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Community and Q&A

Is the foamboard caulked to the WRB/sheathing at the bottom?

michaelbluejay | Posted in Green Building Techniques on

I’ve just spent several hours reading articles and watching videos here on GBA, Fine Homebuilding, YouTube, and elsewhere, trying to learn the best way to install foamboard and flash innie windows when I’m using asphalt paper, but some of what I’ve seen seems incomplete or confusing, prompting this query. I’m sorry if I missed the answers to these questions.

(1) When installing foamboard over OSB/ply, is the inside face of the foamboard on the bottom caulked to the WRB?

I was certain that I read that it should be, to form an air seal and to prevent insects from climbing up, but now I can’t find any such reference. I could see an advantage of leaving it open for drainage.

The bottom of the last page of this article suggests flashing the foamboard seams. It wouldn’t seem to be necessary to try to divert water this way if the foamboard were open at the bottom, so this suggests, that the foamboard is indeed caulked to the WRB.

(2) If the foamboard is open on the bottom (not caulked to the WRB), then why aren’t the windows flashed over the foamboard?

All the articles (e.g., Innie or Outie Windows) refer to flashing the windows over the OSB/ply, which would send water between the OSB/ply and the foamboard, when it seems that we should be directing water over the foamboard and into the rainscreen gap.

One article seems to suggest flashing over the foam to the rainscreen (“Flashings should direct water to drain toward the exterior, usually to the rainscreen gap between the siding and the foam”), but then suggests flashing between the foamboard and the WRB (“[I]f the rough sill directs rain to the exterior face of your WRB, you’ve done a good job.”).

(3) When flashing the seams of the foamboard as described, can I use the housewrap tape? The author suggests housewrap (I’d rather not buy a whole roll just to flash the foamboard seams), or “plastic sheathing”. I can’t find any relevant product called “plastic sheathing”. Perhaps he means “plastic sheeting”?

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  1. Chaubenee | | #1

    Hi Michael, I think you would not want to caulk the foam to the wall. I take it you want to use tar paper instead of house wrap? Here is my wall, starting with OSB (with 3M All-Weather Tape to tape the seams) then Tyvek Drain Wrap overlapped with taped seams vertically. Then the windows are INNIES and the bottom is nailed but not caulked so any water that might get I can drain. Then 2.5 inches of EPS that I happened to buy recycled. The seams on that will be taped with the 3M tape, and then the firring strips. The WRB is integrated with the windows at the sheathing. At the bottom of the foam and firring strips will be an aluminum Z flashing to protect the foam, but any water that gets behind the siding or foam ought be able to weep out the bottom. I like to drill very tiny holes in the bottom of that every four feet or so. We are talking 1/32. Bear in mind my foundation wall has two feet of foundation exposed above the ground so my foam is not close to the ground, and I have two foot overhangs and rakes, and covered porches around more than 60% of my house to keep that area rather dry. Now to your concern about the water going between the foam and the house, I think some might, but the Drainwrap will do its job. I think the trim and the siding will prevent most of it, and the foam and the tape as well, but if you wanted to, I don't think an extra layer of house wrap on the outside of your foam will hurt a bit, if you can squeeze that into your budget. Honestly I don't think there will be a ton of water getting behind the foam at all. Keep in mind that your walls should be the type that are designed to dry inwards.

  2. GBA Editor
    Martin Holladay | | #2

    You raise a lot of questions. I'll tackle them a few at a time.

    Q. "When flashing the seams of the foamboard as described, can I use the housewrap tape? The author suggests housewrap (I’d rather not buy a whole roll just to flash the foamboard seams), or ‘plastic sheathing.’ I can’t find any relevant product called ‘plastic sheathing.’ Perhaps he means ‘plastic sheeting’?”

    A. It took me a while to figure out what author you were talking about. It seems that you are talking about Michael Guertin, the author of a Fine Homebuilding article called “Save Energy With Rigid-Foam Insulation.”

    You have correctly noticed that the article includes a typo; thanks. The Fine Homebuilding art department mislabeled one of the drawings (reproduced below); the words "plastic sheathing" should have read "plastic sheeting" (ordinary polyethylene). Thanks for pointing out the error.

    In this location, you can use a flexible flashing (a peel-and-stick product with a rubberized asphalt or butyl adhesive) instead of housewrap or plastic sheeting if you want. I wouldn't use housewrap tape, which is usually thinner than 6-mil plastic sheeting, but I suppose you could if you wanted.


  3. GBA Editor
    Martin Holladay | | #3

    Q. "When installing foamboard over OSB/plywood, is the inside face of the foamboard on the bottom caulked to the WRB?"

    A. I've seen installations done either way. I would probably leave this joint uncaulked, especially if you are using a crinkly housewrap. If you are using asphalt felt, you can do it either way.

