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Community and Q&A

I am thinking of using Huber’s new Zip System R-Sheathing on new construction

ncjimK | Posted in Green Building Techniques on

I have three questions:
1. What has been users’ experiences with this product?
2. Do I need to install a rainscreen over it if I am installing fiber-cement siding?
3. What’s the best insulation in the 2X6 wall behind it? Is flash & batt overkill?

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  1. kyeser | | #1

    The sheets seem to run slightly irregular in size. Sometimes longer and wider sometimes smaller. Also thickness seems to run a little different between some sheets. All and all it seems to work, just a little funky.

    Also be really carefully around your openings. The sheets have a tendency to compress when nailed. So next time we use it I will check every window and door opening before installation to confirm a uniform width in the r.o.'s. The depths can vary a lot.

    Next I believe when Zip is taped it is considered a WRB so there is no need for an additional layer of house wrap.

    As far as insulatory value R sheathing is polyiso, so in colder climates the actual r value is going to be lower than the stated r value, so we are now adding addional rigid insulation over the r sheathing (1" eps) to help increase overall wall r value and dense packing cellulose in 2x6's. I don't do flash and batt but I think the additional money spent on that instead of cellulose could be spent on more rigid insulation on the outside.

  2. Expert Member

    Kyle is right in that you don't need a separate WRB with ZIP sheathing although it does make flashing a lot easier, but including a rain screen will greatly increase the resiliency of the wall and the lifespan of by the sheathing

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Do I need to install a rainscreen over it if I am installing fiber cement siding?"

    A. Here is a link to an article that may help you decide whether to install a rainscreen: All About Rainscreens.

    Q. "What’s the best insulation in the 2X6 wall behind it? Is flash & batt overkill?"

    A. Zip R-sheathing comes in two versions -- either R-3.6 or R-6.6. If you are framing your walls with 2x6s, neither product is suitable unless you are building in Climate Zones 4 or somewhere warmer. If you are building in Climate Zone 5, the minimum R-value for the rigid foam layer is R-7.5. I suppose that the flash-and-batt method might be one way of combating the low R-value of Zip R sheathing.

    For more information on minimum R-values for exterior rigid foam, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  4. ncjimK | | #4

    Thanks for the reply. I have been reading your postings on GreenBuildingAdvisor for a while and they have been very helpful.
    I am just a homeowner doing the research so that I can the right questions of my builder, energy consultants and designer. I think that I maybe driving them crazy. Especially since some of the information I am finding can be confusing as well as contradictory.
    I am building in the only part of North Carolina that is Climarte Zone 5A.
    Specifications from manufacture of the Hardiboard require 3/8 inch rain screen for commercial applications, but not for residential use although the spec’s for residential use identify it as a best practice. Their Tech people say that it is not required for residential because of eaves on most houses.. Does this sound right? Looking around at most new construction, I see almost no one installing rainscreens around here.
    Wondering if the coating on the exterior of the Zip panel would provide a small drainage layer. Here is what they say in the product literature. “Superior drainage efficiency that channels moisture downward off the panel face”

    Wall R-Value:
    Looks like the local zoning requires R19 walls. The rigid foam plus 6 inches of fiberglass should give us R25 or so. What R value should we be looking at?

  5. GBA Editor
    Martin Holladay | | #5

    As I explained in my earlier answer, Zip R Sheathing does not include enough R-value to be used in your climate zone (Climate Zone 5) over 2x6 walls. If you want to use exterior rigid foam, I suggest that you install the rigid foam separately from your OSB or plywood wall sheathing.

    The sources that tell you that a rainscreen under fiber-cement siding is considered best practice, but is not strictly required, are accurate.

    There is no single answer to the question, "What R-value should we be looking at?" Your local building code provides minimum requirements; exceeding these minimum requirements will lower your energy bills.

  6. Expert Member
    Dana Dorsett | | #6

    Actually, since the foam on ZIP-R is on the interior side, making the foam (not the OSB) the condensing surface, it doesn't really matter what the R-value of the foam is- even if it were R11.25 it wouldn't affect the moisture content of the OB. It a stackup that falls somewhat outside the bounds of IRC prescriptives for insulating sheathing, prescriptives designed as dew-point control for an OSB that is inside the insulating sheathing. Chapter 7 ( ) does not apply, or at least in terms of the R value of the sheathing.

