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Musings of an Energy Nerd

Wrinkled Housewrap Behind Exterior Rigid Foam

If you install housewrap between exterior rigid foam and wall sheathing, does the housewrap need to be one of the wrinkled or dimpled products?

Several manufacturers sell wrinkled or bumpy housewraps. The illustration shows (left to right) samples of Benjamin Obdyke’s HydroGap, Tyvek's DrainWrap, Barricade Building Products’ WeatherTrek, and Kingspan’s GreenGuard Raindrop 3D. Photo courtesy of Fine Homebuilding.

If you plan to install a layer of continuous rigid foam on the exterior side of your wall sheathing, where do you put the housewrap? There are many opinions on this issue, but the usual answer is that the housewrap can be installed either between the sheathing and the rigid foam, or on the exterior side of the rigid foam. Either approach can work, as long as the builder knows which layer will function as the home’s water-resistive barrier (WRB), and as long as the WRB is integrated with the window flashing, door flashing, and penetration flashing. (For more information on this issue, see “Where Does the Housewrap Go?”)

The first time that a builder sandwiches housewrap between his wall sheathing and a layer of rigid foam, he or she may feel uncomfortable. Among the questions we’ve received on this issue are the following: How can the housewrap work as a WRB when there is no clear drainage path for liquid water? Shouldn’t I be installing one of the wrinkled or bumpy housewraps in this location to facilitate drainage? (For more information on wrinkled housewraps, see the Fine Homebuilding article, “Are Drainable Housewraps Enough?”, or the GBA article, “All About Rainscreens.”)

Here’s the short answer: It’s perfectly OK to install a wrinkled housewrap in this location if you want to. But wrinkled housewrap may not be necessary.

To address the question of when wrinkled housewrap is nessesary, it’s helpful to step back and broaden the discussion somewhat—specifically, to establish when walls need a rainscreen gap. Before delving into rainscreens, though, I’d like to tip my hat to building scientist Joseph Lstiburek, on whose analysis I rely heavily. (For those who want to read what Lstiburek has to say on rainscreens and wrinkled housewrap, I recommend…

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16 Comments

  1. User avater
    Armando Cobo | | #1

    Does the insulation values goes down with the type of "air space" created by a 3D WRB? Maybe some more than others?

    1. User avater GBA Editor
      Martin Holladay | | #2

      Armando,
      The answer to your question is in my article. In the next-to-the-last paragraph, I quote Joseph Lstiburek, who wrote: "But won’t the tiny gap cause a loss of thermal performance of the foam sheathing? Yes. How much? About 5 percent of the thermal performance of the foam sheathing (not the entire wall assembly) with the 1/8-inch gap, less with a smaller gap. With ‘crinkly’ stuff you loose next to nothing."

      Translation: If you are installing R-5 foam, then the foam layer will act like R-4.75 foam if you include an 1/8" gap. With R-10 foam, the foam layer will act like R-9.5 foam. If you use "crinkly" (wrinkled) housewrap, there will be virtually no loss in thermal performance.

    2. User avater
      Armando Cobo | | #4

      Sorry, my bad... I guess I missed it being in a hurry to get to Peter Yost's webinar this morning.

  2. Rick Evans | | #3

    Great article, Martin!

    In regards to your "bumpy OSB" reference, Zip sheathing is kind of bumpy as the felt layer is on the rough side of the OSB and has a subtle grid pattern. It's not much, but perhaps enough to matter. I might do an experiment to see if water will drain behind rigid foam when fastened through Zip sheathing.

    I still wouldn't sandwich OSB between closed cell and rigid foam without a larger gap, but perhaps this could constitute a little extra drying for walls with air permeable cavity insulation and exterior rigid foam.

    I'll report back soon.

    1. User avater GBA Editor
      Martin Holladay | | #5

      Rick,
      Matt Risinger already conducted that experiment. Here is a link to his video.

      Spoiler alert: Zip sheathing is bumpy enough to allow drainage.

  3. Rick Evans | | #6

    Haha- thanks Martin!

    Good to know! (Puts away saw, sheathing scrap, and utility knife ...)

  4. Kohta Ueno | | #7

    Adding on to this discussion--Lorne Rickets of RDH recently gave a presentation (Exterior Insulation: A More Layered Understanding), which included their spray testing of walls with exterior insulation and open-joint cladding. Clips of the relevant slides below.

    They had a slick rig to measure where the water ended up draining--off the exterior face of the cladding, the backside of the cladding, the face of the exterior insulation, and the water-resistive barrier behind the insulation.

    Quick takeaway: no drained water measured getting through the insulation, to the water control layer. Basically, all of those layers shed the water before it gets further into the wall.

    Of course, we must design the walls assuming water will get back there--entirely likely with penetrations and other details. But this is an elegant demonstration that the layering of this wall intrinsically limits how much water gets all the way back there.

    Also: the research this is based on: https://www.rdh.com/wp-content/uploads/2017/10/RDH-BSL-Drainage-Balance-Spray-Rack-Report.pdf

    1. User avater GBA Editor
      Martin Holladay | | #8

      Kohta,
      Thanks for providing information on research that confirms the analysis in my article. I appreciate it.

