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Exterior siding layers?

avadon242 | Posted in Building Code Questions on

Hello all, I have a quick siding question.

When we spray foamed my shop with close cell spray foam we sprayed against the tyvek(housewrap). It was only one layer. It seemed like the best option we had at the time as I knew I would down the road drop the T-111 osb finished siding and replace it with plywood siding and housewrap and then a cement overlay (making faux stone work out of the cement)

I asked the spray foamer and he wasn’t sure if i’d be able to get the tyvek paper off the CC foam as the foam adheres to everything in a tremendous way. He said it doesn’t matter to much if you peel the top layer of the foam off because it’s such a strong vapor barrier anyway. Though the top layer does create a skin that helps prevent moisture from getting in but it’s not imperative.

Anyway my question is if it doesn’t seem to peel off is it acceptable to have studs, tyvek, plywood, tyvek(second sheet) and then the stone work? or does trapping a sheet of ply between these two layers encourage rot? My building doesn’t breathe to the inside as the cc foam is a vapor barrier at 2″(probably less actually) and I have a whopping 5.5″ of CC foam in the walls. So am I better off trying to completely remove the first tyvek so i can put the plywood right on the studs and then cover the ply with two sheets of housewrap?

Also once I get my final layer of housewrap on ( i intend two sheets) what more do I need to lay on stone? Do I need a rain screen? flashing? (I have two large carriage doors) Does the metal lath go right onto the housewrap or is that a problem?

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  1. avadon242 | | #1

    I'm kind of at a loss of what is the best housewrap/rainscreen combo available on the market.

    Should I be using something like this?

    I saw other pages that seemed to suggest two layers of asphalt paper. But what are the best reccomended products for doing stone vaneer?

  2. user-2642926 | | #2

    You might check out Keene's products. I almost cancelled the installation of some stone veneer on my house after reading about moisture damage, but their Driwall product made a great deal of sense to me so I incorporated it beneath the scratch coat. (It is only a rain screen - not a house wrap.)

    This has a thin fabric that is installed toward the outside of the wall, intended to prevent the scratch coat from intruding on the air gap, as well as "coiled" plastic that doesn't allow the air gap to be compressed.

    They make it in a few thicknesses.

  3. avadon242 | | #3

    I did see that Keen driwall. Glad to hear a positive review about it. And what is the rollsroyce of housewraps for stone? Is a roll on material much better than a Tyvek?

  4. charlie_sullivan | | #4

    Regarding the original question about whether to remove the tyvek that is stuck to the foam: no need to remove it. The foam is much more of a vapor barrier than the tyvek, so the tyvek doesn't do any harm whatsoever. But you risk damaging the foam and introducing air leaks if you try to peel it off.

  5. avadon242 | | #5

    Charlie, so it's fine to lay the ply over that tyvek and then either use a roll on liquid vapor barrier or another sheet of tyvek? I won't rot out that sheet of ply trapped in the middle?

  6. charlie_sullivan | | #6

    Right. Having the old tyvek on the inside won't make it any more likely to rot than without it. Another layer of tyvek on the outside would be good. You can also use a liquid WRB (water resistive barrier, not a vapor barrier). I don't know which is better.

  7. Expert Member
    Dana Dorsett | | #7

    Tyvek is about 40-60 perms.

    An inch of closed cell foam is about 1 perm, 5" of closed cell is about 0.2 perms. The surface skin of the closed cell foam is meaningless.

    So, the drying capacity to the exterior is about 250 times the drying capacity toward the interior. That's fine, as long as you set it up to be ABLE to dry toward the exterior

    With 40+ perm Tyvek on the exterior of the plywood, as long as there is an air gap between the Tyvek and the masonry vented to the great outdoors the plywood will have no problems drying toward the exterior.

    Even without the closed cell foam in the studwall it would be a bad idea to apply the masonry veneer tight to the plywood without an air gap. Concrete, brick, and many grades of stone can wick & harbor large amounts of moisture, and you need a capillary break to keep the moisture from wicking into the wood. Tyvek is fairly waterproof to liquid water, but it is not a capillary break. Even 1/8" of consistent air gap is an excellent capillary break, but you'd still want at least 1/4" (3/8" is even better) of ventilation gap to allow any super-saturated moist air enough ability to be convected out, keeping the plywood dry.

    The traditional 1-2" cavities in brick veneer walls are a bit overkill in most places, but it really does work.

    At 5" (or even 3") the closed cell foam itself is structural. If you want a less moisture-susceptible sheathing you may want to consider using asphalted fiberboard (eg Celotex) rather than plywood. Fiberboard can be structural too, if you buy the right grade and follow the fastener spacing requirements. It has a pretty good track record for use behind masonry claddings.

