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Community and Q&A

Zip R over existing sheathing

rwesthelle | Posted in GBA Pro Help on

In the planning stages of building an addition and using Zip R Sheathing. While building the addition we are considering replacing  the siding and windows on the existing house.  Question – Is there a risk of potential damage of the existing wall assembly if the Zip R product is run over the existing sheathing to mitigate air leakage and improve R value on the existing walls?  Existing – 1940’s construction, 1/2″ plaster board interior, 2×4 stud, blown in cellulous (1980’s) 3/4″ board sheathing exterior. Steam Heat.  Zone 5 one town away from Zone 6.

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  1. Expert Member
    Dana Dorsett | | #1

    Potential damage to the existing structure, no, as long as there is sufficient R for dew point control where the ZIP-R meets the structural sheathing. For a 2x4 house on the zone 5/6 boundary R6 ZIP-R (1.5" thick) is the thinnest to be considering.

    Potential damage to your WALLET, yes- there are cheaper ways to air seal that wall, and the extra half-inch of depth is probably better used for more foam thickness.

    To bring it up to current code-min, in zone 5 it would require R10 sheathing, which can be had with 1.5" foil faced polyiso. Even if labeled R9 the foil facer adds another R1 if there's at least 3/8" of exterior air gap between the facer & siding. Strapped into place with 1x4 furring through screwed to the studs for mounting the siding adds another 3/4" to the total wall thickness. Even derated for temperature 1.5" polyiso would be adequate for dew point control even at the cold edge of zone 6.

    What sort of siding were you planning on?

    1. rwesthelle | | #2

      Dana - thank you for your reply. We were thinking of Hardy Board or prefinished cedar clap board. 1x4 furring to create a rain screen would be the plan. I would like whatever we end up doing to be consistent outside the sheathing layer (new and existing) to sync details and not make it a science experiment for the contractor.

    2. JZohio | | #4

      Hi Dana, the correct order for this type of application would then be polyiso nailed onto clapboards, house wrap as wrb, then vinyl siding?

    3. JZohio | | #6

      Hi there, am I understanding this right? What you're saying is attach the 1.5 polyiso on top of the clapboard nailing to the studs. Then from there adding strapping 1x4s vertically to nail siding to?

  2. Expert Member
    Dana Dorsett | | #3

    With 1.5" it might be possible to hang some siding with long fasteners through the foam, but since you're rainscreening it that's off the table.

    With a furring & rainscreen approach it might be cheaper to go with 2" reclaimed roofing foam rather than new 1.5" foil-faced goods. Used 2" roofing polyiso is less than half the cost of 1.5" new foil faced goods. Fiber faced roofing foam is typically labeled R11.4, but with derating for temperature with 3.5" of cellulose in the wall it'll only be good for R10-ish in the dead of winter, but R12-ish during the shoulder seasons. It's usually pretty easy to find in my neighborhood. If you don't know where the foam reclaimers are in your area try searching the local Craigslist for rigid + insulation, eg:

  3. JZohio | | #5

    Hi Dana, the correct order for this type of application would then be polyiso nailed onto clapboards, house wrap as wrb, then vinyl siding?

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