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What is the minimum permeability for drying to the exterior in Zone 6?

user-2537291 | Posted in Energy Efficiency and Durability on

Are there guidelines on the minimum permeability you should have in an exterior wall assembly for drying to the outside? For a cold climate, specifically Zone 6 in Halifax. We get a fair amount of rain and I’d imagine it’ll depend on that as well?

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  1. user-2537291 | | #1

    Unrelated question. How do you edit your previous posts? I see I made some spelling errors....

  2. GBA Editor
    Martin Holladay | | #2

    There is no single answer to your question. I notice from another post that you are looking for a vapor-permeable material (ideally 1/2 inch thick) to shim the existing exterior sheathing on your house. If you want to choose a vapor-permeable material, you could choose 1/2 inch fiberboard.

  3. user-2537291 | | #3

    Figured. Thought I"d ask anyways. I never thought about fiberboard. I was looking for something a bit more insulating but fiberboard probably would be a lot better than XPS. Thanks.

  4. Dana1 | | #4

    Crude rules of thumb for zone 6: With a poly vapor barrier on the interior side, ideally you'd want any insulating layers exterior to wood sheathing to be 1-perm or higher.

    If there is a "smart" vapor retarder or a Class-II vapor retarder on the interior side the exterior layers that can be reduced to half, or even lower depending on the ratio of the R value exterior to the sheathing to that on the interior.

    If 1/3 or more of the total R is on the exterior (say, 2" of EPS on the outside of a 2x4 wall with R15 rock wool cavity fill) standard latex paint (3-5 perms) would be fine for the interior vapor retarder, and the vapor retardency of the exterior insulation simply doesn't matter.

    Without exterior insulation if you use fiberboard instead of OSB or plywood for the structural sheathing the assembly dries much faster to the exterior, but not if you put low-perm insulation on the exterior. OSB and plywood are both about 1 perm (when dry), whereas fiberboard is typically 15+ perms. But if you put 1 perm or half-perm foam on the exterior of 15 perm fiberboard it inhibits the drying rate severely, not appreciably faster than OSB or plywood sheathing with the same foam on the exterior.

    XPS is about 1 perm @ 1" thickness (R5) , give or take (it varies with manufacturer), or about a half-perm at 2". Unfaced Type-II (1.5lbs per cubic foot density) EPS is about 2.5-3 perms @ 1" (R4.2) and is still above 1 perm @ 2" (R8.4). With 2" of exterior EPS you would have ample dew point control for a 2x4 assembly to be able to skip the interior vapor barriers, which give it 1+ perm of drying to the exterior, 3+ perms to the interior. With 2" of EPS on a 2x6/R23 assembly it would be wise to use half-perm "vapor barrier latex" primer on the interior and air-tight wallboard methods, or a smart vapor retarder like MemBrain (2-mil nylon sheeting) or Intello ( thicker and lower perm than MemBrain) detailed as an air barrier between the wallboard and the insulation/studs.

    EPS uses a much less damaging blowing agent (pentane) than XPS (a mix of HFCs, predominant component is HFC134a). The global warming potential (GWP) of HFC134a is 200x that of pentane, which is only 7x CO2 (instead of nearly 1400x CO2.) Until the manufacturers move over to one of the low GWP HFO1234(_ _) variant blowing agents to replace the HFCs, it's a lot greener to use EPS. In a handful of decades as the HFCs bleed out the performance of XPS falls to that of EPS of equal density anyway.

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