GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

With Zip board as sheathing/WRB, any danger Intello Plus behind drywall will act as 2nd vapor retarder?

ERIC WHETZEL | Posted in Green Building Techniques on

Our wall assembly will be the following (interior-exterior):

5/8″ drywall
Intello Plus vapor retarder
2×6 studs filled with Roxul batts
Zip board sheathing
4″ Roxul comfort board (2″ + 2″)
1×4 battens
1×6 cedar siding

We’re shooting for a Passive House level of air-tightness, so is there any risk the Intello and Zip board sheathing could trap moisture in the wall assembly?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. user-2310254 | | #1

    Eric. Where are you located?

  2. ERIC WHETZEL | | #2

    Hi Steve.

    I'm in the suburbs of Chicago --- climate zone 5, so we have cold, dry winters and hot, humid summers.

  3. Expert Member
    Michael Maines | | #3

    Eric, Intello opens up to 13 perms or so when the relative humidity exceeds 60%, so as long as your walls are reasonably airtight, you shouldn't have any issues. In fact it's just the kind of think Intello is made for.

  4. ERIC WHETZEL | | #4

    Michael --- Thank you.

    Is there any sense in which the Intello in our wall assembly and climate zone could be construed as redundant or unnecessary?

    We've been intending to use the Intello for awhile, but recently it was suggested to us that it might be overkill, if not potentially risky.

    Based on what I've read, it seems like it's worth the effort/cost to install.

  5. user-2310254 | | #5

    Eric. I'm sure Martin will chime in with some great advice. In the meantime, you might want to read about the "pretty good house" approach. I may be more achievable than shoot for Passive House (.6 ACH/50). See this link for more information:

    Be sure to also check out all the related links as well.

    You should also read this article to learn more about optimal wall assemblies. You can boost the performance of a 2x6 wall by incorporating exterior rigid foam (and maybe get to R-40 Passive House level of performance if that's what you really want).

  6. ERIC WHETZEL | | #6

    Thanks Steve!

    Our house will be a blend of passive house science, the pretty good house concept, and hopefully net zero if we can afford solar on the roof. It will be a single story home with a full basement, so apart from the .6 ACH benchmark (it's a number we're shooting for), we're likely to miss the other PH requirements.

  7. Expert Member
    Michael Maines | | #7

    Eric, as long as the ZIP sheathing is airtight, then the Intello probably is overkill in your assembly. I think it's good insurance, but painted drywall is enough to block vapor diffusion, and you won't have air leaks if you're getting anywhere near 0.6 ACH50 with your sheathing.

  8. ERIC WHETZEL | | #8

    Thanks Michael!

    In regards to air-sealing and insulation, we're trying to do a "belt and suspenders approach" wherever we physically/financially can do so. Therefore, even if the Intello is overkill, I'm still tempted to incorporate it, if only as "good insurance", as you suggest. But it is nice to know we might be able to forgo it without horrible consequences.

    Thank you for the responses --- it means a lot!

  9. Jon_Lawrence | | #9


    Also, ZIP is not a vapor barrier. The OSB perm rating can vary from manufacturer, but BSC lists it 2. Huber says the OSB panel overlay has a perm rating of 12-16, so let's call the ZIP vapor semi-permeable. With Roxul on the exterior and interior, vapor will flow through and will not get trapped. If you were to put Intello on the inside, you will not create a vapor sandwich. If you added exterior foam to increase the R value as Michael suggested, then Intello may actually be needed because that wall won't dry to outside and you want to keep moisture from getting into the wall from the inside.

    Some folks believe that the more air barriers the better. Intello is an air barrier, in addition to a smart vapor barrier, so if you are looking for ways to reduce air flow and beat that .6 ACH, then this could help. Also, you are in PHIUS country and in a heating dominated climate. The heating demand limit for Chicago is 5.8 Kbtu/sf under PHIUS and 4.75 under PHI. If the size of the house in less than 10k sf, then the .05 CFM under PHIUS will be easier to reach than the .6 ACH under PHI. But again, is Passive house what you are shooting for or is it Net Zero or just really comfortable?

    Since you will have a full basement, you may also want to take advantage of a drain water heat recovery system such as Pro Pipe. HPHW systems are also popular, but there is an argument to be made that those should only be used in cooling dominated climates because they emit cold air, which you don't want in your climate in the winter. An expensive and new to the US option is a Sanden CO2 HPHW which has an outdoor condenser so it won't cool your basement in the winter. I am in a heating climate and I plan on using a Sanden in my house. When I ran my WUFI model, switching from a standard HPWH to a Sanden reduced my heating demand much more than it raised my cooling demand.

  10. ERIC WHETZEL | | #10

    Jonathan --- thank you for the detailed answer. I really appreciate all the information.

    It will be a relatively small home, just under 1600 sq. ft. of interior space, not including the full basement.

