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Insulating exterior wall where it meets partition wall

Jon Garbarino | Posted in General Questions on

NH here.

I was planning on using rigid foam insulation in a stud bay where a partition wall for a shower begins.

The rest of the bays use un-faced fiberglass batts and a plastic sheeting as a vapor barrier.

The exterior wall is OSB.

I want to go with the rigid foam because I feel it will be easier to piece around the framing I am adding in the bay to support the partition wall – going above and beyond to make sure there is no movement (in aspects I can control) given that the wall will be tiled.

As I started my research, I can’t seem to pin down weather this is a good idea or not.

Any thoughts?

Any problem running the plastic sheeting over the rigid foam?

Also, I am thinking of replacing the plastic sheeting with “CertainTeed’s MemBrain Continuous Air Barrier & Smart Vapor Retarder” – any thoughts on this?

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  1. GBA Editor
    Martin Holladay | | #1

    We can't answer your question unless you tell use your location or climate zone.

    Unless you live in Alaska or the colder parts of Canada, it's usually a bad idea to install polyethylene (plastic sheeting) on the interior side of your wall.

    For more information on this issue, see these articles:

    Vapor Retarders and Vapor Barriers

    Do I Need a Vapor Retarder?

  2. Jon Garbarino | | #2

    Thanks for the input Martin,

    I'm in New Hampshire - mentioned it in the beginning of the post, but I should have elaborated.

    Looking at the map on this site I'm in zone 5A.

    Also, my house is vinyl sided and there is a thin layer of styrofoam insulation between the siding and the sheathing.

    I've done a lot of reading about vapor barriers in the last couple of days

    I think my conclusion is two fold:

    1) my building inspector wants the barrier so I'm best going with the MemBrain product.


    2) to guard against moisture - my efforts are best put toward guarding against air leakage.

    As for the rigid insulation: If I fill a single stud bay with it - will it cause any moisture problems for the rest of the wall or the OSB sheathing?

  3. Dana1 | | #3

    "Styrofoam" (tm) is blue extruded polystyrene (XPS). If that's what it is, how thick? If that isn't what it is, describe it (color thickness, facers, etc.)

    There are also multiple types of "... rigid foam insulation...", and the type makes a difference. In a stud bay with an interior side smart vapor retarder the safer foam would be to use 1lb per cubic foot nominal density "Type-I" EPS (Expanded PolyStyrene), which would have an obvious macroscopic bead structure, like a cheap picnic cooler. (It's the same stuff, only slightly denser than the cheap cooler.) It should NOT have a plastic or foil facer. This type of insulation isn't carried by box stores.Most wil have some Type-I EPS, but usually with facers. The facers reduce the vapor permenace of the otherwise semi-vapor-open low density foam. It's typically R3.8-R3.9 per inch (a 2" thickness would usually be labeled R7.8 or so, not over R8.) At 1.5lbs density "Type-II" EPS rises to R4.2/inch or so, but the vapor permeance at 3.5" would be under 1 perm. If you can find some unfaced Type-II EPS that would be preferable to Type-I EPS with facers, or any XPS.

    With XPS (pink, blue, green) the vapor permeance at 3.5" thickness would be lower than 0.3 perms, lower than MemBrain even at it's most vapor tight. Using MemBrain might still be "worth it" as an air barrier, but it's not really a drying path for the sheathing.

    Any cut'n'cobbled rigid foam needs to be air sealed into place with expanding can foam. Cut the material 1/2" narrower than the space it's filling to allow enough room to get a foam-can nozzle fully in the slot, and let the expansion fill the gaps. This needs to be the full perimeter of your slab of foam. A surface-seal bead just doesn't cut it, and will create a convection gap to transfer moister-warmer air out to the sheathing.

    For awkward uneven shapes it's generally better/safer to use blown fiber or open cell sprayed polyurethane, in which case the variable vapor retardency of the MemBrain would in fact be useful.

    If it were my house I'd squirt some cellulose in there, not rigid foam, and use MemBrain on the interior side. YMMV. Landscaping fabric stapled every 2-3" about 1/4" in side the inside edges of the studs on both sides is a suitable blowing mesh. Slit the mesh to allow the hose to be pushed to the end of the cavity, and blow until the blower stalls, pull it out 8" or so to get the materiral flowing again, repeat. You should be able to get 2.5-3lb density out of it even with a 3" hose, no "dense packing" tubes, using a single stage rental blower. That's slightly less than ideal but not terrible. At that density it may settle a bit in 25-30 years (or not) depending on it's absolute seasonal moisture cycling peaks. (At 3.5lbs it would be fine pretty much forever in your climate zone.)

