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Inward solar vapor drive with Zip R Sheathing

Patrick Kelly-Decker | Posted in Energy Efficiency and Durability on

I am curious if Zip R sheathing would experience any issue with inward solar vapor drive.

We have an upcoming project that will use Zip R Sheathing (in the polyiso 1 1/2″ variety) – and will also have a reservoir cladding – stone on the exterior. There will be a minimum 1/2″ gap between the back of the stone and the zip – but it won’t be ventilated. Still I was curious if the zip coating and osb might allow vapor to be driven through them only to condense at the outside of the polyiso.

I am thinking this won’t be a problem since the temperature of the outside of the polyiso will be fairly warm even when the inside of the house is being air conditioned. – Also can moisture condense at an interface like this? Thoughts?

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  1. GBA Editor
    Martin Holladay | | #1

    The permeance of Zip R sheathing is less than 1 perm, so you don't have to worry about inward solar vapor drive through Zip R sheathing.

    I hope that your stone cladding is well detailed. Your plan for a 1/2 inch gap will work, but only if the gap is strictly maintained, and only if no mortar is allowed to clog the gap. Obviously, you will also need at least two WRBs and meticulous flashing details. Ventilating the gap would be best, but you have told us that you don't plan to ventilate it.

    Before finalizing your details, you should be aware that stone cladding over OSB and wood framing is one of the riskiest cladding methods ever devised. Many wood-framed walls with stone cladding have moisture problems, so proceed with caution.

  2. Patrick Kelly-Decker | | #2

    Thanks for the response Martin,

    I wan't thinking the inward solar vapor drive would defeat the zip R - since the overall perm rating is so low - rather just if it could get into the outer layer of osb only.

    I am not experienced with stone cladding, but would be more than happy to try to push the assembly design towards best practice.

    Would you recommend a larger air gap - say 1". Also my understanding was that we would not need a second wrb on top of the taped zip with the air gap (only with a stucco or applied stone veneer) - is this incorrect? If so is a high perm or low perm second WRB preferable.

    As for making the air gap ventilated, can you share a detail on how to execute that such that adequate ventilation is provided.


  3. GBA Editor
    Martin Holladay | | #3

    Products manufactured from wood fibers can absorb moisture from the air; that's normal. Condensation problems (and problems with drips and puddles) occur when hot humid air is allowed to contact cold polyethylene sheeting (that is, polyethylene sheeting cooled by an air conditioner). That's the essence of the inward solar vapor drive problem.

    Your Zip OSB is on the exterior side of the insulation. In summer it will be hot. It isn't a condensing surface, so there is no need to worry about the Zip sheathing causing drips or puddles.

    One way to lower the risk of stone cladding over OSB is to use Delta-Dry between your OSB and your cladding. For more information on Delta-Dry, see All About Water-Resistive Barriers.

    Delta-Dry won't work if you can't provide ventilation openings at the bottom and the top of the wall. Consult the manufacturer of Delta-Dry for more information on installation requirements.

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