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Exterior XPS and Tyvek – Will I have moisture problems?

smokey11106 | Posted in Energy Efficiency and Durability on

Sorry I might have posted this as a question in another discussion, because it was somewhat similar. I think its better if I pose this outright considering I’m short on time. I am in the process of doing a renovation currently and I think the contractor might have created a potential problem. The exterior of the house was just finished. The contractor first put tyvek over the old existing plywood sheathing, then installed 1″ xps foam board, then put vinyl siding over the foam. Am I going to have moisture problems now becase of the tyvek? We still havent put insulation in the walls yet, but had planned on R-13 fiberglass faced/backed in the 2×4 bays. Is that also a mistake? Finally, the contractor proposed as an alternative to the fiberglass bat to spray foam the interior side of the sheathing with closed cell foam( half inch) and fill the rest of the cavity with blown in insulation. Does that sound even worse with the closed cell foam creating a barrier between itself and the xps with the sheathing between it, oh and I forgot the tyvek too? Any ideas before I close up the walls? Thanks. So, just to clarify, As it is now am I going to have a problem with the tyvek and xps? If I put in the fiberglass should it be without backing/ vb. Does the foam seem like a good idea. Unfortunately, we are closing the walls in week. I still might have time to do something to correct any problems. Any input would be great.

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  1. Riversong | | #1

    Any of those options will work.

  2. smokey11106 | | #2

    Thats great. So if I go with the half inch of closed cell spray foam then the rest fiberglass(unbacked) in the cavity this is not a problem? Does the foam seem like a good idea? I figured the benefit was more for the the tighter envelope due to foam and less due to the R value of only a half inch, or am I just wating money and stick with the backed fibewrglass R-13?

  3. GBA Editor
    Martin Holladay | | #3

    I just posted this answer on the other page. But since you asked twice, I will answer twice.

    1. As usual, I have to remind questioners that it is often hard to answer your question if you don't provide a location or climate.

    2. Whenever you install exterior foam insulation, you have to be sure it is thick enough. To find out whether your contractor chose thick enough insulation, we need to know your climate. Find out more here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    3. There's nothing wrong with the Tyvek; your contractor did the right thing by installing Tyvek over plywood. Tyvek is vapor-permeable, so it doesn't trap moisture.

    4. Installing spay foam on the interior of your plywood is probably a bad idea; you don't want to create a foam sandwich with plywood in between, or the plywood won't be able to dry out if it ever gets wet. The best insulation to use in your stud bays is probably dense-packed cellulose, although carefully installed fiberglass batts will also work.

  4. smokey11106 | | #4

    Thank you martin. Sorry I didnt post the climate. I live in New York so most of the system will be dedicated to heating. If I go with the fiberglass should it have the paper backing or without? Would a vapor barrier be almost as bad as the foam on the interior? It's very confusing. Is there concern about the dew point? Thanks.

  5. GBA Editor
    Martin Holladay | | #5

    I still don't know your climate zone. New York has two zones: 5 and 6.

    If you live in climate zone 5, the thickness of your exterior foam is adequate. If you live in climate zone 6, your exterior foam should have been thicker (R-7.5 minimum R-value). You guessed correctly: the reason for the thicker foam is to be sure that your sheathing is above the dew point.

    You don't need a vapor barrier; interior polyethylene would be a mistake. If your foam is thick enough, you can get by with latex paint as your vapor retarder.

    Kraft-faced batts probably won't do any harm, but unfaced batts make more sense.

  6. Riversong | | #6


    A field for questioners to enter their geographic area and climate zone are still notably absent from the short list of website improvements. It would also be helpful to have an easy-to-find graphic of the US climate zones that can be linked to from the Ask a Question page.

  7. GBA Editor
    Martin Holladay | | #7

    Great suggestions.

  8. Riversong | | #8


    If you're (rightly) concerned about trapping moisture with a double vapor barrier, and you want to air seal the house as well as possible, you might consider a flash coat of open-cell foam, though a ½" of closed cell will still allow some moisture diffusion and is not likely to cause a problem, particularly given that you have plywood - and not OSB - sheathing. I would not install more than 1" of interior foam, however. The less the better in this application.

    And I would avoid any facing material on the fiberglass batts (if you must use batts), since it only makes it more difficult to do a good installation. Unfaced batts are a little wider, for a friction fit, and so tend to fill the stud cavities better, also making it easier to see how well it fits.

  9. smokey11106 | | #9

    Sorry about the lack of zone information. The house is in westchester NY and zoned for 4a, but am on the border of 5a, so my 1" of xps on the exterior looks good in regard to the dew point issue, correct?. I was hoping to to get the house air tight, but do worry now that I cant accomplish that. If I went with the flash coating of open cell foam or no more than 1/2" closed cell foam with no interior vapor barrier would that be acceptable? How do you see that playing out as Robert stated ? You guys are great by the way!
    I dont have a rain shield between the tyvek and the xps.

  10. GBA Editor
    Martin Holladay | | #10

    Did you see my last post? Perhaps you didn't understand it?

    What makes you think that you can't get your house very close to airtight?

  11. smokey11106 | | #11

    Sorry Martin, I do and I dont.. know how to get it almost air tight and neither does my contractor or the people pushing there products in my area. I'll go with dense pack cellulose or carefully placed unfaced fiberglass batting. I'm using 5/8 sheetrock on the interior walls and using air tight electrical boxes and regular latex paint on the walls. I'm also using pliable window spray foam between window frame and studded openings. Any other suggestions? How about the roof? I have ridge vents and soffit vents in the attic. Dont mean to be a pain, just confused.

  12. Riversong | | #12


    Densepack cellulose is far more air tight than fiberglass batts, and would not require the foam "flash".

    The most important places to air-seal, however, are the top and bottom of the house: the basement, mudsills and bandjoists and the upstairs ceiling (which is typically easier to seal from above after all existing insulation is removed by a thin layer of spray foam - or by piling up cellulose on top of whatever is already there). Pay special attention to any mechanical penetrations.

  13. GBA Editor
    Martin Holladay | | #13

    The answer to air sealing is careful attention to details.

    Think it through. Know where your air barrier is. Consider each transition from one material to another as a potential leakage path. Choose from among available air sealing materials, including gaskets, caulk, and canned foam.

    Hire a contractor to perform a blower-door test. If it's your first blower-door test, tell the contractor you intend to perform some blower-door-directed air sealing work. Ask the blower-door contractor if he or she can help you with this goal.

  14. smokey11106 | | #14

    10-4 I'm getting it a little more. So keeping the wall as air-tight as possible yet permiable, right? If I go with the dense-pack cellulose and try to keep the wall as air tight as possible. My only issue is how do I get past the code which I believe requires me to have a vapor barrier on the interrior? Stupid idea, but could I put Tyvek facing inward over the insulated bays? I know, stupid.

  15. Riversong | | #15

    Code does not require a warm side vapor barrier, only a 1 perm vapor retarder like Benjamin Moore Super Spec primer.

    And you're still confused about housewraps like Tyvek, which are highly vapor-open. The are weather barriers (wind and rain) not vapor barriers.

  16. GBA Editor
    Martin Holladay | | #16

    I have never seen a residential building code that requires a vapor barrier. In some climates, however, a vapor retarder is required. To learn the difference between a vapor barrier and a vapor retarder, and to learn more about the code, see Vapor Retarders and Vapor Barriers.

  17. smokey11106 | | #17

    Well I have to deside now. Unfortunately, I didnt find about this sight until it was too late, or was it just in time. I will consider the dense-pack cellulose. First, I will try to have the seems on the interior side of the sheating caulked inside the bays. Is that helpful and if so, what type of caulk is good? Next, should I have the cellulose installed while wall is open with the netting or wait till drywall is up for better packing, if I understand correctly? Third, Do I then caulk all the 2x4 stud wall surfaces to which I then place the drywall against to prevent any air movement or gaps between the drywall and 5/8th drywall. Like I'm gluing the drywall to the studs? Use flange type oulet boxes (any type of other gaskets for the outlet?). Once completed is there any other caulking or sealing around the drywall other than the usual taping and skim coating? Oh...and the the primer for the vapor retarder, correct? Anything else? Thanks again.

