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Vapor permeance of thick foam sheathing and board seams

Sash057 | Posted in Energy Efficiency and Durability on

I have read that at a certain thickness, all foam boards (eps, xps, and polyiso) lose their vapor permeance, effectively rendering them vapor retarders with very low perm ratings. Does anyone have a link to the research behind these findings and if so can I be pointed in the right direction?

I am hopeful that the effective thickness at which this condition occurs for each foam is stated on the research but I will hold further questions till I have read up on it.

A secondary question to the above: if xps becomes a vapor retarder at 1″ thick (for instance) and my entire exterior wall is cover by 2″ thick 4×8 boards without the seams being taped, is my wall vapor open to the outside or not?

I have heard the arguement that since the seams are not taped, vapor will magically finds its way to these micro channels where the boards meet and escape to the outside. I, however, am skeptical as my seams may be vapor open but my 32 sq ft board is acting like a massive vapor retarder. So the question stands: at thick exterior foam, without the seams taped, am I looking at a vapor open wall or have I effectively reduced my drying capacity so much that I should consider this wall vapor closed to the exterior and dry inward?

Thank you in advance, this community has taught me a tremendous amount already!

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  1. Sash057 | | #1

    Clarification and support:

    Sorry I do not see an edit button. I meant unfaced polyiso, obviously aluminum facing on any foam renders it vapor closed.

    As for the support, is this not the exact condition that has caused us to start back ventilating caulk boards and marker boards in the south (high cooling loads)? The majority of the walls are open but these large expanses of vapor closed areas cause problems with drying to the interior, same with large mirrors etc...

  2. Expert Member


    Both XPS and EPS foam can become significant vapour-retarders quite quickly. EPS is 2 to 6 perms per inch. XPS is 1.1 perms per inch. Un-faced Polyiso however is 26 perms per inch, so you would need an awfully thick layer for the wall to become vapour-closed.

    To answer your second question: No, you are right there is no magical mechanism to get the vapour to find the cracks and diffuse through them. The cracks and holes are incidental from a vapour point of view, but not from an air-sealing one.

  3. GBA Editor
    Martin Holladay | | #3

    Be careful: When Malcolm wrote about "perms per inch," he really meant, "the vapor permeance at one inch thickness is as follows." Doubling the thickness does not increase the vapor permeance -- it decreases the vapor permeance. Vapor permeance and foam thickness are inversely proportional.

    For the reference work you seek, see "Choosing Rigid Foam." Here are the relevant sentences:

    "One inch of unfaced EPS is relatively permeable; it has a permeance of 2.0 to 5.8 perms. If you are looking for vapor-permeable EPS, though, be careful: if the EPS is faced with polyethylene or foil, it will have a very low vapor permeance.

    "One inch of XPS has a permeance of 1.1, while a 2-inch-thick sample of XPS has a permeance of 0.55. XPS is classified as a semi-impermeable material.

    "One inch of foil-faced polyiso has a very low permeance of 0.03 perm. Polyiso with a glass-mat facing or a felted-paper facing has a permeance ranging from 1 to 10 perms, depending on the type of facing used."

    Here's the short version: If the rigid foam is of useful thickness, don't expect much exterior drying.

    Remember that a wall with exterior rigid foam sheathing is not supposed to dry to the exterior. It is supposed to dry to the interior. The problem you refer to -- moisture accumulation behind a wall-mounted mirror in an air-conditioned home -- does not occur in a properly designed wall with a continuous layer of exterior rigid foam. For more information on the theory behind this type of wall, see "Calculating the Minimum Thickness of Rigid Foam Sheathing."

    Builders who talk about leaving the seams of exterior rigid foam untaped, "to let the wall breathe," are making at least two mistakes. They are undermining the airtightness of the wall assembly -- always a mistake -- as well as engaging in magical thinking. The vapor permeance of the wall assembly is a function of the percentage of the wall covered by a relatively impermeable material, so omitting the tape doesn't help improve vapor permeance.

    Finally, the idea that you want a wall to be "vapor-open" in both directions is misguided. There is no absolute value to building assemblies being vapor-open. In fact, in many cases, having a vapor-open assembly causes problems -- the classic example being a slab floor. You never want your slab floor to be vapor-open.

