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Community and Q&A

Poly on interior wall

CTSNicholas | Posted in Energy Efficiency and Durability on

I was talking to a builder who said the key to having an energy efficient interior is to use 6 mil poly on the walls before installing drywall. He said you carefully tape around all outlets and seal off the holes.

We are in Zone 5, and although that sounds like it would be a great air barrier, I fear I would be making a mistake to follow his advice.

I am planning a house build with 2×6 studs and the following details. I will use a cheap vinyl siding, good tyvek house wrap, 3M tape on the seams, 5/8 OSB, 2×6 studs, mineral wool or fiberglass rolled insulation, then drywall.

I would like to install 2″ of rigid foam some day when I decide to replace the cheap vinyl siding once I have the savings for it.

Should I use poly on the interior if I have Tyvek on the outside?

The other question is, if I plan on using 2″ rigid foam in the future on the exterior wall, won’t that cause an issue if I do use poly on the interior side?

Lastly, when installing 2″ rigid foam, would it be an issue to have the tyvek housewrap under instead of over the rigid foam? I didn’t think it was an issue, but want to make sure.

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  1. davidmeiland | | #1

    No poly! Whoever told you to use it doesn't understand air sealing or vapor movement. I would upgrade the sheathing to Zip or CDX plywood. A lot of people put the wrap under the foam but I like it over.

  2. iLikeDirt | | #2

    Poly vapor barriers: the mistake that just won't die.

  3. CTSNicholas | | #3

    So I assume I am correct to not use a poly vapor barrier for the walls. Same goes for the ceiling, right? The attic will be vented.

  4. iLikeDirt | | #4

    Correct. Seal your drywall as the air barrier instead. Also, if money is tight and you're planning to install rigid foam later, you might consider downgrading to 2x4 walls and use the money you'll save toward your foam budget. Foam outside the sheathing is going to make a bigger impact than more mineral wool between the studs. The best bang-for-buck in that department will likely be a bunch of EPS foam, in which case, you should go thicker than 2." Might be so cheap that you can afford to do it now if you use 2x4s.

    And keep in mind that applying foam after the house is finished is going to be more of a pain than before. Consider how you're going to handle the windows, doors, soffit vents, bump-outs, electric meter, exterior outlets, etc.

  5. GBA Editor
    Martin Holladay | | #5

    The key to a good wall or good ceiling is having a good air barrier. Worry about air leaks; don't worry about vapor diffusion.

    Here are links to three articles on vapor barriers:

    Vapor Retarders and Vapor Barriers

    Forget Vapor Diffusion — Stop the Air Leaks!

    Do I Need a Vapor Retarder?

    If you decide to install rigid foam on the exterior of your wall, the housewrap can either go under the foam or over the foam. For more information on this issue, see Where Does the Housewrap Go?

    I agree with Nathaniel: If you think that you will install rigid foam in the future, it's a lot easier to do now than it will be later.

  6. CTSNicholas | | #6

    Alright, sounds like I won't touch any vapor barrier system. Thanks for the articles. It does lead to a question about insulation. Fiberglass with or without face? Would it be wise to use unfaced fiberglass, especially if rigid foam is planned for the exterior?

    Next to the air sealing, from the articles it's obvious any holes need sealed around. What above a bead of caulking along the drywall joints, not every butt joint, but more so along the ceiling where it meets the top plate?

    The house size is 44x30 with a 26x36 attached garage. A bit of rigid foam, but I may talk myself into putting it on a 0% APR credit card if my budget doesn't allow it in the build to pay off later in increments.

    Otherwise if I don't do rigid foam for now, could I 'prep' for the foam in a way, such as having 2" deep window and door jams temporarily with the windows installed 2" further out to account for future foam depth?

    I really like the extra room in 2 x 6 walls, the cost difference between the two on a single story (+concrete basement) would not seem to be much difference.

  7. Expert Member

    I'm not recommending the OP use poly but I think the case against it is a lot more nuanced than posters here generally accept. In a climate without cooling loads, where the walls are designed to dry to the outside, what is the downside of using poly as a combined air and vapour barrier?
    It has some very real advantages over airtight drywall in that it can be installed, inspected, tested and problems remediated before the drywall is put on. Many intersections, like that at the floor/sill plate and openings for plumbing and electrical are much easier to seal. It is continuous over the entire ceiling avoiding leakage at interior walls.
    The other advantage it presently has in areas where it is widely used is that people generally recognize it as an air or vapour barrier in a way they don't when working on a house where some other element serves that purpose. So a future owner or renovator doesn't have to wonder whether the sheathing has some other role than structural.
    Sure smart membranes or other air sealing strategies can offer a better solution but I bet the contractor who suggested it did so because the local alternative is to eliminate the poly and not use any alternative air sealing strategy in its place. It might also be worth remembering that the tightest house so far tested in the world used poly to achieve its goal.

