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Zip System R-Sheathing or Alternatives

mitcheta | Posted in Green Building Techniques on

Hi All
I recently purchased a 1977 home in Chicago Suburbs (Climate Zone 5) and looking at a ‘quasi deep energy retrofit’

Exterior walls are 2×4 (fiberglass batts)
1st floor is cladded with brick
2nd floor has Stucco board with some kind of hardboard sheathing (and dry rotted tudor trim boards)

I’d like to resheath and reside 2nd floor for both asethics and energy savings.
Because of the depth of the exterior 1st floor brick in certain places I probably only have 1.5 inches of thickness from the outside of the 2×4 stud to the exterior face of the brick.

I was considering:
LP Smart side
tyvek drain wrap
osb (continuous caulked)
Blown in cellulose in 2×4 wall studs (from exterior)


LP Smartside
Zip system r sheathing (either R-3 or r-6)
Blown in cellulose in 2×4 wall studs (from exterior)

Obviously i’d like to get some additional insulation factor but i see this ongoing debate on the dew point issues in zone 5 and making sure i get enough foam on the exterior.
Can someone comment on options and the potential risks of using r-sheathing on 2×4 walls?
Better off polyiso with strapping for siding?

Thanks in advance

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  1. GBA Editor
    Martin Holladay | | #1

    Here is a link to an article that explains what you need to know about the minimum R-value for exterior rigid foam: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    In Climate Zone 5, the minimum R-value for foam installed on the exterior side of a 2x4 wall is R-5.

    There are three types of Zip R panels that would work in this application:
    -- The R-6.6 panels are 1 1/2 inch thick, and include a 1-inch layer of polyiso.
    -- The R-9.6 panels are 2 inches thick, and include a 1 1/2-inch layer of polyiso.
    -- The R-12.6 panels are 2 1/2 inches thick, and include a 2 inch layer of polyios.

    If you want to keep the thickness and the expense to a minimum, choose the R-6.6 product.

  2. Expert Member
    Dana Dorsett | | #2

    In this application & climate zone don't count on the 1.5" ZIP-R to deliver the full R6.6, due to the cold temperature derating aspects of polyisocyanurate, which drops in performance fairly substantially when the average temp through the foam is below ~45F, which it will be in winter. I'm not convinced the 1.5" /R6.6 product will really deliver R5 from an dew point control perspective in your climate, but the 2" ZIP-R (R9.6) should be enough though, even after derating.

    EPS increases in performance at lower temps, and will run ~R4.5/inch when the mean temp is 40F, ~R4.7/inch when the mean temp through the foam layer is 25F. A reliable mid-winter R5+ can be had with 1.25" of foam. If you prefer pre-laminated structural sheathing clad in EPS, the Atlas LCi-SS series is worth looking at:

    LCi-SS R5 is only 1-5/8" thick and would meet code from a thermal perspective, but it's marginal.

    The 2" thick LCi-SS R7.5 is preferable, and would likely outperform 2" ZIP-R during the coldest hours of winter but underperform it during the shoulder seasons.

    A major difference between LCi-SS and ZIP-R is that LCi-SS has the foam on the exterior side of the OSB, whereas with ZIP-R the foam on the interior side. Having the foam on the interior of the OSB offers less structural rigidity at the thicker versions, and puts the OSB on the "cold" side of the assembly. Huber ZIP can take the cold & rain better than most due to their proprietary factory applied WRB and other processes, but it's likely to be more expensive too (I haven't priced them head to head.)

    [edited to correct with late breaking information]

    See the LCi-SS R-value discussion- the R5 is the value of the EPS @ 75F, not the whole product, due to their use of graphite loaded EPS, as clarified in this thread:

    With that information the 1-5/8" thick LCi-SS R5 product should have sufficient margin, no need to go to 2" for dew point control.

  3. GBA Editor
    Martin Holladay | | #3

    For more information on the products that Dana is discussing, see Nailbase Panels for Walls.

  4. user-2310254 | | #4

    Martin. The Zip R panels place the insulation on the interior. I know from talking to Huber that its engineers are not wild about the idea of using exterior foam in combination with the R products. (You can do it but...) Wouldn't Tom be better off with OSB, taped seams, and 1.5 to 2 inches of EPS on the outside of the sheathing?

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