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Community and Q&A

Roof Zip Sheathing and Vapor Control

adam_nw | Posted in General Questions on

Hi All,

Location: Seattle
Climate Zone: 4C

Roof Assembly:
-5/8″ Zip roof sheathing
-1 1/2″ continuous ventilation channel from eave to ridge
-1/2″ CDX ply, taped for air-sealing

I recently re-built the roof of my garage.  The day of the standing seam install was clear, but very cold.  There was frost on the roof and in some places even ice.  The installers hit the roof with a torch and a blower and got the bulk water off, but the Zip was definitely still damp when the I&WS went on.  The installers seemed unconcerned and I was anxious to keep the project moving.  I know what the product literature says, but I also understand that it’s the PNW and we sometimes have to work with the conditions we’re given.  I’m curious if this was a bad decision.

Will water vapor find it’s way out of this assembly when the weather warms up?  The Zip facing has a relatively high perm rating, but the OSB is only 1-2 perms.  Can vapor make it’s way through the Zip panel and exit through the ventilation channel or is this now a moisture sandwich that will never dry and lead to rot?

Any input would be greatly appreciated.  Cheers,


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  1. Expert Member
    Akos | | #1

    Since you have a vent channel, any moisture will move through the ZIP and eventually evaporate.

  2. Expert Member
    Michael Maines | | #2

    I'll second Akos' thoughts. If you were spray-foaming the rafter cavities I would be concerned about moisture in the OSB, but with a vent space, there's nothing to worry about.

  3. adam_nw | | #3

    Thanks to both of you for easing my concerns. Greatly appreciated!



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