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tylerjay | Posted in Energy Efficiency and Durability on

Alright we living in central Iowa (zone 5)and we are remodeling a house and put an addition on. We’re down to the insulation for the walls, on the exterior of the house it’s a mix, on the north it’s a 2 x 6 wall cavity with vinyl siding,house wrap and sheathing. On the southe side it’s 2×4 wall cavity with house wrap green fan fold insulation I believe is what they called it and what my contractor calls manure board? So the decision was to use r15 on 2×4 and r21 on 2×6 walls all Kraft faced. He also said he puts poly up as extra barrier for moisture. Needed thoughts on this second barrier. Is it needed or not and reason why. Thanks!

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  1. Expert Member
    Dana Dorsett | | #1

    Fan fold XPS or EPS needs to be a PERFORATED variety to deliver a sufficient drying capacity toward the exterior under the vented cladding (= vinyl siding). Also R-value isn't anywhere near sufficient for dew point control at the sheathing.

    Unperforated versions GreenGuard fan fold runs between 1.3-1.7 perms, which isn't super tight, but tight enough to require something more vapor tight than standard interior latex, and won't tolerate air leaks to the interior very well:

    Kraft facers are "smart" vapor retarders- less than 1 perm when dry, more vapor open than standard interior latex paint when the cavity is humid enough to support mold. That is good enough in zone 5 as long as the assembly is air tight to the interior. A layer of polyethylene runs the risk of creating a moisture trap (particularly with un-perforated or foil faced fan-fold) and should be avoided. If a broad-sheet product is used it needs to be detailed as an air barrier.

    I'd personally be more comfortable with using 2-mil nylon (Certainteed MemBrain) than using 4 or 6 mil polyethylene. Like kraft facers it will be < 1 perm in winter, but more vapor open than latex paint when the assembly needs to dry. Polyethylene is cheap (a bit less than half the cost of 2 mil nylon), but always less than 0.1 perms- a true vapor barrier. Polyethylene vapor barriers can become a condensing surface in the sticky days of summer when air conditioning to temperatures lower than the outdoor dew points, which can hit the 70s for several days time in places like Des Moines:

    When those events happen liquid moisture can form on the polyethylene- if there are leaks to the outdoors there can be enough liquid to accumulate even over a few days of torrid weather to get mold going. When moisture accumulates in a cavity, the more vapor permeable nylon will let it dry out an order of magnitude faster, while still providing excellent protection from wintertime interior moisture drives.

    1. tylerjay | | #2

      So are you saying that since we are using kraft face we don’t need even the 2 mil certainteed membrain? Or Do you recommend using both

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