  8. kshenefiel | | #8

    Nicholas, Interior polyethylene can trap the inward flow of water vapor in summer in locals with hot humid summers. This is a cause for some concern in the more humid parts of zone 5a especially with growing AC usage but is not a concern in the less humid zone 5b. And not much of a concern in the far northeastern or western portions of zone 5a. The vapor permeability of poly is excessively low, and it has draw backs as an air barrier as well.

    Adding 2 inches of foam to the exterior of a 2x6 wall poses some problems. Two inches of XPS or polyiso will inhibit or block outward drying of the wall yet provide inadequate insulation proportion to insure good inward drying. Some siding requires firring to the exterior of more than 1” of foam. The cost of modifying all the widows and doors will make adding insulation when you upgrade siding unlikely, unless you pad out the windows and doors during the original construction to accommodate the thickness of the planned insulation. Windows set for 1 inch of insulation to be added later could look nice but set for 2” would look rather awkward.

  9. CTSNicholas | | #9

    I worried about that. We seldom have wet summers...but they do happen.

    Would 1" be a better choice of XPS or EPS instead of 2" It would be cheaper for me to do only 1". I wasn't sure why I thought I needed 2". Something about a certain R value in proportion to how much interior insulation there is. (planned minimum of R19, possibly R23 with mineral wool in a 2x6 stud cavity)

  10. iLikeDirt | | #10

    Code says with 2x6 walls in zone 5, you need R-7.5. That means at least 1.5" of foam, possibly 2 if you use EPS.

    If you go down to 2x4s you only need R-5, or 1" XPS...

  11. GBA Editor
    Martin Holladay | | #11

    You wrote, "In a climate without cooling loads, where the walls are designed to dry to the outside, what is the downside of using poly as a combined air and vapour barrier?"

    You have listed the issues that are concerning. If someone builds a house in Climate Zone 5, can the builder guarantee that the house will never be air conditioned? No.

    Will the walls of this house be designed to dry to the exterior, for the life of the house? Maybe -- but future homeowners may be tempted to install exterior rigid foam.

  12. CTSNicholas | | #12

    I feel it would be counter intuitive to reduce down to 2x4 walls to be able to use 1" of Rigid. In this case, 2x6 walls with NO rigid would be more cost effective. More R value in stud bay, less R value from thermal bridging, all equaling about the same but minus the cost of any Rigid Foam.

    I'd be concerned 1.5" to get R-7.5 would be risky and 2" would be the insurance. I did plan on using single sheets 2" thick. It would cost me 33% more to use two 1" sheets of Rigid Foam. Am I shooting myself in the foot by not using two pieces with staggered joints?

  13. Expert Member
    MALCOLM TAYLOR | | #13

    Martin, I prefaced my general remarks by saying "I'm not recommending the OP use poly". While the industry transitions to new air sealing techniques, I think there are still circumstances where poly provides a good alternative. The PNW climate with the universal use of rain screen being one of them. I don't think a general interdiction against poly is useful at this point.

  14. GBA Editor
    Martin Holladay | | #14

    1. Polyethylene is extremely useful under slabs and to cover the dirt floor of a crawl space.

    2. Interior polyethylene can also be used on the interior of walls and ceilings in very cold climates (northern Ontario, northern Quebec, northern Manitoba, interior of Alaska) where air conditioning is never used.

    Elsewhere, use vapor barrier paint, kraft facing, or a smart retarder like MemBrain.

    3. Air barriers are useful in all climates.

  15. CTSNicholas | | #15

    Martin, I was hoping I could bug you about the earlier two questions I had.
    1.) It's cheaper to use a single 2" thick sheet versus (2) 1" thick sheets of foam. Will I be okay using just a single sheet, or is it worth the extra 30-35% in cost for the ability to overlap joints?

    2.) Should the fiberglass, (if chosen over mineral wool), be faced or unfaced assuming there will be rigid on the exterior?

  16. GBA Editor
    Martin Holladay | | #16

    Q. "Will I be okay using just a single sheet, or is it worth the extra 30-35% in cost for the ability to overlap joints?"

    A. Yes, you will be OK -- although two layers with overlapped joints is better. Only you can decide whether the additional cost is worth it.

    Q. "Should the fiberglass, (if chosen over mineral wool), be faced or unfaced assuming there will be rigid on the exterior?"

    A. Either will work.

  17. Dana1 | | #17

    "I feel it would be counter intuitive to reduce down to 2x4 walls to be able to use 1" of Rigid. In this case, 2x6 walls with NO rigid would be more cost effective. More R value in stud bay, less R value from thermal bridging, all equaling about the same but minus the cost of any Rigid Foam."

    A code-min 2x6 /R20 16" o.c. wall with wood siding and half-inch gypsum comes in at about R14 "whole wall" after factoring in the thermal bridging of the framing.

    A 2x4 with the same siding & interior materials comes in at about R10 whole-wall. Add an inch XPS and your're at about R15 whole-wall only slightly better than the 2x6

    If you add 2" of EPS to the 2x4 wall you end up with R18+ whole-wall performance at the same wall thickness as a 2x6 wall, but with substantially better performance, and excellent dew-point margin at the sheathing.

