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Can plywood furring be placed between two layers of WRB?

Jean Sebastien Grise | Posted in Building Code Questions on

Here’s my exterior wall composition:
– Drywall
– 1×3 furring strips
– Vapor Barrier
– 2×6 @ 16″ insulated with R20 pink wool
– Insulated BP R4 Structural Panel (1″ foam laminated on a tentest sheating)
– BP AirLock WRB (similar to Typar)
– 5/8″ CDX Exterior plywood as a nailing base for the white cedar shingle siding.

Question: Before nailing the shingles, what should be applied on the plywood to assure that the plywood won’t get wet if the siding fail and that it can breathe at the same time? Can adding a 2nd WRB be good?

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Replies

  1. Anthony Ratliffe | | #1

    Why not use a nailbase (one sided) 2" to 4" SIP directly over the studs? That seems to me to simplify the work considerably, upgrades the insulation standard, and lets you apply Typar and then siding to the exterior SIP's surface.

    Tony.

  2. Jean Sebastien Grise | | #2

    Thanks for the reply Tony, the thing is that the wall is already built the to plywood. What would be your suggestion then?

  3. GBA Editor
    Martin Holladay | | #3

    Jean Sebastien,
    First of all, I hope you live somewhere warm. If you live in climate zone 5, 6, 7, or 8, your exterior foam sheathing has too low an R-value to keep your stud bays above the dew point in winter. (For more information in this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)

    It won't cause any problems to install a WRB on the exterior of your plywood sheathing; in fact, it is a code requirement.

  4. Jean Sebastien Grise | | #4

    Thank you, I just wanted to make sure that adding a 2nd WRB would not be an issue.

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