    But the " Vented cladding over wood structural panels." portion for zones 5 & under IS relevant. With even a 1/4" of rainscreen the rainscreen alone is sufficiently protective of OSB according to the IRC prescriptives for US climate zone 5 meeting "Vented cladding over wood structural panels." condition.
    If you don't want to use furring, 6mm Obdyke Rainslicker mesh between the ZIP and siding works.

    Then you have to consider how much if any moisture will accumulate inside the fiber insulation:

    The vapor retardency of the foam is about 1.5-2 perms according to the manufacturer (


    R-sheathing is a multi layered product. The combined system permeance is 0.8-1.1 perms. The permeance of each layer when measured by ASTM E 96 Procedure B (Wet Cup) is as follows: • WRB Overlay 12-16 perms • 7/16” OSB 2-3 perms • Coated Glass Facer 45-50 perms • 1” Polyisocyanurate 1.5-2 perms • Coated Glass Facer 45-50 perms"

    With a total exterior side permeance of 0.8-1.1 perms there is still substantial drying into the rainscreen gap even in winter, but not so much that it's a no-brainer to use latex paint as the only interior-side vapor retarder. A conservative approach would be to use either cellulose as the cavity insulation (for buffering wintertime moisture drives without loss of function or damage) and/or an interior side "smart" vapor retarder such as Certanteed MemBrain or Intello Plus, detailed as an air barrier between the wallboard & studs would make the assembly even more resilient. That's cheaper than a flash & fill approach providing sufficient total R of the ZIP-R + flash-foam for dew point control to keep the insulation dry. It's greener too, since it doesn't use any HFC245fa-blown foam.

    Of course, in zone 5a building with standard ZIP and 2-3" of exterior EPS or rock wool panel would be even better and you wouldn't need to worry about interior side vapor retardency.. With 3" of EPS outside the foam you could use just about any cavity-fill you like, but with 2" it's still a bit more resilient by going with cellulose.

    If wall thickness is an issue, going with 2.5" of exterior EPS and 2 x 4 framing delivers a higher thermal performance (about R20 whole-wall) and more resilient wall than 1" ZIP-R at the same wall thickness. (R19 whole-wall, a bit less when it's below freezing out, due to the necessary derating of the polyiso layer.)

  7. ncjimK | | #7

    Thanks Dana:

    So is what you are saying is perhaps a better way to go would be a wall assembly that looked like the following::
    2 inch of EPS
    plywood or OSB
    cellulose fill

  8. Expert Member
    Dana Dorsett | | #8

    That is a stackup that works for zone 5.

    Two inches of Type-II EPS would be about R8.4 when it's not too hot or cold out, but when it's 25F outside (which is when the dew point of the interior air starts to matter more) it's performance will be about R9, and incrementally higher on those single-digits days, which gives you some margin over the IRC prescriptive R7.5 minimum. Don't try to cheat the system and cut back to 1.5"- if anything, take it up to 3". That extra inch will cost 40 cents per square foot more, but it buys you huge dew point margin. If you're heating the place with propane it'll still pay back on fuel use too- maybe not if natural gas.

    IRC 2012 code-min for zone 5 would be R20(cavity) + R5 (condinous insulation) which is about the performance you get with the 1" ZIP-R. That's not an energy-pig, but it's the crummiest wall you'd be allowed to build. The 2" foam gives you R3 better than code (and higher resilience), but there is still an lifecycle economic rationale for more- (quite a bit more, in fact).

    With 3" (R12.6) foam on the exterior of a 2x6 cellulose wall the whole-wall R will be about R26-R27 after factoring in the thermal bridging of the studs. Depending on your insulation & energy costs there may still be a financial rationale for something north of R30 whole-wall, according to the anlaysts at Building Science Corp. Read the whole first chapter, but see the recommended starting points in Table 1, p.10 of this document: (Those are "whole-assembly" R values, not center-cavity.)

