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Membrain in walls, poly in attic. OK or not?

Alex House | Posted in Energy Efficiency and Durability on

I join hundreds of others in trying to wrap my head around the nuances of vapor retarders.

In this thread in comment #32 Martin notes that ceiling poly is unlikely to cause much trouble and then in comment #34 he observes that another person should use Membrain in their walls.

Is there ever a need for Membrain in the ceiling or can cheaper poly always be used?

One other question.

Poly in walls should be on the warm side, Martin quoted the following from the same article:

“In all framed walls, floors, and roof/ceilings comprising elements of the building thermal envelope, a vapor retarder shall be installed on the warm-in-winter side of the insulation.”

I’d like to avoid any penetrations in the vapor retarder, and if the vapor retarder is placed on the outboard plane of an inner wall (in a double wall system) or the inboard plane of the exterior wall does that count as the “warm in winter side of the insulation” in other words, can some portion of the wall insulation be on the interior side of the poly and still meet the code or does every scrap of insulation have to be on the exterior side of the poly?

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  1. GBA Editor
    Martin Holladay | | #1

    You seem to be confusing vapor barriers and vapor retarders. In some climates, building codes require an interior vapor retarder. In no case is a vapor barrier (interior polyethylene) required in the U.S., although some building inspectors are under the mistaken impression that it is.

    In some parts of Canada, the code is written in such a way as to support the idea the interior polyethylene is necessary. If you live in Canada, get local advice on this issue.

    The kraft facing on fiberglass batts and vapor-retarder paint both satisfy code requirements for a vapor retarder. So does MemBrain, although MemBrain is more expensive.

    Worries about vapor diffusion are not based on science. It's far more important to have a good air barrier than a vapor retarder.

    In very cold climates -- northern Minnesota, the prairie provinces of Canada, northern Quebec, and interior Alaska -- the use of interior polyethylene is fairly common and won't do any harm. In my opinion, interior polyethylene should never be installed in a house that will be air conditioned in summer.

    For more information on these issues, see:

    Vapor Retarders and Vapor Barriers

    Forget Vapor Diffusion — Stop the Air Leaks!

    Do I Need a Vapor Retarder?

  2. Expert Member
    Dana Dorsett | | #2

    With a vented attic in US climate zones 6 or warmer there is no point to installing a vapor retarder unless there is a low vapor-permeance top side air barrier to the attic insulation (such as a plywood or OSB subfloor on top of joists). With open blown fiber insulation with nothing but attic are above it air-tight ceiling below is sufficient- the attic ventilation purges any moisture that gets into the insulation via diffusion, even through unpainted wallboard (though painting it cuts the vapor diffusion a full order of magnitude.)

    Most wood framed wall assemblies have OSB or plywood sheathing panels on the exterior. At about 1 perm, the vapor retardency of the sheathing is lower than standard latex paint, and the wood is susceptible to moisture. If there is a vented rainscreen gap between the sheathing & siding it has sufficient drying capacity to the exterior that there isn't much risk to the sheathing in US climate zones 5 or warmer, as long as the interior drywall is air-tight. But in zones 6 or colder there is at least some risk, even with ventilated siding, and either exterior insulation sufficient to keep the sheathing warm enough, or a vapor retarder of 1 perm or less is needed to mitigate that risk.

    Using smart vapor retarders rather limits the rate of moisture accumulation, since below 30- 35% relative humidity it vapor retardency is under 1 perm. That is a reasonable & comfortable humidity level for indoor air in winter. During the warmer months and higher relative humidity (either inside the cavity air or the conditioned space air) the vapor retarder becomes dramatically more vapor open, which allows it to dry at a much faster rate than it was taking on moisture during winter. This makes it a far superior product than 6-mil polyethylene sheeting, which is very low permeance ( about 0.05 perms) under all conditions, and can thus trap moisture in the wall cavity.

    Using poly sheeting in air conditioned buildings is primarily a problem in locations where the outdoor dew points are above 75F much of the summer, or on brick or stucco clad buildings. The masonry sidings store a lot of rain & dew moisture, which gets released in intense bursts when heated by the sun, temporarily raising the dew point of the air on the interior side to temperatures much higher than conditioned space air. With a polyethylene vapor barrier that can result in liquid water condensing, running down the sheet, then being absorbed by the bottom plate of the framing. For houses without moisture-reservoir siding types in climates where outdoor dew points rarely exceed 70F those condensation events are rare enough to never become a problem.

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