    If it's just one stud bay it's probably not worth the headache & expense of doing it with cellulose. It would be cheaper to buy a 12 board foot FrothPak kit and give it a 0.5-1" full coverage insulate & seal over the OSB, and stuff the rest with a compressed batt. The sprayed foam would form a non-wicking and vapor retardent condensing surface at a point within the total R that it averages above the dew point of wintertime conditioned space air, which would by-itself be protective of the OSB (with or without the MemBrain.)

  4. Jon Garbarino | | #4

    Thanks for the reply, D.

    I was thinking of using the pink XPS.

    The attached pic is an exterior rendering of the bay - sheathing removed.

    I could get a continuous 1.5" thick sheet between the sheathing and the blocking for the partition wall, and then I would piece XPS between the blocking, filling the 2x6" cavity.

    Or I could just go with un-faced fiberglass.

    I like the idea of using the FrothPak or something like it, but it is cost prohibitive.

    As for the stuff I called styrofoam on the exterior - it is foil faced white foam that looks identical to packing styrofoam and is 1/2" thick at most.

    I should also mention that the bay is currently wide open as I wanted to get the first layer in before placing the blocking.

  5. Dana1 | | #5

    Unless the foil facer on the exterior EPS is perforated on a fine grid pattern it's vapor permeance is extremely low, on the order of 0.05 perms or less, which means effectively zero drying capacity toward the exterior. If it's only 1/2" thick it's woefully short of R value for dew point control at the sheathing for an insulated 2x4 wall, let alone a 2x6 stackup.

    With foil faced EPS on the exterior doing a cut'n'cobble of XPS would be a mistake, due to the very low vapor permeance & inability of the XPS to wick moisture away from the wood to redistributing the moisture load. On the already built sections with interior polyethylene an air leak to the interior would potentially become a problem, since it's a moisture trap. If you ever re-side the place, bumping the exterior R up by increasing the total exterior EPS thickness to 1.5" would improve it's resilience, 2" would be even better. If you ever pull the interior wallboard in any of those rooms it's worth inspecting, air sealing the framing to the sheathing,re-doing the insulation, and installing MemBrain on the interior in lieu of polyethylene.

    Is the blocking is 2x4 (3.5" ) and the studs are 2x6" (5.5")??

    If yes, buy some R15 rock wool or fiberglass batts, and split off some 2-inchers (shoot for 2.25") to slip in snugly behind the blocking, then carefully fit the R15s between the 2x4 blocking. A sheet of MemBrain on the interior side detailed as an air barrier would at least give it a drying path if it ever gets damp.

  6. GBA Editor
    Martin Holladay | | #6

    I'm sorry that I missed your reference to your location in my first response.

    It's too bad that there are still building inspectors in New Hampshire who insist on interior polyethylene. In fact, all the building code requires is a vapor retarder, not a vapor barrier. In some cases, when the code provisions are shown to a local code official, they are willing to concede this point (once they read the code).

    I agree with Dana Dorsett that your first order of business is determining what type of rigid foam was installed on the exterior side of your OSB. If the rigid foam is a vapor barrier, that's a problem -- because it's a wrong-side vapor barrier.

    Assuming that the exterior foam has a perforated facing, and is somewhat vapor-permeable, I think you can go ahead and insulate the stud bay that you are worried about with ordinary fluffy insulation (for example, fiberglass batts). After all, if the rest of your stud bays are being insulated with fiberglass batts, there is no particular reason to treat this stud bay differently.

    In any case, pay attention to air sealing when you are insulating your wall cavities.

  7. Jon Garbarino | | #7

    Thanks for the advice. I decided to go with un-faced fiberglass and pieced it in.

    As I mentioned this partition wall will be a tile wall - as part of a shower.

    I have read numerous sites regarding the use of a vapor barrier behind shower walls, yet can't seem to find a consistent opinion.

    If I use cement board with an interior impermeable water-proofing - should I forgo the vapor barrier in that area?

    Also, I have been playing with the idea of putting a continuous piece of the 2" thick pink rigid foam on the inside exterior wall of the shower to help with thermal transfer from the studs and better insulation/air barrier in general.

    Good idea or bad idea or what considerations should I make?

  8. GBA Editor
    Martin Holladay | | #8

    You might not want to include a continuous layer of interior rigid foam in your shower area, because you need your cement board to be very rigid. Tile doesn't like movement, so I don't think you want any rigid foam between the cement board and the studs.

    Your waterproofing layer is already a vapor barrier, I'm sure, so there is no need to add another vapor barrier.

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