    2. Malcolm Taylor | | #9

      Kohta,

      Thanks for the link. Apart from the conclusions they draw, my big take-away is not to use an open-joint cladding.

  5. User avater
    Armando Cobo | | #10

    Kohta or Martin - Occasionally we install taped sheathed walls with taped polyiso, a black WRB on top of the taped insulation, and open-joint cladding with Ipe or cedar board or Hardie (panel and board) siding, over 1x4 rainscreens, for a modern look. I assume this still ok, right?

    1. User avater GBA Editor
      Martin Holladay | | #11

      Armando,
      Malcolm Taylor's comment -- "my big take-away is not to use an open-joint cladding" -- is apparently a reaction to the researchers' obervations on how much water reaches the insulation layer when a wall with open-joint cladding is subjected to driving rain. The researchers wrote:

      "There is a decrease in the total water collected when the joints are taped, and a further decrease for the vinyl siding, as more water was reflected from the higher percentage of cladding, and splashed over the collection trough onto the floor. This test result means that if the cladding is not a high-percentage open-jointed rainscreen, the amount of water actually coming into contact with the insulation layer in a wall assembly with a realistic driving rain is likely very small, assuming there are no gross deficiencies in construction."

      Open-joint cladding obviously lets in the rain. It's an architectural conceit, chosen by a designer to satisfy a desire for a certain look. The open joints obviously undermine the principles of water management.

      That said, if a client insists on having that fashionable look, the wall can be flashed in such a way that the water is managed. You just have to be very careful with your flashing and WRB installation.

      The other issues with open-joint cladding have to do with finding a fastening method that prevents curling or warping, and the question of cladding durability.

  6. Dan Kolbert | | #12

    I use a rain screen as a matter of course (that's a clever play on words).

    On new construction, we pretty much always build double wall with dense pack cellulose. We try to remain as vapor open to the exterior as possible, so we want to make sure it can easily escape.

    On renovations, especially in the historic district where we frequently work, it's usually not possible to increase the thickness of the assembly with a real rainscreen, so we try to use the crinkly or bumpy stuff there.

  7. Patrick McFarlane | | #13

    This is a super useful article. I was wondering about whether vapour barriers might be another reason to use a drainable WRB.

    Here in Canada, interior vapour barriers (poly sheeting usually) are found in the vast majority of houses built since the early 80's and the code in most (maybe all?) provinces has requirements for vapour retarders. Building permit officials are so used to assemblies with poly that you just can't build any sort of wall without showing them where the vapour barrier is going to go.

    I would think that the presence of an interior vapour barrier would be another occasion for the mandatory use of a drainable WRB. Like closed cell foam, the vapour barrier would prevent interior drying of the sheathing. Best solution would be to omit the vapour barrier but that's probably too costly (remove all the drywall or sheathing + cavity insulation?) and might not get through the building department - a problem for a new build as well.

    1. User avater GBA Editor
      Martin Holladay | | #14

      Patrick,
      If we're talking about houses with exterior rigid foam, the rigid foam slows down exterior drying to such an extent that wrinkled housewrap is irrelevant to the discussion of exterior drying rates.

      I think your worries about interior polyethylene are unwarranted. To learn why, read this article: "Rethinking the Rules on Minimum Foam Thickness."

      1. Patrick McFarlane | | #15

        Thanks for the pointer to that article. I get the logic about how an interior vapor barrier actually reduces issues that could be caused by too thin foam. But what if everything isn't done perfectly...like say window leaks or some interior air sealing wasn't perfect? Then you've got moisture in walls and no way to get out. Seems risky.

        I'm not sure I'm following you regarding the inability to dry outward with exterior foam. I thought that was the logic for including a wrinkled house wrap when you have exterior rigid foam and closed cell between the studs. Joe's "Mind the gap, Eh!" article seems to suggest that the wrinkled wrap provides just enough "hygric redistribution" to allow some drying to the exterior so that the OSB sheathing can dry in at least one direction. I don't see why this wouldn't also work with an interior vapour barrier.

        1. User avater GBA Editor
          Martin Holladay | | #16

          Patrick,
          Q. "What if everything isn't done perfect--like say window leaks or some interior air sealing wasn't perfect? Then you've got moisture in walls and no way to get out. Seems risky."

          A. That would be risky. I never said that the wall you described wasn't risky. But in the case you are talking about, the main problem would be the defective window flashing, not the interior polyethylene.

          Q. "Joe's 'Mind the gap, Eh!' article seems to suggest that the wrinkled wrap provides just enough hygric redistribution to allow some drying to the exterior so that the OSB sheathing can dry in at least one direction."

          A. You're basically right--although the issue of hygric redistribution is a little more complicated than the way you stated it. Removing concentrated moisture and redistributing it so that the moisture is "averaged out" over a larger area is often all you need to do to avoid OSB rot. I'm not going to pretend that I understand all of the physics involved--but suffice it to say that wall performance improvements occur even if the water never makes it all the way to the exterior. I agree with you that wrinkled housewrap can't hurt, and might help somewhat, in the case you're talking about.

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