    If you use 3/4" fiberboard it even adds another ~R1.5 to the whole-wall R value, which nearly what you gained by using closed cell foam in the wall cavities rather than R20 batts (the very high center-cavity-R notwithstanding.)

    If you can get an engineer to sign off on the structural aspects of the closed cell foam, you could also use 1" EPS and gain R3.5 of whole-wall performance over what you'd have with half-inch CDX, which would be a very significant performance improvement. The stud edges can still dry into the cavity with unfaced 1" EPS (or even 2" unfaced EPS). And unlike Tyvek, EPS is a pretty good capillary break. Taping the stud edges with housewrap tape before putting up the EPS would be even more protective.

  8. avadon242 | | #8

    Thank you for the great response Dana,

    What about CDX vs Exterior plywood vs Marine Plywood? Those are all fairly expensive options but i'm only doing the face of a 28' long build and it has two large 7'tallx8'wide garage doors openings in it.

    What about one of those types of plywood, with a roll on WRB, and then Keene driwall rainscreen. I'm sure that's overkill but it sounds super formidable.

    Is a roll on wrb going to trap any moisture that's already in the plywood? or is that virtually nill. I just wonder because as you mention it virtually can't dry to the inside and only the outside. So I just hope I wouldn't be rotting the plywood if I put up one of the three aforementioned plywoods and then coat it with a roll-on WRB. Then I would go on to get the rainscreen which I found out I might be able to order the driwall rainscreen from home depot.

    Thanks so much for this help. No one in my area knows much about going beyond felt paper.

  9. Expert Member
    Dana Dorsett | | #9

    WRB does not trap moisture- it can't. While WRBs are fairly waterproof to liquid water, they are VERY permeable to water vapor, both of which are intentional aspects of the product design.

    There's nothing wrong with #15 felt, which is something of a "smart" vapor retarder, but not a vapor barrier. Under sustained humid conditions it's more vapor permable than a thin coat of latex paint, but when it's bone-dry it's as impermeable as 2" of closed cell foam.

    With 5" of closed cell foam in the cavities you don't really need structural sheathing, and with 3/4" Celotex or Stedi-R or similar asphalted fiberboard you also don't need the WRB. It's cheap stuff compared to any grade of plywood, comparable to or cheaper than half-inch OSB, and is commonly (& effectively ) used under brick and stone cladding in the sticky humid southeastern US where summertime exterior moisture drives are high.

    Your only concerns are exterior moisture drives affecting the sheathing & framing. The closed cell foam protects it completely from interior moisture drives, which are already pretty low in a shop building compared to a residence. The moisture drive from masonry claddings is very high compared to other types of siding, and using a moisture-insensitive sheathing seems like the "right" approach. Putting plywood out there serves no structural purpose, but is susceptible to moisture. If you get rid of the plywood, you've gotten rid of the problem. You could also use an exterior grade fiberglass faced gypsum board (GP DensGlass, etc), but that's more expensive than fiberboard, and doesn't add as much R-value.

    If a building inspector isn't willing to sign off on closed cell foam providing the structural rigidity, if you follow the fastener specs for structural fiberboard they'll pretty much have to give that a pass (even though the 5" of 2lb foam is providing as much or more structure than the fiberboard.)

    Most fiberboard is no tighter than 5 perms when dry, 15+ perms under sustained humidity. With a rainscreen mesh to dry into, it'll do just fine.

  10. avadon242 | | #10

    Hi Dana,

    Thanks for the great information. I researched everything you said and your links. Unfortunately I keep thinking at some point there needs to be wood on the outside of the building. Not for sheer strength, as I have 1" of ply on the inside of the building (1" subfloor non-tongue and groove plywood) and two layers of 5/8's drywall with green glue in between. This is all for soundproofing and adding mass and isolation. So I don't need sheathing on the outside for strength but I'm afraid if I just do the Densglass and sto gold treatment then I won't have anything but studs to screw into. I am concerned about trying to screw up lath, use masonry anchors or have any kind of flashing. Maybe the best solution really is to use 5/8's PT plywood, 15LB felt, and then densglass/sto-guard. Or at that point is it overkill? From my reading I really liked the densglass/stoguard system. I'm sure it's not cheap but I don't have an enormous area to do. And if you have to use wood is PT plywood better than marine plywood?

  11. GBA Editor
    Martin Holladay | | #11

    You don't need marine plywood. Nor do you need pressure-treated plywood. Ordinary CDX is all you need.

    However, it is vitally important that you include a ventilated air gap between your sheathing or water-resistive barrier (Tyvek) and your stone veneer. Stone veneer over wood framing is a very risky way to clad your building, so get your details right if you don't want your walls to rot.

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