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Convince me to pay for Intello over poly

lance_p | Posted in General Questions on

I understand the attraction of an intelligent Vapor Retarder like Intello, Membrain or Majrex allowing walls to dry to the interior.  I’ve done a ton of reading and was convinced I’d use Intello on my build.

However, I’m not building an average house.  I have a 12″ double stud wall with dense packed cellulose and an interior 2.5″ service cavity.  My vapor retarder will go between the 12″ cellulose wall and the service cavity.

The house is designed to avoid penetrations in the vapor retarder.  Only the bare minimum of penetrations will be made, and those that will be made will be done in planned areas that are easy to access and properly seal.  I will be doing or overseeing all the work myself.

The “wet sheething” issue in thick walls seems to stem mostly from interior moisture getting into the walls and not being able to properly dry.  The attraction of the variable retarders is that the moisture can dry to the interior (as well as the exterior, ideally).

Since no variable retarder is as vapor tight as good old “Super Six” poly, using poly will keep more moisture out of the wall than a variable retarder will.  Since my build strictly emphasizes having a minimum number of properly detailed penetrations in my vapor retarder, I’m starting to think I might be better off just sticking with Poly?

The house has 36″ roof overhangs all around, and we plan to install a steel roof.  I don’t anticipate exterior water leaks being an issue.

Tell me I’m wrong!  Intello is sexy stuff and I’d love to justify its use, but I’m having a little trouble justifying the (considerable) extra expense. CZ 6A

Replies

  1. Jon_R | | #1

    Definitely avoid poly if you are ever going to run AC. Even if not, Class II is recommended . If not Intello, then Membrane or vapor retarder paint (which is surprisingly "smart").

    There are many other factors that contribute to double stud wall risk. Interior side air sealing, cellulose, exterior perms, plywood, ventilation gap, humidity, etc.

    1. lance_p | | #3

      I understand concerns about AC use and interior vapor barriers, but with a well detailed exterior sheething-based air barrier I don't see much air movement happening through a dense-packed wall. Our relatively hot and fairly humid summer only lasts 6-8 weeks, and our heating season lasts 7 months (Ottawa, Ontario).

      I know many builders think differently, but I can't bring myself to think of drywall as anything more than a fragile cosmetic coating. I will not plan to rely on it as part of my enclosure.

      A ventilation gap between the siding and sheething is also planned.

  2. Aedi | | #2

    Hi Lance,

    I believe you are correct in stating that a variable vapor retarder is unnecessary, but there may be better ways incorporating an air/vapor barrier than poly.

    But first, even in the worst case scenario of a long heating season with prolonged periods of cold (and perhaps wet) sheathing and minimal vapor barriers, double-stud construction usually holds up fine. The reason, it is hypothesized, is the ability of the cellulose to absorb and redistribute moisture, which keeps the sheathing and frame dry enough to prevent damage and mold, and the added borates which further discourage mold growth. See this study from Building America, where a wall was tested with only latex paint as the vapor barrier:

    https://buildingscience.com/documents/bareports/ba-1501-monitor-double-stud-moisture-conditions-northeast/view

    Even though the walls reached moisture conditions that would imply failure, when disassembled the walls showed little evidence of water damage or mold growth. See also Martin's overview of available data/studies:

    https://www.greenbuildingadvisor.com/article/monitoring-moisture-levels-in-double-stud-walls

    Using poly as an air barrier should lead to better results, as the moisture prevented from reaching the sheathing will far outweigh the minor benefits from more drying potential. However, there may be other ways to build the wall safely, as rigid products generally make better air barriers. Joe Lstiburek prefers this assembly, incorporating plywood as an air barrier:

    https://buildingscience.com/documents/enclosures-that-work/high-r-value-wall-assemblies/high-r-value-double-stud-wall-construction?topic=doctypes/enclosures-that-work/etw-high-r-value-enclosure-assemblie

    he defends his choice in this article:
    https://www.energyvanguard.com/blog/77996/Joe-Lstiburek-s-Ideal-Double-Stud-Wall-Design

    Including this especially relevant passage:

    "Some builders in cold climates put a layer of 6 mil polyethylene inside a double-stud wall instead of the OSB or plywood that Lstiburek prefers. I asked Joe for his thoughts on that technique: 'In terms of the physics, it's great. In terms of practicality, it's a very fragile way of doing things. That's why I like sheet goods. We did this in the early '80s and it was difficult to build.'"

