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Plywood Sheathing Outward Drying Potential

user-1140204522 | Posted in General Questions on

I have a post and beam house with site built insulation “panels” that are really just 2×6 walls, outside the posts, built as follows, from inside to out:
Sheetrock, airspace w/ electrical, 3″  closed cell spray foam, 1″ continuous styrofoam, 5/8″ plywood sheathing, Tyvek, cedar clapboards.

By the nature of these being outside the frame, it is hard now to access some of it.

Recently I noticed some wet sheathing (some rotting). That is being replaced now. With blower door testing, some air leaks (gaps) through the sheetrock (especially in areas hidden behind beams) have been found.  I can caulk those and retest with blower door, that’s a good thing.

I’m trying to learn more about how wall components can dry, it seems the sheathing is the critical item in my walls that needs to dry (learning a lot on this site).  I guess one good thing is that nothing is blocking air and moisture leaving the sheathing to outside except the tyvek and clapboards.  I understand a rain screen would have been helpful, but its tricky to add one now.

I will seal every leak I can find, and maybe then some.  But as added insurance, what would be the best thing I could do to make it less likely the sheathing will get soaked again, and not be able to dry?

Is this a case where maybe asphalt building paper would allow more drying than tyvek?

What would the result be of adding dense packed celulose be in the 2.5 inch air space/electrical area behind the sheetrock?  (colder sheathing but less airflow?)

Add better ventilation with rainscreen or vinyl siding?

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  1. andyfrog | | #1

    There are other more knowledgeable posters but here is what I have gathered:

    1) which direction do you think the moisture was coming from (inside or out) that contributed to the rotting sheathing? The two main sources would be interior moist air condensing on the backside of the cold sheathing, or bulk water from the exterior.

    2) ideally your spray foam job would have been done perfectly, and therefore serve as the air barrier to the sheathing preventing backside condensation, but if it wasn't, I believe you are correct in attempting to make your drywall (and associated penetrations e.g. can lights, electrical sockets, bathroom fans) as airtight as possible.

    3) if you were going to the trouble of removing all the siding to replace the tyvek, you'd be better off doing it with something like a self-adhered or mechanically fastened non-porous vapor permeable membrane ( than with felt paper as these will resist bulk water better. With regard to the vapor permeance, I'm not sure what is ideal--I have heard you want to stay around 10-15 perms, as too permeable means vapor drive inwards can happen too fast and wet the sheathing, but too impermeable means it can't dry outwards.

    4) if you're going through the trouble of removing all the siding and replacing the Tyvek, it might be worth adding a layer of exterior insulation.

    5) a rainscreen would be ideal, although vinyl siding supposedly sort of acts like one because it has so many gaps anyways.

    6) dense pack cellulose won't meaningfully reduce the risk of condensation via airflow, although it might reduce airflow somewhat, while also certainly increases the risk of condensation on the backside of your sheathing. Still you are better off making a discrete layer function as your air barrier e.g. the drywall.

    1. maine_tyler | | #2

      Are you familiar with SIP failures. The issue stems from them being 'panels' and thus if the joints are not properly sealed on the inside, interior air can exfiltrate and condense on the cold sheathing. If your described assembly was constructed as 'panels,' it sounds like they have seams, and could also have the air leakage issue. Did you notice the rot at these seams? The field of the panel sounds robust.

      As Andyfrog says, you do want to eliminate exterior leaks as the cause.

      As far as drying potential of the sheathing, it's hard to see that as being the issue and about the only thing I can think to make it better is a rainscreen.

  2. user-1140204522 | | #3

    thanks andy
    - item 1. I am thinking moist air is traveling from the house out, but its hard to say none from the other direction. I don't really trust the Ceder claps hard up against an un-taped, not so continuous layer of tyvek, hard against the plywood.
    - item 2. We thought when we did the spray foam 15 years ago that would be the air barrier, and not the sheetrock. Therefore opportunities were missed (!) in making sure the sheetrock was fully sealed. Now its hard to do so because some is hidden, but I can make a lot of improvement to it anyway.

    The sheathing around the damaged area will be exposed in the spring. This will be an opportunity to at least make changes to the WRB.

    1. andyfrog | | #4

      I did miss a critical detail in my original reply--if as tyler is saying, the spray foam was applied prior to the panels being erected into place, or if it was applied imperfectly to panels with a lot of butt joints between that 1" of styrafoam, then you probably-leaning-towards-definitely have the 3D airflow network problem

      Another thing you might consider is Aerobarrier, although it's not clear if you could find someone willing to apply it to an inhabited structure.

  3. Expert Member


    - As other posters have pointed out the main problem is air-leakage, and the secondary one is a combination of capillary driven moisture from the siding, and the lack of drying to the exterior.

    - Unfortunately the air-leakage won't be helped by adding cellulose, you need to pick a plane - the sheathing, or foam - and make it form a completely continuous air-barrier.

    - While adding a full rain-screen might be too onerous for you, adding a mesh underlay would provide a capillary break between the WRB (stick with Tyvek, which is high perm) and the siding. It will also promote drying.

    1. user-1140204522 | | #11

      Malcolm, do you have any recommendations on a mesh underlay to use?

      1. Expert Member
        MALCOLM TAYLOR | | #12


        Cedar Breather or something similar. I would avoid dimpled WRBs. They often have low perm ratings and don't effectively provide a capillary break, or a wide enough vent space.