    My advice: Don't overthink this. There is very little chance, if you do a good job, that there will ever be any liquid water between your rigid foam and your WRB. Even if you did get a few drops of water in this location, it is physically impossible for the water to drain by gravity and reach the bottom of the wall, unless you choose to install a crinkly housewrap as your WRB.

    Liquid water reaches this location in your wall assembly -- if it ever does -- from the exterior. How many ways does your wall have to keep this water away from the WRB? Lots and lots of ways.

    1. First, your siding deflects 98% of the rain that hits it; perhaps the percentage is even higher.

    2. The remaining water is handled by your rainscreen gap. The small amount of water that gets past the siding either runs down the back of the siding or (in rare cases) reaches the outer face of your rigid foam. The water rapidly evaporates -- in most cases, evaporation happens before enough water can accumulate to drain by gravity to the bottom of the rainscreen gap.

    3. If you have taped your foam seams, how is water going to get past your rigid foam?

    4. Your asphalt felt is going to sit there forever: warm, happy, and dry.

  4. GBA Editor
    Martin Holladay | | #4

    Q. "If the foamboard is open on the bottom (not caulked to the WRB), then why aren’t the windows flashed over the foamboard?"

    A. They sometimes are -- and if you want to do it that way, you can. It's often harder -- depending on where you want to locate your windows -- but it's always possible.

    If you feel better about draining your windows on the exterior side of the rigid foam, then you would want to install your WRB on the exterior side of the rigid foam. If you go this route -- it's perfectly acceptable -- then it's easier to do with housewrap than with asphalt felt. I would find it awkward to install asphalt felt with long cap nails through rigid foam.

    It's even possible to use rigid foam as a WRB, if you want to. For more information on this approach, see Using Rigid Foam As a Water-Resistive Barrier.

  5. michaelbluejay | | #5

    Why is the standard practice to flash the sills so that water is directed between the foamboard and the WRB, if "it is physically impossible for the water to drain by gravity and reach the bottom of the wall"?

    In the other thread you advised, "No matter what approach you take, the construction sequence needs to be anticipated, and every step thought through carefully, before the details are finalized." In this one you tell me, "Don't overthink this."

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Why is the standard practice to flash the sills so that water is directed between the foamboard and the WRB?"

    A. I don't think that the method you describe is necessarily "standard practice." Actually, I think that most builders who install exterior rigid foam go with outie windows, and choose to install the WRB (housewrap) on the exterior side of the rigid foam.

    I think that builders who install a WRB between the rigid foam and the wall sheathing are in the minority.

    When I wrote an article about all of the different approaches to detailing this type of wall -- the article is called, Where Does the Housewrap Go? -- I interviewed building scientist Joe Lstiburek. He listed the benefits of sandwiching the housewrap between the rigid foam and the wall sheathing: namely, that the rigid foam protects the housewrap from temperature extremes (thereby ensuring its longevity) and protects the housewrap from "bellowing" caused by changes in wind pressure.

    You may be unconvinced by these advantages. That's fine. You don't have to do it that way. The great thing about building a house is that you can build it the way you prefer.

    If you are losing sleep over these details, you are also free to install crinkly housewrap.

    Q. "In the other thread you advised, ‘No matter what approach you take, the construction sequence needs to be anticipated, and every step thought through carefully, before the details are finalized.’ In this one you tell me, ‘Don't overthink this.’"

    A. Some things are worth worrying about, and others aren't. I think that it is definitely worth thinking through your window details before you build the wall. You need to be absolutely certain about the window rough opening size (to accommodate flashing and/or the sill pan); you need to know where the window flanges will go (if the window has flanges) and how they will be fastened to the wall; and you need to know how the window flashing will be integrated with your WRB. These details are important; if you think them through, and get them right, then you won't have to worry.

    Once you've done all that, I don't think you need to worry about liquid water between your rigid foam and your housewrap. If your details are good, you can sleep well at night. Worrying about drops of water that aren't there falls into the category of "overthinking," in my book -- but perhaps that comment was a little flippant. In general, I'm happier to hear from builders who overthink than from builders who don't think at all -- so it's fine to keep asking questions.

  7. michaelbluejay | | #7

    I don't know how to ask this any more directly. In your innie/outie article, IF one is going the innie route, the directions are not to flash the window over the foam to the rainscreen, but instead to flash the window onto the WRB, which is then covered by foam. That makes no sense to me (since as you say, water that gets between the foam and the WRB can't drain unless the WRB is crinkly), and I don't feel I'm any closer to an answer about this. If water is to be directed to somewhere it can't drain, then why flash at all?

