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Community and Q&A

Zip Sheathing durability vs plwyood and self-adhered WRB

rocksteadily | Posted in Energy Efficiency and Durability on

Hey everyone,

After reading countless posts on this site, I am so excited to post here myself! My name is Derek and I am still in the early planning stages for a small 2 story 3 bedroom house (1300 square feet) in zone 5 that I plan on building myself. My priorities are affordability, durability, energy efficiency, and ease of construction, in that order. I am weighing benefits for some components of my wall assembly. I am somewhat committed to the following assembly: Fiber cement siding, 3/4 inch rain screen, 2 layers of 1.5″ comfortboard 80, A WRB/air barrier of some sort, 2×6 Studs 24″ O.C. with mineral wool batts and finally 5/8 drywall. 

At first, I was leaning towards zip sheathing as the WRB and air barrier with huber’s liquid flash product sealing the nail holes, joints between sheathing, and around and on my window bucks. My concern is in the lack of durability (priority #2) and permeability of the Zip sheathing. Yes, I know the assembly can always dry to the interior regardless of how permeable the Zip is, but I want a vapor open wall that can ideally dry in both directions. Right? The consensus seems to be that plywood is both more durable when exposed to moisture and more vapor permeable. This makes me lean towards a self-adhered WRB/air barrier with 1/2″ CDX as the sheathing. Would this be a significantly safer route if and when my house has wet sheathing?

Switching from Zip sheathing to plywood and a self-adhered WRB would be a more expensive and time consuming assembly. If the CDX and WRB assembly is in fact more durable, is the improved durability significant enough to justify the decrease in affordability (priority #1)?

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Replies

  1. Jon_R | | #1

    > a more expensive and time consuming assembly

    I'm curious, what is the cost for Zip vs plywood+self adhered WRB?

  2. rocksteadily | | #2

    The cost difference between Zip and the CDX I would want to use is relatively small. So, depending on which WRB I choose is what creates the price difference.

    Zip: $20.15
    1/2 inch 5 Ply CDX: $20.02

    Tyvek: .66 per square foot
    Henry Blueskin Vp100: .72 per square foot
    Solitex Adhero: 1.02 per square foot

    The total WRB area is 2,434.66

    For financial context, I would like to build the entire house for around 110$ per square foot meaning my budget is around $ 150,000. I understand that the WRB--even if I go for the most expensive option--is only around 2% of the overall budget, but things can add up.

  3. user-1072251 | | #3

    I'd eliminate Tyvek from your list, to start. I'm unclear what the overall difference is between one SA membrane and the other, so I won't comment on that. And I'd definitely go with the fir plywood! I would only caution you that I've had serious warping issues with SYP CDX, so avoid that.

  4. Expert Member
    AKOS TOTH | | #4

    I think you have the decimal point on the regular house-warp off a bit, up here in the great white north, it is $0.10 /sqft (that is Canadian funzo bills).

    Generally plywood/osb with taped seams with regular house wrap is your lowest cost well sealed assembly.

    The fully adhered membrane is only really worth it if you are doing something silly like open cladding or zero overhang with no gutters.

    Rigid mineral wool is also not a budget assembly as the 1.5" board is a 6" batt squished down, so you are paying for R25 of material but getting only R6 insulation. Generally roofing polyiso, especially if you can find it reclaimed, is your best $/R value.

    One could argue that a 2x8 24OC wall with R30 batts and no exterior insulation is good enough and a lot cheaper/simpler.

  5. insaneirish | | #5

    Personally, I think you're making much ado about nothing. Zip is a good product and is widely used, and I'm using it (R-sheathing, to be specific) on my own house.

    We pontificate here about ideal assemblies, and I am as guilty as the rest as far as my own personal hypothetical designs. The rude wake up call for me on my own project was realizing that getting a contractor to execute (correctly) anything out of the ordinary is near impossible.

    If your contractors haven't installed a self-adhered WRB before, prepare to be a guinea pig. Before I decided on Zip R all around, I actually toyed with the idea of having a friend who installs vinyl signs and vinyl wraps do a self-adhered WRB for me. It's not that it's difficult, but having experience handling a floppy, sticky sheet makes all the difference if you care about a clean install.

    Maybe your labor market is better than mine, but the commodity building market is a much more utilitarian place than we sometimes wish it were.

    Also: if affordability is truly your #1 priority, I don't think exterior mineral wool is your best choice. You're in zone 5 (i.e. not extreme). Do you really need 3" of exterior insulation? Or is that money better spent put into a PV array?

  6. JC72 | | #6

    ZIP overall is fine In addition with the rain screen detail and mineral wool you really don't have anything to worry about because you'll have very little vapor drive into the sheathing.

    The advantage behind ZIP is to save time getting the house up and being able to leave it exposed to the elements for (IIRC) up to 6 months. Housewrap doesn't typically afford that amount of exposure time.

  7. GBA Editor
    Brian Pontolilo | | #7

    Hi Derek.

    Many builders take a similar approach to what you are thinking, using plywood instead of OSB because it seems to hold up to wetting and drying cycles better (obviously, many builders also believe that ZIP sheathing is an excellent product). It's a judgement call, I guess. If the sheathing doesn't get wet, you won't have a problem (build the house well) and if the sheathing can dry when it does get wet, you won't have a problem (design the assembly right).