    So at least think about it. Two inches would be OK from a dew-point perspective, but the labor cost for going 3" is about the same as for 2", its primarily a change in material cost.

    Air seal the OSB/ply to the studs with caulk prior to insulating, and don't forget to caulk between doubled-up top plates, and under the bottom plate, etc. Air sealing as you go is cheaper and more reliable than trying to retrofit it to tight levels after the fact. On the foam, two layers is better than one, with seams taped on both layers, and overlapped by a foot. Over then next 40 years the EPS is going to shrink a bit, but the performance hit won't be too bad if the seams are overlapped and still reasonably air-tight.

  9. CBG Passive | | #9

    To pickup on this conversation…I'm also trying to determine the best fit for wall construction for a new home I'm designing & building. I'm a builder (with an architecture degree) and would like to use advanced framed 2x6 walls with r-sheathing (r-6.6) in climate zone 5. I'm shooting for an overall r-40 performance and would like to use full depth closed cell foam on the interior (r-30+). The r-sheathing, for me, keeps costs low and details straight forward (window and trim installation) and I love the multi-level benefits of it. I'm trying to identify a way to use less layers with sophisticated products. If the primary use of the r-sheathing, in this case, is to achieve a thermal break, do I need to worry about additional density having to do with the dew point conversation?
    Overall, my preferred system would be:
    hardi plank
    rain screen
    1.5" r-sheathing
    2x6 walls with closed cell foam

  10. Expert Member
    Dana Dorsett | | #10

    With 3-5" of air-impermeable closed cell foam in the cavities you don't have a dew-point issue. The closed cell foam is a class-II vapor retarder, and there is zero risk of air transported moisture getting in.

    Instead what you would have is a severe lifecycle global warming problem, due to the blowing agents used for closed cell foam, the vast majority of which is blown with HFC245fa (at about 1000x CO2 global warming potential.)

    There are a very few water-blown closed cell foams out there (Aloha Energy's 1.8 lb foam, Icynene's MD-R-210), but almost everyone else is using HFC245fa (under a number of trade names). There are at least two HFO1234yf based blowing agents out there with a global warming potential less than 4x CO2 (Honeywell's Solstice, and DuPont's FEA-1100), but they do not have huge market penetration. (I can't name a single vendor who is using either in the US.)

    The large cost of closed cell foam as cavity fill is also not a great value, since the performance of high-R/inch foam is cut by the 20-25% of the wall area that is comprise of R1.2/inch timber. The whole-wall R (all thermal bridging and sheathing R accounted for) of a 25% framing fraction 2x6 wall with 5" of R6.5/inch foam (not 5.5", since it can't easily be trimmed flush in any practical way, it must be under-filled) , with the R6.6 ZIP-R and 1/2" gypsum on the interior is about R22.5.

    And that is despite a center-cavity R (all layers included) of about R39.5.

    If you went with cellulose or open cell foam as the cavity fill and did the full 5.5" you'd be at a whole-wall R of about R20.5.

    That's a pretty expensive R2 improvement, which could be made up for with as little as 0.5" of EPS on the exterior of the sheathing.

  11. CBG Passive | | #11

    Thank you Dana. When I start to move away from the class II vapor retarder that is closed cell foam (and instead add more EPS to the exterior side of the sheathing) that puts the osb back as the condensing surface, in which case, it would be imperative to meet the R-7.5 minimum for class-5, correct?

  12. Expert Member
    MALCOLM TAYLOR | | #12

    Matthew, Gregory La Vandera had an interesting blog which peripherally referred to Swedish framing, a technique that may hold more appeal than Advanced Framing for you.

  13. CBG Passive | | #13

    Thanks Malcolm. Chasing that blog and its tangents lead me to be considering Roxul's Comfort Board IS (3") on the exterior side of the sheathing, in lieu of eps or xps. After reading all the material I didn't know about stone wool (and also some of the downsides of foam), I'm also considering the batts for the 24"oc 2x6 bays. Of course I could go further using a more swedish approach. Thoughts? For Zone 5, vapor barrier behind gwb right?

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