    With 2" of EPS (R8.4 if Type-II /1.5lb density foam) on the 2x6 wall you'd be looking at R22+ for whole-wall performance, and would still have some dew point control margin at the sheathing.

    Batts with foil facers are to be avoided, but kraft facers are OK. Facers make the quality of the installation a bit harder to inspect than with unfaced. Rock wool is preferable to fiberglass on a fire-resistance and overall lower environmental impact basis, but it may cost substantially more in some locations. R21 fiberglass is still a very good product. Blown/sprayed cellulose would be even better, and helps protect the structural wood by sharing the moisture burden, wicking moisture away from the wood (without loss of function or damage to the cellulose itself, within reasonable limits.)

    Note: In a zone 5 climate if the siding is back-ventilated, even the bare-bones 2x6 wall (no foam) does not need an interior side vapor retarder. If you added a skinny 1" of rigid rock wool to the exterior you'd come in around R18 whole-wall, and still not need an interior side vapor retarder, since rigid rock wool is highly vapor permeable at any thickness (more than 25 perrms- more vapor permeable than some housewraps.)

  18. CTSNicholas | | #18

    Mr Dorsett,

    From your post, I understand how even 1" on 2x4 walls would benefit me.

    I definitely plan on using 2 x 6 construction. I would use R21 Kraft Faced Fiberglass. With code, it is determined I would need 2" of EPS/XPS foam. (Planning on Dow Blue Board Extruded polystyrene since it is sold locally).

    I will use a Tyvek or Typar housewrap and tape the seams. I will also plan on using 3M tape on the foam board joints if I do opt to get foam board.

    Should I plan on taping the plywood or OSB joints prior to house wrap?

  19. Dana1 | | #19

    Dow XPS is currently blown with a heavy fraction of HFC134a, which has a 100-year global warming potential of about 1400x CO2. As it leaks out the thermal performance takes a hit too, and by ~50 years it's performance will have fallen to that of EPS of equal density. The vapor permeance at 2" is about 0.6 perms, which while it isn't a true moisture trap, it inhibits the drying rate toward the exterior.

    EPS is widely available in a number of densities and from multiple vendors. I'm sure you can find a local source (maybe even a local manufacturer.) In this application 1.5lb "Type -II" density goods would be the right choice. At 2" it's tested-labeled performance is R8.4, but it has a but more than twice the drying capacity- well over 1 perm, but not more than 1.5 perms. This gives it pretty reasonable drying rates to the exterior.

    For the additional and temporary R1.6 that 2" XPS would deliver over 2" EPS, the environmental hit doesn't seem "worth it". Sheet EPS is also usually about 26-30% less expensive than XPS at any given R-value.

    Taping foam-board does not provide a long-term air seal, due to seasonal dimensional changes of the foam and poor adhesion. Some have taken to putting duct-mastic over the tape to keep it from peeling over time, but there isn't a long history on that method, hard to say for sure if it'll still be air-tight in 50 years.

    If the housewrap is going between the foam and sheathing, it's worth using a crinkle type of material such as Tyvek DrainWrap, which provides a bit of capillary break and allows bulk water directed onto the housewrap by the window flashing to actually drain rather than spread out, clinging by surface tension to the foam and ending up in the sheathing. If the housewrap & flashing are going to be on the exterior side of the foam, between foam & siding the flat housewraps are fine.

    Taping the sheathing seams works well if you use a high quality acrylic latex primer on the wood as a bonding surface for the tape adhesive.

  20. CTSNicholas | | #20

    Menards is the only supplier nearby. A search for EPS only yields what appears to be Foamular XPS. There is a white generic brand of syrofoam that looks like the stuff used in packaging and is about 60% cheaper than Foamular and DOW. I can't find specs on it that dictate if it is EPS or XPS.

    Edit: It appears to be EPS. There is not much information on it. Here is the link:

    I will have the housewrap between the foam and wood sheathing. I will choose a wrap that offers the ripples used for drainage. Perhaps using a foam safe caulking to seal the joints rather than tape or ducting mastic would be a better option? I have seen specific tapes used by installers on all butt joints. I suppose over time the tape loses any adhesion from hot/cold shrinkage.

    I will have to look into a primer for the wood sheathing.

  21. iLikeDirt | | #21

    "Styrofoam" is a trade name for EPS. That product is "Expanded Polystyrene"; that's what EPS stands for. :) I'm jealous of your having a Menards nearby! I love that place.

  22. CTSNicholas | | #22

    They are 70 miles away, but well worth the drive. We don't have a lowes or home depot, but Menards has really good sales. I will have to look into their EPS product. One thing I feel like Menards lacks is knowledge over their products. Sometimes I catch someone witty, the next time I am am the one that is witty. :)

    Builders around here stick their nose up to using any form of rigid foam on the exterior. So I am left to learn about the product through here and similar webpages.

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