work.

    There is absolutely no evidence for your contention that "the foam will leak air." In fact, your suggested insulation material -- batt insulation -- is far more likely to leak air than closed-cell spray foam.

  6. Ken Levenson | | #6

    Not trying to convince anyone - just trying to provide an alternative to foam. Americans create lots of problems up front and then look for a techno fix..... it will surely continue. However there is much evidence that spray foam installations leak air - easy to find with a blower door and thermal camera. Foam panels expand/contract and the joints can leak - particularly to dissimilar materials. One Passive House project initially passed the blower door test but failed later and it was concluded that an imperfect spray foam job was to blame.
    Many Americans couldn't imagine living without cigarettes just a few decades ago, yet today we mostly are. Sometimes things change for the better. Cheers.

  7. Ken Levenson | | #7

    Quick note: Suggestion of batt insulation was only as insulation. The airtightness comes from the membrane.

  8. GBA Editor
    Martin Holladay | | #8

    The air between the fibers of your batt insulation will have the same moisture content as the air in the crawl space. That's the problem -- all that moist air in your batt.

  9. Ken Levenson | | #9

    Not true. Our proposed construction for Chris, out to in, is: crawlspace/vapor retarding airtight membrane/batt insulation/plywood subfloor/vapor open flooring. The insulation will dry to the inside, remaining dryer than the crawlspace.

  10. GBA Editor
    Martin Holladay | | #10

    Going around in circles here.

    "The insulation will dry to the inside" -- through laminate flooring?

  11. Ken Levenson | | #11

    No, not through the laminate flooring.....that's not what i wrote.
    (I'm guilty of suggesting Chris change his approach to avoid foam.)
    Like a merry-go-round? ;)

  12. Expert Member
    Armando Cobo | | #12

    I would follow Martin's advise. CC foam is vapor impermeable and will serve as an effective moisture and air barrier, preventing cold subfloors when the AC is running and warm humid air flow from creating condensation, mold and decay problems. Of course, installation is always the key with any product.
    A membrane of 1.43 perms is clearly a vapor semi-permeable material, which means that eventually the moisture from the crawl space will pass thru to the batt insulation and framing cavity. If the flooring and adhesives are vapor impermeable, that humidity will get trapped in the subfloor and the framing cavity. If the flooring and adhesives are semi-permeable, eventually some of that humidity will get through to the inside of the home; but also the rate of permeability may also allow for moisture problems in the cavity.

  13. Aj Builder, Upstate NY Zone 6a | | #13

    Martin, rigid foam placed on joists with a captured airspace sucks and is never to be done. Small leaks make the whole assembly worthless. Rigid foam applied in many methods is nonsense. Some multiple layers placed tight to other airtight layers will stop bypass air... but I see most installs have problems.

    The close cell idea would be best. I have actual experience, not arm chair-ing said subject.


  14. Expert Member
    Dana Dorsett | | #14

    Armando: Closed cell foam is NOT vapor impermeable- it takes at least 2" of 2lb polyurethane to hit the mid-range of Class-II vapor retardency, and at 1" is usually still in the 1.2-1.3 perm range- a class-III vapor retarder.

    To hit a minimal class 1 vapor retardency you'd need about a foot of 2lb foam, and to approach the vapor retardency of the evil 6-mil poly you'd need at least THREE FEET of foam.

    For a FL climate, a flash-inch of 2lb polyurethane under the plywood, filling 2x 8 joist bays with fiber works from a dew-point and moisture migration point of view. With deeper joists bump that to 2". In an air-conditioned house the bottom edges of 2 x 8 or shallower joists can drop below the dew point of the crawlspace air unless you add an inch of unfaced EPS.

    For all the trouble & expense of doing it right it may be cheaper to put a vapor retarder on the ground and insulate the crawlspace walls with 2" of EPS or closed cell polyurethane, sealing off any exterior venting, then vent the crawlspace with a small amount of conditioned space air. It's usually less material and easier to get right.

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