  8. GBA Editor
    Martin Holladay | | #8

    In the case you discuss, the point of the WRB is to keep the sheathing dry. It protects the sheathing.

    Any liquid water that ends up in the sill pan (the pan that protects the rough sill) will eventually evaporate. The amount of liquid water is small. Experience shows that this approach works.

    Walls can rot because of window leaks, but this type of rot only occurs in houses without sill pans. If you have a sill pan and a properly installed WRB, the wall is protected. As I say, job-site experience bears this out.

    I think that you are hung up on this because you are imagining larger quantities of water than are actually involved.

    All of this said, I understand your obsession, because (like you), I often obsess about building details that keep me up at night. This detail bugs you -- so don't do it.

    Either flash your sill pans to a WRB installed on the exterior side of the foam (near the ventilated rainscreen gap), or install crinkly houswrap with tiny drainage channels.

    You should definitely choose a detail that you are comfortable with -- not a detail that strikes you as non-sensical.

  9. michaelbluejay | | #9

    The alternatives you suggest are also not ideal for me. I don't want to put the WRB outside the foam because I already have asphalt, and you discourage asphalt on top of the foam, and because I'm more comfortable with a WRB covering the sheathing directly. What seems to make sense is to keep the WRB between the sheathing and the foam, and then flash over the foam into the rainscreen gap, but you've never suggested that for some reason. That seems like an intuitive solution; why is it never suggested, let alone advocated? Maybe it's discouraged because the foam isn't a WRB, but if water is flashed/directed to between the WRB and the foam, then water is still in contact with the foam. And in your innie/outie article, for outies you advise flashing over the foam and then installing the WRB. So I'm still perplexed.

  10. 52Dave | | #10


    I am not as experienced as the many of the individuals posting on this site, but I would like to take a stab at helping you come to an answer. I do, however, have a clarifying questions or two for you.

    The windows you are installing, will they have mounting flanges?

    If these windows are flanged, and installed as a innie window, you would flash to the WRB. Although this does may seem counter-intuitive, I feel you are not accounting for a few things that Martin pointed out.

    First and foremost, bulk water intrusion is handled by your cladding of choice and the rain screen gap created by the furring strips. As the foam-board will have taped seams,any water intrusion will be isolated to the surface of the foam-board. Secondly, the trim, sill, and details around the exterior of the window direct any water to the exterior of the foam or cladding, away from the window and WRB. Any water that makes it past these systems, in-between the foam-board and the WRB, will be negligible and will evaporate.

    I am unsure if this clarifies anything for you or clouds it further. In any case, I am in the midst of a renovation of an existing home. My windows are installed as innie windows. I am using Tyvek as a WRB, against the OSB. The foam-board is fastened over this. I will be using Vinyl siding as my cladding.

  11. michaelbluejay | | #11

    My windows have nail fins, currently planned is innies, but I might change that.

    I'm well aware that most water is deflected by the siding, trim, and sill, and that drainage and evaporation is facilitated by the rainscreen. It still makes no sense to me why we would flash to between the WRB and the foamboard, rather than flashing into the rainscreen. Okay, maybe the amount of water directed to between the WRB and foamboard by the flashing is little to non-existent, I understand, but if we're flashing in the first place it's because we want to direct water IN CASE there's any water to direct, and because we want to direct it to a safe place. So it makes no sense to me why we flash to somewhere where the water can't drain (unless we have crinkly wrap), and why we don't flash to the rainscreen (which is supposed to help us facilitate water drainage in the first place).

  12. GBA Editor
    Martin Holladay | | #12

    I think that we've explained the pluses and minuses of all the various approaches. If you want to direct the water from your window sill pans to the exterior face of your rigid foam, that approach would also work. There really aren't any downsides to that approach if you prefer it.

    It's up to you to decide which approach you are most comfortable with. Good luck.

  13. Expert Member
    MALCOLM TAYLOR | | #13

    Here in coastal British Columbia our code and recommended installation guide has developed details for that it terms "high moisture index windows". It suggests two approaches specifically for flanged windows to insure they can drain easily. One is to mount the flanges outboard of your rain screen strapping so there is a gap left. The other is to drill holes in the bottom flange. In both cases the sill pan drains into the rain screen cavity - perhaps because they don't anticipate exterior foam - but thinking about it I can't see how you would integrate flashing into the sill with a flanged window that would move the water out over the foam. Could you perhaps post a sketch or link to what that would look like?

  14. JC72 | | #14


    Have you taken a look at ZIP-R Sheathing? This product may give you the confidence you're looking for.

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