    One last thing I will note is that a rainscreen gap is a very good idea for any double stud wall.

    Edit: Also, recent research has cast doubts on OSB's ability to act as an air barrier, so go with plywood if you intend to follow Joe's assembly.

  3. Expert Member
    MALCOLM TAYLOR | | #4

    Lance,

    I agree with Aedi's arguments. The fragility of the poly is offset by the location you have chosen.

    1. Expert Member
      Dana Dorsett | | #5

      +1!!

      Most half inch OSB meets the NBC code definition of a "...vapour barrier...", and is quite rugged. Detailing it as an air barrier would be required (maybe even some latex paint would be needed to seal up some OSB sheathing, which can sometimes leak surprising amounts of air if uncoated.) This is a standard approach taken in many Passive House wall designs without resorting to broadsheet flexible goods as air barriers/vapor barriers.

      BTW: Is "sheathing" really spelled "sheething" in some locations?

      1. Expert Member
        MALCOLM TAYLOR | | #7

        Yeth.

        1. lance_p | | #9

          The leaky OSB thing has me pretty scared of using it as an air barrier of any sorts. Winding up with thousands of dollars worth of unplanned peel-n-stick WRB to solve air leakage issues is not what I would call a pleasant surprise!

          Sheething vs sheathing. Well, I'm probably ignorant of the real background. I always thought it was called exterior sheeting, so I added the H just to fit in I guess. Lo and behold, sheathing as a verb is a thing! Who knew? Not me I guess. From now on I'll just call it sheathing. :-)

          1. this_page_left_blank | | #13

            Two options to alleviate your concerns: One, use 3/8" plywood instead of OSB. Alternatively, proactively paint the OSB with a semi-vaopour-permeable latex paint. I see Dana already mentioned this. If prepared for and done in advance, it wouldn't be that much more work or expense.

  4. gstan | | #6

    GSTAN
    There is a far more rational way to construct a Double
    stud wall - outer stud wall 2x6 with bats (mineral wool?)
    then 2 or 3 inches of polyiso fastened to the inner face
    of the 2x6 studs - then construct a 2x4 inside wall against
    the inner face of the polyiso. The inner wall becomes the
    service cavity and sheet rock hanger - the polyiso (Foil Faced)
    is the air/moisture barrier and is protected from physical
    damage by the inner wall and from thermal rate drop by
    the outer 2x6 wall - of course you have to tape the seams
    on the inner side of the polyiso and seal it to the floor and
    ceiling with some combo of caulk, tape, or spray on. With 3
    inches of polyiso the total wall thickness will be 12 inches
    (stud face to stud face) and the actual R value will be 36
    plus or minus - want more R value fill the service cavity
    with bats or blow in for an R-46 total. You won't see any
    moisture problems with this and it's simple to construct
    although it may offend some peoples concept of "Green".
    Why fumble around with expensive vapor retarders?

    1. lance_p | | #10

      If expensive is something you're trying to avoid, a 3" layer of polyiso would be a good place to start! ;-)

  5. RMaglad | | #8

    Hi Lance, just wrapping a build in Ottawa with Intello in my walls. We did 2" ext EPS, zip sheathing, 2x6 w/ roxul batts, intello, 2x4 wall with roxul batts.

    Contractor found framing to be straight foward and easy. We basically balloon framed the house, whereby the 2x6 walls were erected, trusses installed, and the intello walls were taped and air sealed with the poly ceiling VB. afterwards, 2x3 ceiling strapping was used, and all interior walls were framed. the strapping and interior walls took 2 days of labour to complete.

    1. lance_p | | #11

      Thanks for the follow-up, Ryan! Hopefully your build went according to plan. What was your final blower door test numbers? Was this your personal build, or for a client?

  6. RMaglad | | #12

    personal build. Final blower door not yet completed. I really wanted to have one completed pre-drywall, but i ran into delays, and my patio door and front door were delayed, and i couldnt put the project on hold for the test. We did our best to to really inspect and try to patch up the vapour barriers and windows.

