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Sprayfoam house and real stone cladding. Wall sheathing?

Mark Helmrich | Posted in General Questions on

Good day,

This is my first post, but I want to comment how much I’ve enjoyed reading topics on this q&a forum.

My question is regarding a new house I’m building. Zone 8 climate (cold!) similar to Alaska, 2×6 framing 16″ OC walls, 1/2″ OSB sheathing, no basement. Right now I’m just finishing up framing.

I’ve been dead set on using closed cell spray foam all around…on the interior wall cavity at 3″ thick, 4″ on the roof deck and 1.5″ on the floor.

I’ve always been concerned about moisture and stories of wood rot from using spray foam. At the same time, I like that its a vapor barrier and I know that its well bonded to the studs/sheathing.

My question specifically is regarding application of real stone (travertine, 1-1/2″ thick tight stack cladding) on the outside of the entire building. I was hoping to use a product similar to Wedi board to cover the entire perimeter of OSB sheathing and then affix the stone directly to the Wedi with a exterior rated medium bed mortar. The Wedi board seams are sealed with polyurethane sealant to make it waterproof.

For those not familiar with Wedi, it is a 1/2″ thick XPS board with cement coating and fibremesh on both sides. I know its not vapor proof, but I’m unsure of its perm rating. I’d imagine around PERM 1 or so.

I’m worried now that the OSB will become sandwiched between the Spray Foam on the inside and the Wedi strofoam on the outside causing the OSB in the middle to rot over time.

I was set on using the Wedi XPS on the outside so that I could provide a thermal break to the studs, but I’m starting to think it may not be the best choice.

I’ve been told that perhaps a air gap, perhaps 1×4 strapping or similar before applying the Wedi to the outside might make it a better solution so the OSB can better dry should it get wet?

Sorry for the long post. I look forward to any replies.

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  1. GBA Editor
    Martin Holladay | | #1

    In general, I always prefer to see foam insulation installed on the exterior side of wall sheathing rather than between the studs, for two reasons: (a) when installed on the exterior of the house, the insulation helps keep the wall sheathing warm and dry; and (b) insulation between the studs doesn't address thermal bridging through the studs.

    If you install closed-cell spray foam between your studs, you will prevent the OSB wall sheathing from drying inward.

    You might want to consider a different approach: installing R-15 of rigid foam on the exterior side of your wall sheathing, and filling your stud bays with dense-packed cellulose. That way your OSB would stay warm and dry, and the OSB could always dry inward if there were a wetting incident.

    If you instead go ahead with your current plan, it's worth noting that stone veneer over OSB tends to be a risky wall assembly. I would certainly urge you to include a ventilated air gap between the OSB sheathing and the stone veneer. If you take this advice, the insulating value (which in any case is minor) of the 1/2 inch XPS would be lost, but the OSB would be far less likely to rot.

  2. Richard Beyer | | #2

    "My question specifically is regarding application of real stone (travertine, 1-1/2" thick tight stack cladding) on the outside of the entire building."

    I would NEVER recommend the use of Travertine in a freeze thaw environment. In my opinion your asking for trouble.

  3. Mark Helmrich | | #3

    Thanks Richard and Martin.

    Richard. Let me thank you for your answer. I've got an interesting predicament for sure. You see in my part of the country everybody builds 2x6 with OSB and insulation from the inside. You are lucky to get mineral wool instead of standard fiberglass insulation.

    So with that said, spray foam is the only real option I can think of now because adding R15 foam to the exterior I would imagine would not be possible without first allowing for the extra thickness on the outside (windows and such).

    Of course any suggestions would be greatly appreciated.

    In regards to the ventilated air gap, I've been suggested using chicken wire and then to staple cement board over top of it. I'm thinking 1x4 strapping might be a better solution with better airflow?

    Richard. Thank you as well for your reply.

    Yes, I agree that Travertine may not be the best suited option for freeze thaw but from my research so far I've found that there has not been much testing in cold climates and therefor no-one does it.

    I've talked to people who have used slate cladding outside with no issues in my area. A good application of sealer every few years is apparently all that is needed.

    To test I've installed some travertine outside on a south wall on cement board. I'm them seeing how it does. I will even throw a bucket of water at it and see how it reacts. This is with an un-sealed piece.

    Please keep the advice coming. I appreciate any help I can get :-)

  4. GBA Editor
    Martin Holladay | | #4

    I wouldn't do what you are doing, but it's your house.

    I always find that it's particularly poignant to hear that someone is choosing details that are both expensive and inadvisable. If you are going to spend money on expensive siding and spray foam, it's really a shame that you can't come up with better details.

    You definitely want a real ventilated air gap -- not just chicken-wire. The problems with adhered stone veneer over OSB are comparable in some respects to the problems with stucco over OSB. You may want to read more on these issues here: To Install Stucco Right, Include an Air Gap.

  5. Rick Van Handel | | #5

    Is spray your only insulation? Sounds like a fairly expensive house. Why such low r values?

  6. Mark Helmrich | | #6

    Thank you again for your replies.

    Martin. Funny thing is, it was one of the top builders in town AND the building inspector that came up with the chicken wire idea! I'm not so fond of this ideal for obvious reasons. As for better details...that's why I signed up for the find them :-)

    I read the article you posted, but I'm not sure if the identical situation is relevant with stone. I do want to leave a proper ventilated air gap, but because I want to attach cement board on top still...i'm not sure if the typical ventilated air gap plastic drainage mat would compress when the cement board was screwed to it.

    Would 1x4 or 1x3 strapping 16" OC direct to the studs with the cement board on-top work? I'm worried there might be to much deflection?

  7. Mark Helmrich | | #7

    Hi Rick. Thanks for your reply as well.

    Discussing Travertine as a siding sounds expensive and lead those to think of a "expensive" house, but I'm actually doing it as cost savings.

    I'd prefer not to use vinyl siding. Stucco goes for about $20 sq/ft supplied and installed in my area. Cultured stone near $30. That includes tar paper, lathe directly over the osb. Yes, they don't use ANY sort of air ventilation if you go with the pro's.

    I'm a tile installer for 15+ years, and I can supply cement board and the travertine (not including labor) for similar price to the vinyl siding.

    I was planning for spray foam to be my only insulation because its 100% vapor barrier. As I was planning to do the roof deck, walls and slab with the stuff, I figured I could build an air tight house that would be good quality. I'm option to any options though.

    Looking forward to any suggestions.

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