    If you think back to your example of mold behind a wall-mounted mirror, you will realize that the origin of the problem is that the wall was vapor-open to the exterior. If the wall had a layer of exterior rigid foam (a vapor-closed layer), there never would have been any inward solar vapor drive in the first place, and the "mold behind the mirror" problem would have been avoided.

    1. user-553778 | | #8

      Why not use rock wool insulation on the exterior if rigid foam prevents outward drying of the wall? Is it because it is difficult or impossible to install furring strips over rock wool? I’m not sure why everyone seems to use rigid insulation which also shrinks over time increasing gaps between panels.

      1. Jon_R | | #9

        Indeed, in cold weather, for the same R value and wrt moisture, exterior rock wool will typically outperform unfaced EPS which will outperform foil faced rigid foam. But the difference may not be significant. And in warm weather with AC, the rockwool may be too vapor permeable.

        1. user-553778 | | #10

          How can it be too vapor permeable? You want it to be vapor permeable. It’s not a WRB, it’s just insulation.

          1. Jon_R | | #11

            Too vapor permeable can let too much water vapor from outside into the wall where it may cause excessive humidity on the cool interior side. Agreed, there are ways of addressing this concern (eg, lower than typical perm WRB).

  4. GBA Editor
    Martin Holladay | | #4

    Your understanding is off.

    With any moisture-related problem, the first question that needs to be answered is, "What is the source of the moisture?"

    In the case you're talking about -- mold behind wall-mounted mirrors (or vinyl wallpaper) in hot, humid climates -- the source of the moisture is either exterior air or wind-driven rain that soaks the siding. Inward solar vapor drive pushes exterior moisture into the wall assembly. Air conditioning cools the mirror or the vinyl wallpaper, transforming the mirror or the wallpaper into a condensing surface. Mold forms on the back of the mirror or the wallpaper.

    In an existing building, it's expensive to retrofit a layer of exterior rigid foam, so the quick-and-dirty solution is to remove the mirror and re-mount the mirror on stand-offs, so you have some air circulation behind the mirror.

    If you are designing a new building, there are a variety of tactics that can be used to minimize inward solar vapor drive. These include:

    1. Choosing a type of siding (for example, vinyl siding) that isn't a "reservoir cladding."

    2. Including a generously sized ventilated rainscreen gap between the siding and the WRB.

    3. Installing an adequately thick layer of continuous rigid foam on the exterior side of the wall sheathing.

    For more information on inward solar vapor drive, see "When Sunshine Drives Moisture Into Walls."

  5. Expert Member

    Martin, Thank you for the clarification.

  6. Sash057 | | #6

    Thank you Malcolm and Martin for taking the time to respond.

    I was also able to dig up this guide from the BSC regarding the vapor permeance of foam if anyone else needs it:

    There is a useful section at the end that talks about perm ratings of the materials and supports the information Martin provided. The table also shows how density of foam affects perm rating and the perms decreasing with thickness is also discussed above it.

    Martin, my example of back ventilated mirrors was anecdotal evidence from a talk Dr. Lstiburek gave where I was given the impression that even a wall with exterior insulation, closed to air and vapor to the exterior (drying only to the interior) in a high cooling load climate would be subjected to potential problems drying to the inside if large expanses of impermeable material is adhered directly to the wall. It was this reason, I believe, that he stated we are starting to back ventilate blackboards/marker boards/casework in classrooms located in the southern USA. As in to say that any potential vapor in the wall wouldn't be able to find and move to the vapor open areas, but be trapped behind the vapor closed material. Was my understanding off?

  7. Sash057 | | #7

    Thank you for explaining Martin, and I appreciate all of the time you've put into this great resource.

    I was mistaken on my original recollection. The discussion was indeed on exterior moisture making its way into the wall where we would want it to leave as quickly as possible, thus back ventilating mirrors / blackboards / etc.

  8. jimg13 | | #12

    I have a related question that not only pertains to my current project, but to about a zillion others that the world will have to confront in the coming decades. I am residing an old building. I want to do the responsible thing, by adding continuous exterior insulation. I do not know what vapor barrier now exists on the interior side of the walls. I also do not know how well insulated the walls are. I am fairly sure that the insulation and vapor barriers are inconsistent throughout the building, because there have been several modifications made throughout the building's long life. How does one deal with the question of vapor permeance when adding continuous exterior insulation in a situation such as this?

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