        1. user-1140204522 | | #13

          Thanks Malcolm, I see. 1/4" gap, maintained under pressure. That should be do-able without changing dimensions too much to meet up with unchanged areas.
          I'm thinking through the options, trying to make sure I do enough here. I think this and a good WRB and flashing should take care of exterior concerns. I appreciate your help and will look at the VP100 as well. Thanks so much for your comments. Here is mfg notes on using under Cedar Breather under siding

          1. user-1140204522 | | #14

            VP100 looks like a very good air barrier as well as having 33 perm vapor transmission, literature says it is self sealing around nails, so this all sounds very positive.

            Reading details on cedar breather, the mfg says it can be used for siding but also seems to be steering customers toward slicker classic rainscreen because it has vertical drainage channels.

            Any preference for one of those products over the other?

          2. andyfrog | | #15

            RE: VP100 - it probably depends on climate zone. I have heard that 10-15 perms is the "sweet spot" for WRBs, although more or less permeance may be more optimal depending on the climate.

            In humid and hot areas, more permeance means more vapor drive into the wall. It's also more vapor drive out of the wall, if the exterior is less humid than the interior. However, _without sufficient energy flux, this might not be enough to actually dry things out_.

          3. Expert Member
            MALCOLM TAYLOR | | #16


            Sorry - yes Slicker is probably preferable. The real difference in performance would be between any of the mesh products and drain-able house wraps - which I don't have as much confidence in.

          4. user-1140204522 | | #17

            Thanks Andy, I am in a heat dominated climate, southern New Hampshire. We are just at the bottom of climate zone six.

  4. user-1140204522 | | #6

    Malcolm - Adding a mesh with a more continuous WRB (Tyvek then) is doable as we will have the siding off in the problem area, and it won't be hard to do this.
    So that is one thing that will be on the list.

    Andy - I have already discovered a gap where the wall "panels" meet the roof, hidden behind large beams. I will be able to seal most or all of this by sealing the gap between the beam and the ceiling. I have found other gaps in sheetrock as well, mostly where sheetrock meets something else. So I know this is at least part of it.
    I will look at the 3D Network article tonight, thanks -

    BTW, where the worst damage to plywood was, there was a hole in the sprayfoam, which we were able to fill in with canned foam. But, it seems to reinforce the airpath causing damage.

    1. Expert Member
      MALCOLM TAYLOR | | #10

      Thinking about it again: If you are removing the siding in the problem areas, I'd be tempted to replace the Tyvek there with a self-adhered WRB like Blueskin VP 100, which would also act as an effective air-barrier

  5. maine_tyler | | #7

    Why do you refer to the tyvek as 'not so continuous'? If you have claps direct against sheathing, it's certainly worth considering exterior water as a source of concern.

    Still not clear based on your latest responses if your spray foam was done in place and continuously, or were panels put together post spray.

    1. user-1140204522 | | #8

      more details.
      The Tyvek has some issues.
      1. There apparently was a problem when it was installed 30 years ago with a reaction between cedar and tyvek that made the tyvek weak and flimsy.
      2. The installers weren't careful with it, there are many smaller pieces, only recent work is taped.

      "Panels" - These are big. Basically one wall of the house has panels built in place, full width of house. 2 or 3 panels stacked on top of each other, 1 per floor or part floor.
      These were sprayed in place, from the house out, so against the 1" styrofoam inside the sheathing. Spraying straight into the panels is fine. Where they had to spray up or down into the panel cavities behind the upper and lower beams not perfect. I tried, and may have succeeded to fill the top of cavities with canned foam.

      1. user-1140204522 | | #9

        Tyler, per your earlier question, I did notice rot at top of top panel which may be related to a gap at top of sheetrock for that panel. This one I believe I can close.

  6. user-1140204522 | | #18

    Talking about this a little more, here is the situation. We will remove siding from the top half of this (East) wall to replace a window and repair some plywood and possibly framing with some rot. I believe the chief culprit was exfiltrating humid air. I have found some gaps where the wall meets the roof and will look for any others with a blower door and seal these.

    I was thinking about replacing the original tyvek in this area, which isn't great after 30 years in contact with cedar clapboards and also installing slicker clasic rainscreen for more ventilation.

    However, we realize this will bump out the siding 1/4" which will then require some type of a transition back to the original level below. It will probably mean changes to trim on this and adjacent walls, and may look funny where the two parts meet. (also, continuing this later to the lower part of this wall, or any other wall will mean windows may now not trim properly as there is not much clearance from clapboards now)

    Therefore, I'm thinking about maybe scaling back to just installing one of the drainable housewraps. I found one with 1.5 mm bumps, which is 50% more than the original one I looked at. This is Tamlynwrap.

    This should install without any real adjustment to meet the old portion.
    I don't really think the original problem was exterior water, however I think the tyvek is failing and I like the idea of some separation between the sheathing and the siding.

    Any thoughts on this?

    1. Expert Member
      MALCOLM TAYLOR | | #20


      Your plan sounds fine. Good luck going forward.

      1. user-1140204522 | | #21

        OK, thanks Malcolm. I understand the rain screen would be best, this will be better than it was, and I’ll probably do a fresh assessment once we get a good look at the wall opened up.

  7. user-1140204522 | | #19

    I’m wrapping this up for now, just wanted to check to see if there were any last comments. Thanks for any ideas.

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