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Use regular OSB or Zip Wall OSB in conjunction with 2″ XPS?

mogilski | Posted in Green Building Techniques on

My wall plan consists of:
1- fiber cement siding
2- 3/4″ air gap/ 1×4 firing strips
3- 15# felt (or possibly Tyvek housewrap) for general protection of the XPS
4- 2″ rigid XPS with taped seams and caulked at butt joints
5- 7/16″ OSB (possibly taped Zip Wall)
6- 2×4 24OC stud wall with mineral wool or fiberglass insulation
7- 1/2″ drywall sealed at top and bottom and around device boxes and penetrations

My concern is if it is necessary (or recommended) to use the taped Zip Wall OSB, or should the general protection of the felt and the perm rating of 2 inches of XPS be sufficient for proper vapor retarding and proper drying. All the framing and sheathing will be caulked and sealed to achieve a tight house. The reason Tyvek housewrap is not speced to go between the OSB and 2″ XPS, is because we plan on running a bead of OSI Draft & Acoustical caulk on the back of the XPS around the perimeter and a horizontal bead every 2′ to create the look of a ladder on the back of the panel; this will have the XPS and OSB be sealed together with minimal air movement. I feel the button caps and the space in front of and behind the Tyvek would lend to air movement between the OSB and the XPS if I installed it between them. Any thoughts?

Thanks, Nathan Mogilski

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  1. GBA Editor
    Martin Holladay | | #1

    Whether or not to invest in Zip sheathing depends on (a) whether you care to use the Zip System (including Zip System tape) for its air sealing benefits, and (b) whether you trust ordinary OSB to be a durable material over the next 100 years.

    Some conservative builders and building scientists say that the jury is still out on OSB longevity. These old-school conservatives prefer plywood to OSB. If you want to choose an OSB, almost everyone agrees that Zip System OSB is a better product than ordinary OSB.

    Zip System plus Zip System tape is a good, fast way to create an air barrier.

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