    I do require the test to be completed as part of the final occupancy for the City of Ottawa, so i can report back then. My siding contractor got hurt, and i (we) haven't been successfull in finding a replacement. 90% exterior cladding is also a requirement for occupancy. I am also not able to pass my plumbing final "bowl" test, which after many many many hours of trying to resolve, i am convinced it has to do with the american standard low flow dual flush toilets that we purchased. Looking in to alternative tests to satisfy the inspector. We are living in the home now, super comfortable house, LOVE the triple pane windows. Need the occupancy permit in order for the lawyers to release my 10% holdback.

    1. this_page_left_blank | | #15

      The exterior cladding is a pretty weird requirement for occupancy.

      What is the plumbing final "bowl" test? I didn't have to do that. If it's back filling the drain so that the bowl fills and fails to drain out, then I'm not sure how the toilet itself can be to blame. Unless there's actually a leak in the toilet. If you fill the toilet bowl, there's really only two places for the water to go - down the drain, or through a leak. I would suspect the toilet seal. Maybe pull the toilet off, insert a drain test tube and fill that up. If that holds, then you know it's either the toilet or the seal. In either case it's something that needs to be fixed.

    2. lance_p | | #16

      Wow, the fun I'm looking at in my near future! Well I wish you the best in getting your occupancy certificate. I've heard the City of Ottawa inspectors are very, shall we say, meticulous.

      I can't wait to have that "super comfy house" feeling. Congratulations! I've been dreaming and planning ours for a couple of years now, and come spring the shovels should be hitting the ground.

    3. Solar_Bill | | #19

      Just realized how old the post is so this is probably silly. But my guess on a weak bowl test is a bottleneck in the air vent, the stack. Water can't go OUT unless air comes IN. Blocked by a bird's nest? Or error in layout, air burbling through water? Good luck!

  7. lance_p | | #14

    Thanks for everyone's thoughts so far.

    So I came across a paper published by the University of Alaska Fairbanks that pretty much nails my thought process on the use of poly in a cold climate (Permeability of Common Building Material to Water Vapor). Their take is, the less moisture getting into the wall the better, and in cold climates the vapor pressure across the retarder is very high.

    http://cespubs.uaf.edu/index.php/download_file/1012/

    They give the calculations for moisture through a substrate based on vapor pressure and permeability. Their example uses 6 mil poly (0.06 perms) at 0.1 perm to account for installation imperfections, 40% indoor RH (my desired indoor RH) and -10F outdoors which is a very common overnight low in Ottawa Dec-Feb. Based on 3900 sqft of material under those conditions, 1.4 gallons of water will work its way through per month.

    Intello starts at 0.13 perms in dry conditions and goes up from there. At 40% indoor RH it is likely higher than 0.13, but even at that you're looking at 30% more moisture than their example, or 1.8 gallons per month, if perfectly detailed with no unintended air movement. A similarly well detailed poly installation would limit that to 0.8 gallons.

    I understand the concerns over interior poly and air conditioned buildings in humid environments, but in my case I will be using an R10 insulated service cavity on the inside of the vapor retarder which should keep any summertime moisture accumulation concerns at bay. Poly below cellulose on the attic floor might also be seen as a concern, but that's how my leaky air conditioned townhouse is built and I didn't find any evidence of moisture damage to the truss bottom chords when I was up there blowing insulation last fall. I was surprised at how good everything looked up there, actually, given that I use a humidifier in the winter months.

    As with most items in our build, I'm looking at value and what everything brings to the party. In my case I'll need roughly 7000 sqft of Intello if I do the walls and attic floor, which is 10 rolls at CDN$421, or $4210. That's a lot of money compared to the 5 rolls of poly I'd need instead at about $400. I could certainly think of somewhere I could spend that extra $3800.

    Reduced moisture drive in the walls in cold weather, and minimal risk of moisture in summer. Have I missed anything?

    1. this_page_left_blank | | #17

      Please remind me, what is your inner sheathing, if any? If you are already going to have OSB there, then the cheapest option would be to just paint it and tape the seams. I would argue that it's a better option than poly, cost aside.

      1. lance_p | | #18

        I'm planning to use steel T bracing for shear resistance on my inner wall. There are a couple of walls with large windows at the front of the house that may require OSB if I can't squeeze the T bracing in, but other than that only drywall on the interior.

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