  18. GBA Editor
    Martin Holladay | | #18

    1. It's hard to air-seal wall sheathing from the inside. (The seams should be located over the framing, so you can't see the seams.) If you can see any horizontal sheathing seams spanning the studs, you can tape those with housewrap tape or peel-and-stick tape.

    2. If you really want to air-seal the sheathing from the inside, you can consider using the new Owens Corning EnergyComplete system. This system uses a latex spray for air sealing the sheathing from the interior.

    3. If you aren't sure how to install the cellulose in your walls, you should talk to your insulation contractor. Some contractors prefer damp-spray cellulose, while others prefer to install cellulose behind netting or to blow the cellulose through holes in the drywall. You need to have a conversation with a contractor to determine the best way to proceed.

    4. If you want to follow the Airtight Drywall Approach (ADA), you don't have to caulk every stud. You just have to use gaskets or caulk at critical locations to prevent air leakage. To learn more about the ADA, see Airtight Drywall.

  19. smokey11106 | | #19

    Thanks, Martin. I think I will look at the Dense-pack. I have two installers coming out tomarrow. Contractor said he will detail as I say. Saw something interesting on something called Air Krete. Any opinions about it. Sounds kind of pricey even more than spray foam if I'm correct. The cellulose seemed a little expensive too. What do you ,think the average going price would be per board foot for dense-pack? Thanks again.

  20. Eric Schroeder | | #20

    Martin, I have a couple of areas in the walls that are unevenly shaped and might be hard to fill with cellulose. Would Air-Krete against the inside sheathing be just as bad as the open cell in my situation? In seems to be permiable, but there are no values for that as far as I know.

  21. Riversong | | #21


    Cellulose will fill every space if it's properly installed. Just make sure that, if it's blown after drywall, that all odd spaces are marked and drilled for filling.

    An excellent way to blow after drywall, is to staple a 12" wide strip of Insulweb or filter fabric horizontally across the center of each wall, leave a 4"-6" horizontal gap between the upper and lower sheets, blow through this gap, and then fill the gap with 3/8" (or one size thinner) DW for a seamless finish. Additional holes will have to be drilled above and below windows or for other odd spaces.

  22. Eric Schroeder | | #22

    Thanks Robert. I'm probably having a cellulose contractor do it for me. Our town is very stringent. They will not allow the cellulose to be blown in behind the drywall, after the drywall is put up. The village has to inspect the walls and insulation before the drywall goes up. I guess the insulweb across the studs is a must. One contractor stated that no matter how tight the web it still bulges and 1/2" drywall might be a problem, because of that. I'm installing 5/8" anyway and according to him extra screws to prevent any popping. While we're still on the topic can you reccomend a good and resonably priced Polyurethane caulk thats low or No VOC for sealing the drywall, and what are your thoughts on Air-Krete. I dont want to upset Martin, but if Air-Krete were used on the interior of the sheathing is that just as bad as sandwiching the sheathing between open cell and the exterior XPS? From what I have read it seems pretty permiable. Thanks.

  23. Riversong | | #23

    I would fight that policy. Even the VT Energy Star program allows blowing behind drywall, since it's the best way to assure good density and requires no more than ½" drywall, even with framing on 24" centers.

    If you want to seal the drywall to the plates and window rough openings, you need to use an acoustic caulk, which doesn't harden. I've always used Tremco butyl caulk and Lescco polypans. OSI makes a GreenSeries latex acoustical caulk, but I've never used it.

    Air-Krete is a flow-in non-expanding cementitious foam for retrofitting. It cannot be installed in an open cavity.

  24. smokey11106 | | #24

    Thanks , Robert I'll check out the acoustic caulk. As far as the Air-Krete, one of the videos on there sight shows them blowing the mix through stapled mesh on open studs, also saw video of them where they used tyvek with slits on interior for better curing. An authorized installer also said no problem for open stud walls before the drywall goes up, but needs 3 to 4 days to dry. It must still be a bad thing if my sheathing is between the exterior XPS and this stuff, right?

  25. Riversong | | #25

    Air-Krete does not list a perm rating and apparently has never been tested for permeance.

  26. smokey11106 | | #26


  27. smokey11106 | | #27

    This spam filter is killing me, sorry.

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