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Community and Q&A

Martin’s recent article “Combining Exterior Rigid Foam With Fluffy Insulation”

MarcYoung | Posted in Energy Efficiency and Durability on

Hello All,

This is a question regarding outboard and inboard insulation and Martin’s recent article “Combining Exterior Rigid Foam With Fluffy Insulation”.

I’m up in Canada – vapour barriers are the norm.

Assuming one is ignoring overall R-values and thermal bridging, if one installs a perfect vapour barrier (and I know this is not possible) then, in theory, one does not need any outboard insulation. If I understand correctly, a perfect vapour barrier will stop vapour drive AND air flow towards the exterior so there will be no warm humid air to condense on the sheathing and no need to keep said sheathing warm.

Am I understanding the theory correctly?


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  1. GBA Editor
    Martin Holladay | | #1

    In the type of wall you describe -- a wall without any exterior rigid foam -- the wall sheathing will get damp in February and March. Many researchers have confirmed this phenomenon.

    Cold sheathing tends to be wetter than warm sheathing; this is due to physics. There are three sources for the moisture that accumulates in cold wall sheathing in February:

    1. Some of the moisture taken up by the sheathing comes from the exterior air.

    2. Some of the moisture taken up the the sheathing comes from interior air that has migrated into the wall assembly due to air leaks.

    3. Some of the moisture taken up by the sheathing comes from water vapor that has diffused through the wall assembly from the interior toward the exterior.

    The inclusion of interior polyethylene does not stop these phenomena, because it is impossible to install a perfectly airtight layer of polyethylene. Researchers have measured increases in the moisture content of sheathing in February, even in Canada, and even if the wall includes a layer of interior polyethylene.

  2. Expert Member

    Yes you are right: In a perfect world walls would not need to dry because they would never get wet. However, as Martin has pointed out when writing about this in the past, no house has ever recorded a zero blower door test. Even tight houses leak air at a rate that makes installing air barriers on both the inside and exterior a risky proposition.

  3. GBA Editor
    Martin Holladay | | #3

    I think it's OK to install two air barriers -- one on the interior of a wall assembly, and one on the exterior. There is no downside to the increased airtightness that results from this approach.

    [Later edit: Looks like Malcolm meant to write "vapor barriers," not "air barriers." I agree that you usually don't want a wood-framed wall to have foil-faced polyiso on the exterior as well as polyethylene on the interior. A smart vapor retarder like MemBrain would work on the interior, though.]

  4. MarcYoung | | #4

    Perhaps Malcolm is referring to two vapour barriers and having moisture get in between...

    Thx for your replies Martin and Malcolm

  5. Expert Member
    Dana Dorsett | | #5

    Using smart vapor retarders (detailed as an air barrier) rather than true vapor barriers adds a lot of resilience to the structure, either with or without outboard insulation.

    Plywood or OSB sheathing are themselves smart vapor retarders, but with exterior foam the drying to the exterior is limited (or eliminated, if foil-faced). In cold/very-cold climates with short drying seasons and long moisture-loading seasons, using rigid rock wool on the exterior in lieu of foam offers the dew-point protection of foam of equivalent-R, but without inhibiting drying toward the exterior.

  6. Expert Member

    Mitchell, Yes, just half awake this morning.

  7. MarcYoung | | #7

    Hello again,

    The article is only referring to air-permeable insulation...

    What if one wanted to use closed cell spray foam insulation?
    Would the same percentages of insulation inboard and outboard of the condensing surface apply?
    If so, would rockwool be a better choice for outboard insuatiuon (as it may permit more drying to the exterior than eps, xps, or poly would) ?

    Any comments...suggestions...caveats...Thx

  8. GBA Editor
    Martin Holladay | | #8

    If you want to install exterior rigid foam, I don't recommend the use of closed-cell spray foam insulation between the studs. To learn why, see How to Design a Wall.

    If you want to install closed-cell spray foam insulation between the studs in spite of its high cost and contribution to global warming, then it certainly makes more sense to install mineral wool on the exterior side of the sheathing rather an a low-permeance layer like EPS, XPS, or polyiso.

  9. charlie_sullivan | | #9

    But if you are going to install rockwool as the exterior insulation, the system will work just fine without spray foam on the inside--you might as well use cellulose or rockwool inside the studs.

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