    I like the idea/theory of a PH level of air-sealing, so that's why we're shooting for the .6 ACH, and we're in the ballpark in terms of a PH level of insulation (R 16-20-38-64; slab to attic), but as you point out, it's really about durability and comfort in the end, so we're not obsessing about reaching the other PH requirements.

    We also like the idea of Net Zero, and it might be possible financially, so we're excited about the possibility of living in an all-electric, utility free home with a PV array on the roof.

    I will definitely look into the Pro Pipe system you mentioned.

    I think I saw the Sanden CO2 HPHW system in a Hammer and Hand video. I will definitely check and see if it's something we could afford financially. Our PH consultant did mention the HPHW in the basement might be an issue in the winter, as you pointed out.

    Thanks again for the info and ideas.

  11. GBA Editor
    Martin Holladay | | #11

    The short answer is that the Intello Plus membrane will do no harm. I don't think it's necessary from a vapor-control perspective -- all you need is vapor retarder paint.

    If you want to improve the airtightness of your wall assembly (after taping the seams of the Zip sheathing), I think that it makes more sense to simply tighten up the drywall layer than to add an expensive membrane behind the drywall. Pay attention to the perimeter of the drywall field and electrical boxes. (These are standard details for the Airtight Drywall Approach).

    The vapor permeance of Huber Zip sheathing is 2 to 3 perms.

  12. ERIC WHETZEL | | #12

    Thank you Martin, for weighing in. Really appreciate all the info everyone offered. Thanks!

  13. Expert Member
    Dana Dorsett | | #13

    Of course Intello Plus behaves like a vapor retarder, but it's vapor retardency varies with the relative humidity of the cavity air next to it. At mold-inducing levels of moisture inside the cavity it will be sufficiently vapor open to dry toward the interior.

    But when the sheathing is cold, the RH of the cavity air next to the Intello is quite cold, since the moisture in the cavity air becomes adsorbed into the ZIP, which making the Intello much more vapor tight.

    You do NOT need vapor retarder paint here (which would actually impede drying) , nor do you need Intello (which at least does no harm.) Standard latex paint on reasonably air tight drywall will do.

    In climate zone 5 with R16 rock wool on the exterior of the ZIP you already have more than sufficient dew point margin at the sheathing keeping it warm enough protect it from excess wintertime moisture uptake using only standard interior latex paint for the interior vapor retarder. (You have more than 2x the exterior R that the IRC prescribes for 2x6 framing in that climate!)

    AND you have the 3/4" air gap between the highly vapor permeable rigid rock wool and the siding for the ZIP to dry toward the exterior. Either one of those alone would have been sufficient to protect the sheathing from mold & rot.

    Do you really need triple redundancy here?

    The moisture management priority should be (as always) getting the flashing details right, followed by (a truly distant second at your dew point margins) air tightness of the sheathing and wallboard.

  14. GBA Editor
    Martin Holladay | | #14

    In many cases it's easier to install vapor retarder paint than it is to explain to a code official why it's unnecessary.

  15. Expert Member
    Dana Dorsett | | #15

    True, that, and half-perm paint is a heluva lot cheaper than Intello.

    I'd personally be inclined to explain it to the code official first, since with R16 outside the sheathing it's not even close to being necessary. And in this case half-perm paint is not neutral- it would slightly reduce rather than enhance the overall resilience.

  16. charlie_sullivan | | #16

    Maybe we could convince that company that makes a bogus insulated housewrap to make an equally bogus "vapor retarder" and get it certified by the phony lab they use, so that builders can use it in these situations to satisfy code officials without actually having any impact on the moisture transport in the building.

  17. ERIC WHETZEL | | #17

    What about the notion that ideally 2/3 of all insulation in the wall assembly should be on the exterior side of the air barrier (Zip board in our case)? I'm assuming this approach/theory comes out of WUFI - PHPP energy modeling?

    Since slightly more of our insulation (5.5" in the studs) will be on the interior side of the Zip (vs. 4" exterior), no serious moisture implications without the Intello?

    Is the 2/3 approach based on the idea that thicker insulation on the exterior side of the air barrier means warmer sheathing in winter, therefore less chance of moisture/condensation getting behind the Zip?

  18. GBA Editor
    Martin Holladay | | #18

    The rule of thumb you are talking about -- the "2/3 and 1/3 rule" -- was an old Canadian rule of thumb to guide people trying to figure out where to put a polyethylene vapor barrier. It was always a crude rule of thumb.

    A more sophisticated approach for anyone who plans to install exterior rigid foam (which prevents a wall assembly from drying outwards) is to make sure that the first condensing surface (either the interior face of the wall sheathing or the interior face of the rigid foam) stays above the dew point during the winter. This is done by making sure that the exterior rigid foam has an R-value that meets certain minimum percentage requirements that vary by climate zone. For more information, see Combining Exterior Rigid Foam With Fluffy Insulation.

  19. ERIC WHETZEL | | #19

    Thank you for the clarification, Martin.

    And thanks again to everyone who submitted comments!

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |