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Rigid exterior insulation over existing OSB siding?

Max Anthony | Posted in Building Code Questions on

Hi Everyone,

I live in Redmond, Oregon Zone 5B and I am working on a 24W x 36L shop that I am trying to shoot Closed Cell spray foam into (7″ in rafters and 4.5″ in walls) hopefully before winter really hits. I read a lot of the articles on this site so I feel somewhat well informed but I still have a few questions as to my specific project/dilemma.

What I want to do on the walls of my shop is have 2″ of rigid on the exterior and have about 4.5′ feet height of manufactured stone veneer. Above that I’d like to do cedar shake or stained wood hardy board style siding. I know stone over rigid insulation complicates the process.

What I have right now is 6″ bottom and top plate, 2×4 staggered stud wall (8″ on center/ every 8″ a stud). Previous owner put Tyvek on the studs and then did 4’x8′ OSB grooved paneling siding. Apparently that is the siding and the structure for the wall together. It’s a very cheap setup. I wouldn’t have done it that way, but it was like that when I bought the place.

I know about installing SIP’s (Structural Insulated Panels) but as I live in the country, outside of a small city, I think the chance of getting SIP’s in town is going to be difficult. I’d have to have them trucked in from Portland OR and that I think is going to be pricey. Probably quite a bit more expensive then building something similar on-site.

What I came up with is this wall system. Hoping to get some input if this possible?

– 2×4 staggered stud wall w/4.5″ C.C Spray Foam
– Existing Tyvek over studs (not easy to take off it’s under the osb siding)
– Existing 4’x8′ osb grooved paneling
– 2″ XPS rigid foam insulation R10 (or comparable) fastened with long screws through osb into studs
– 5/8’s OSB fastened through foam, 1’st layer of osb, and into studs.
-Tyvek wrap (or should I use a rain screen product?)
– Metal Lath, scratch coat and Manf. stone and board siding above it.

How would this compromise work?

Ideally I would have done this:

– 2×4 staggered stud wall w/4.5″ C.C Spray Foam
– 1/2″ Plywood sheeting
– 2″ XPS rigid foam insulation R10 (or comparable) fastened with long screws through osb into studs
– 5/8’s OSB fastened through foam, 1’st layer of osb, and into studs.
-Tyvek wrap (or should I use a rain screen product?)
– Metal Lath, scratch coat and Manf. stone and board siding above it.

So I guess one of my most important questions is can the tyvek over the studs stay? Does it hurt to have more than one WRB? Can I also use that osb grooved siding. It doesn’t exactly have a smooth surface to lay the foam onto. It has sort of a rough surface. Also, I don’t know if those grooves help or hinder. They could channel water away, but the tops and bottoms have to be sealed to prevent air/water getting in there. To take off all the siding and the tyvek and start from scratch (like new construction) is going to be a pain in my hiney and set me back weeks before I can get to spray foaming.

I was hoping to get some wall system and exterior rigid insulation experts to chime in on the best way to do this. Also what kind of fasteners do i need to use on these layers? One of my first ideas was foam the shop, then next spring when it warms up, take off the siding and try to peel the tyvek from the C.C. spray foam. I’ve since learned that foam really sticks to anything and everything and it’s really best to not try to change siding after foaming or you may destroy the foam or pull it from the studs.

Thanks in advance.
Max

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Max,
    There are two problems with your plan. First of all, it's not a good idea to sandwich wooden components between two layers of impermeable foam. The name for the siding/sheathing on your building is T-111. T-111 is a type of plywood, not OSB. There is nothing wrong with leaving the existing housewrap on the interior side of the T-111.

    But you don't want to sandwich the T-111 between rigid foam on the exterior and closed-cell foam on the interior. The exterior rigid foam makes the most sense. I would advise that you install cellulose between the studs. If you must use spray foam between the studs, choose open-cell foam, not closed cell foam, so the T-111 can dry to the interior.

    The second problem concerns your plan to use manufactured stone veneer, a type of siding that has been associated with a very large number of failures and wet-wall disasters. I would advise you to avoid this siding type if possible. But if you insist on using it, you must include a ventilated air gap between the stone veneer and the rigid foam. The easiest way to do this is with one of the dimpled drainage mat products; it needs to have one or two layers of housewrap or asphalt felt on the exterior side to keep the mortar or adhesive from clogging the drainage channels.

    More information here: Stone and Synthetic Stone.

  2. Max Anthony | | #2

    Thanks for the response Martin.

    Is the issue the T-111 itself or it just that that anything in between the rigid insulation panel and the C.C. foam is going to be an issue?

    Actually the closed cell foam is the only thing I do want to keep in the wall system. Is it just as bad to remove the T-111 and tyvek and from the studs, put the rigid foam on, then the 5/8's osb, then the tyvek, then rainscreen product, then lath? Or even just remove the T-111 and leave existing tyvek and put rigid foam over that existing tyvek?

    Just trying to figure out other options..

    Thanks,
    Max

  3. GBA Editor
    Martin Holladay | | #3

    Max,
    It's always better to put foam on the exterior side of the studs rather than between the studs. But it's up to you how to proceed.

    The "sandwich" issue applies to all wood-based components, including OSB and plywood.

    Q. "Is it just as bad to remove the T-111 and Tyvek and from the studs, put the rigid foam on, then the 5/8" OSB, then the Tyvek, then rainscreen product, then lath?"

    A. The OSB layer needs to be nailed to the studs, because it helps brace the walls. If you want to go this route, you need to put the rigid foam on the exterior side of the OSB. Needless to say, this approach means that you shouldn't use closed-cell foam between the studs.

    Q. "Or even just remove the T-111 and leave existing Tyvek and put rigid foam over that existing Tyvek?"

    A. Again, you have to think about wall bracing. Here is an article with more information: Four Options for Shear Bracing Foam-Sheathed Walls.

  4. Max Anthony | | #4

    Thanks Martin,

    It kind of seems like I am in a catch 22. Where I can't sandwich wood between the foam's, but I also need the wood for structural bracing. I did read that article days ago on the bracing. very informative. It pretty much sounds like if you do closed cell foam, you don't do rigid exterior. Or you have to do one of those wall bracing methods, none of which I really like very much compared to a full sheathing with osb/plywood.

    I guess what I'll probably do is just stick with my original C.C spray foam plan and if I want more insulation just add more C.C foam in the wall.

    Before I hang it up can I ask one last question from you. ? :)
    Would the T-111 be an okay siding to use to hold stone? What would that system look like?
    Would be it be
    Studs (filled with C.C Foam)
    Tyvek
    T-111
    Rainscreen product
    Tyvek
    metal lath
    scratch coat
    mortar and stone

    Would this be acceptable?

    Thanks for all your help,
    Max

  5. GBA Editor
    Martin Holladay | | #5

    Max,
    You wrote, "It pretty much sounds like if you do closed-cell foam, you don't do rigid exterior." That's correct -- unless you skip the OSB or plywood sheathing altogether. I usually tell people, "If you want to use foam, just choose one place to put it. And the best place is always on the exterior side of your sheathing."

    Q. "Would the T-111 be an okay siding [sheathing] to use to hold stone?"

    A. Maybe, as long as it is structurally sound, not delaminating, and not rotting.

    Q. "Would this be acceptable: Studs (filled with C.C Foam) / Tyvek / T-111 / Rainscreen product / Tyvek / metal lath / scratch coat / mortar and stone?"

    A. First of all, you need a WRB (Tyvek would work) on the exterior side of the T-111. On the exterior side of the drainage mat, I recommend asphalt felt -- 2 layers are better than one -- instead of Tyvek, since Tyvek can be damaged by the alkalinity of mortar. Finally, adhered stone is always a risky siding, especially over plywood, OSB, or T-111. The best substrate for adhered stone is rigid foam, because it interrupts inward solar vapor drive, and is more resistant to wetting than plywood or OSB.

  6. Max Anthony | | #6

    Excellent. That helps a great deal. So then it would stack up like cc foam, tyvek, T-111, another wrb like tyvek or delta dry(?), drainage mat, asphalt felt (2layers).

    It doesn't matter that the T-111 is sandwiched between two wrb's?

    I live in extremely dry country. Dry both because it rains less than 12" a year and dry because there is generally very little moisture in the air, even in winter. I'm in high desert country. Hopefully a lot of these vapor issues won't be a problem, even though I'm going the extra mile to prevent them.

  7. GBA Editor
    Martin Holladay | | #7

    Max,
    Q. "It doesn't matter that the T-111 is sandwiched between two WRBs?"

    A. No, because the WRBs are vapor-permeable.

  8. Max Anthony | | #8

    As this is a shop and I was intending on doing two layers of plywood sheathing on the inside (electrical in conduit on the surface of that), can the first layer of interior sheathing count as my structural sheathing? Does the code allow for structural sheathing from the inside (versus being done on the outside?) I was thinking that if it does, I could then drop the T-111 and then put the rigid foam over the studs (with the already in place tyvek between the two)?

    Then it would look like

    -Studs
    -Tyvek
    -Rigid foam
    -WRB
    -Drainage mat
    -Asphalt Felt
    -Metal Lath

    Does this sound like a workable scheme. Thanks a great deal for helping me figure this out. I'm definitely going to subscribe here. Your articles are amazing. I tell everyone I know about this place.

  9. GBA Editor
    Martin Holladay | | #9

    Max,
    It's possible to provide bracing on the interior walls, using plywood as you suggest, as long as your local building inspector agrees with your plan. In some cases, you'll have to hire an engineer to specify the nailing schedule for the plywood and to approve the approach. Interior bracing doesn't tie the wall sheathing to the floor assemblies above and below the wall as effectively as exterior bracing, so the method may not be suitable in high wind areas. (Disclaimer: I am not an engineer.)

  10. Max Anthony | | #10

    Thanks Martin for all this help. I think I'm set to go explain what I want to do to the guys down at the bldng dept. I really appreciate all the help.

    Sincerely,
    Max

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