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Polyethylene inside if my contractor used foil-sided graphite EPS?

user-6700812 | Posted in Green Building Techniques on

Our wall construction is the following (from exterior to interior)

SIGA-Majvest with taped seams
1/2″ OSB
1″ double foil faced GPS (Graphite EPS – R5.1 at 25degC)
2×6 Wall with batt insulation.

This is where we are at, should we include the poly membrane on the inside? My concern is that the foil face (which we should not have had, but here we are) will trap moisture on the inside, if I use this. If I do not use the poly will it fix my problem? Do I have a problem?

If it matters – We are slab-on-grade with R-20 XPS below and R-10 XPS outside the stem wall. The slab does not touch the stem wall (SIGA felt tape between slab and stem wall). Taped seams from outside XPS to top of Stem.

Climate Zone 5 dry

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  1. GBA Editor
    Martin Holladay | | #1

    To make this wall safe, you have to increase the thickness of the exterior rigid foam, which needs a minimum R-value of R-7.5 for a 2x6 wall in Climate Zone 5. For information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    This type of wall is designed to dry to the interior, so the wall shouldn't have interior polyethylene. Of course, an interior air barrier (usually the taped drywall) is always desirable.

  2. Expert Member
    Dana Dorsett | | #2

    The statement "This is where we are at..." presumably means the siding, foam, sheathing and batts are already in place?

    If yes, there's no 100% true answer to the question- it's now a risk mitigation problem. If the sheathing is already reasonably dry (<<20% moisture content, measured with a 2 prong moisture meter) it's not TOO risky to add air-tight polyethylene sheeting on the inside despite insufficient foam-R for dew point control.

    But rather than impermeable polyethylene on the interior, it would be better to use a smart vapor retarder such as 2-mil nylon (Certainteed MemBrain) or a more rugged, somewhat less permeable smart vapor retarder such as Intello Plus on the interior. Smart vapor retarders are very low permeance when the entrained air in the fiber insulation is dry, which it will be when the sheathing is cold enough to take on moisture as adsorb. But it becomes dramatically more vapor open when the air becomes humid, which will happen when the sheathing warms up, releasing it's accumulated wintertime moisture. This allows the assembly to dry dramatically faster than it can through polyethylene sheeting, while still limiting the rate of moisture diffusion into the wall when the sheathing is cold. By slowing the moisture up-take and speeding up the drying the sheathing's moisture level hopefully will stay <<20%, but even if it crosses that threshold it won't stay high enough long enough to matter once the sheathing reaches temperatures high enough for rapid mold growth.

    It's less than ideal- not as robust as adding more R on the exterior, but it'll still work.

  3. user-6700812 | | #3


    Thank you for your answer. So the OSB should be dry, it has been exposed to 100deg temps for 2 months prior to the siding.

    I would prefer no interior membrane at this point. Would what you described slow or stop the vapor from entering the wall cavity?

  4. Expert Member
    Dana Dorsett | | #4

    Nothing will "...stop the vapor from entering the wall cavity...", not even 6 mil polyethylene- it's really all about the rate of diffusion, both during the accumulation phase and the drying phase. But with polyethylene the rate of water vapor diffusion doesn't change much (or at least not enough to matter)- it's pretty vapor-tight, blocking both wetting & drying phases.

    Smart vapor retarders are well under 1 perm (class-II vapor retarders) when the air next to it is less than ~35% which it will be for most of the winter, and that's good enough, as long as you aren't keeping the ROOM's relative humidity at some high level, creating a higher vapor-pressure difference. Keeping it under 40% RH for the 10 coldest weeks of winter by adjusting the ventilation rates works just fine. But unlike polyethylene, when humidity levels in the cavity are high enough to support mold the smart vapor retarders become fairly vapor-open- more than 5 perms, which is more vapor open than typical interior latex paint finishes which becomes the ultimate limitation on the drying rate. Interior latex paints are on the order of 50-100 times more vapor open than 6-mil polyethylene, which allows the cavity to purge moisture quickly via diffusion.

    So with a smart vapor retarder it slows the moisture accumulation rate to something like 2-5x that of a true vapor barrier like polyethylene, but still allows the assembly to dry 50-100x as fast. It'll be something like 90% LESS moisture accumulation than through 3-5 perm standard interior latex, but still more wintertime accumulation than with 6 mil poly. But moisture will always be able to get out at a reasonable rate via diffusion. That's pretty decent protection even if you didn't have R5 on the exterior but instead had an exterior side vapor barrier. With the R5 foam the number of hours that the sheathing will dwell below the dew point of the conditioned space air (which is what drives vapor diffusion into the wall) are much fewer than if it had no foam, but still isn't few enough to be well protected without some sort of interior vapor retarder.

    The cheap way out would be air-tight wallboard and "vapor barrier latex" primer, which runs about 0.5 perms. That slows the accumulation rates to about the same level as a smart vapor retarder, but it takes a month of warmer weather to release what a smart vapor retarder would let go in 2-3 days.

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