    I think it is worth noting that it doesn't make sense to compare the permeability of ZIP to another WRB. It only makes sense to compare the permeability of ZIP to another WRB plus whatever sheathing it will be installed over.

    OSB is rated at .75 perms to 2 perms (dry cup and wet cup testing). CDX is .75 to 3.5, so both are class II to class III vapor retarders depending on certain conditions (you can find that info here: https://www.buildingscience.com/documents/information-sheets/building-materials-property-table).

    The coating on a ZIP panel is about 12-14 perms, which is vapor open. In other words, the OSB in a ZIP panel can dry to the exterior just like a plywood sheathed wall with another WRB.

    1. Jon_R | | #11

      > OSB in a ZIP panel can dry to the exterior just like a plywood sheathed wall with another WRB.

      No, Zip is much lower perms than plywood + WRB. Perhaps < 1 for Zip vs ~3.2 for plywood+WRB (both "wet" perms). And a much greater difference when saturated.

      Derek:

      But it's likely that the difference won't be of any practical significance to you.

      Consider unfaced EPS instead of Comfortboard. Lower perms, but fine for your case and lower cost.

  8. rocksteadily | | #8

    Thank you so much for the replies! I very much appreciate all of the insight and expertise. I have a basic understanding of vapor permeability, which I would sum up as "more is better." comfortboard, especially when compared to similar products, is overpriced. However, its vapor permeability and dimensional stability make me more inclined to choose comfortboard over foam. I know it has less compressive strength than foam, but I am confident in my ability to install it.

    My decision to go with 3 total inches of comfortboard was due to the thickness that is readily available in my area in conjunction with the thickness required to avoid sorption of the sheathing (R7.5 for 2x6 walls in zone 5). I know that I am over-shooting the continuous insulation in two different ways: the continuous insulation is vapor permeable so the exterior insulation r-value prescription does not apply, and obviously r-12 is significantly more than r-7.5. Something about this excess feels good though. I am generally reasonable, however, so if you think I'm crazy please help me see the light. I am here to learn.

  9. Expert Member
    AKOS TOTH | | #9

    For the overall build, the wall assembly cost is pretty minimal, you can build whichever way you want especially if you are not paying for labour.

    I would recommend that you do a small mockup up the wall with the mineral wool to get a feel for it. It is not the easiest material to work with as it is still squishy and more importantly, unevenly so. This means to get a flat surface for your siding, you need to adjust every screw or use beefier strapping such as 2x4 on flat.

    If you must have exterior mineral wool, a cheaper assembly might be to cross strap your wall with 2x3/2x4 on edge and use regular mineral wool batts in between (Rockwool AFB is available in 2.5" thickness). This would have about the same R value as your assembly but use much cheaper insulation plus give a solid wood structure to attach your rain screen strapping to.

    In zone5, an R25 wall gets you most of your energy savings (even R20 is good enough). Unless you can drastically downsize your mechanical equipment, going much above that is not worth it. There is no ROI based on operating costs.

  10. Don_Christensen | | #10

    Akos - I wondered about a cross-strapped 2x6 + 2x4 assembly, such as you described above, after noting that ComfortBatt and ComfortBoard 80 have about the same R-value / inch thickness, while the ComfortBatt is only about 1/4 the cost per sq. ft. as ComfortBoard of equal thickness. (The BuilditSolar site featured a project by Montana builder Andrew Ray that used a similar assembly, although he used fiberglass batts.)

    In Derek's case, would you put structural (and air-tight) sheathing between the load bearing 'inner' 2x6 wall and cross-strapped wall? Would the WRB go there as well, or would it be better located on the outside of the cross-strapped wall under the rain-screen strapping? You could move the sheathing all the way to the outside, but then it starts to become a different type of wall.

    Also - Do the outer batts need to be enclosed by something to minimize wind-washing and realize expected R value? That would support moving the WRB, and possibly the sheathing, to the outside. Window placement, flashing, etc., need to be figured out too. It starts to look a bit complicated, but if you got everything right it could yield pretty good performance at reasonable cost. Assuming DIY or finding a willing builder, that is.

    1. Expert Member
      MALCOLM TAYLOR | | #12

      Donald,

      The amount that wind-washing de-rates mineral wool batts isn't significant enough to worry about.
      https://www.rdh.com/resource/wind-washing-effects-on-mineral-wool-insulated-sheathings/

    2. Expert Member
      AKOS TOTH | | #13

      You would build everything as with the rigid insulation. This means the plywood+wrb over your studs. The cross strapping gets attached afterwards. You can either do a couple of small saw cuts on the back (just lay out all the 2x on the ground and run a skillsaw over all of them) to allow for a bit of drainge or you can space them slightly off the wrb with some thicker washers.

      It is not any more complicated than the rigid mineral wool, you detail your windows exactly the same way.

      You do need to pre-drill the holes for the mounting screws but that is about it.

      I've done a similar assembly for over-roof. Pretty straight forward. The cross strapping is about 90% as efficient for reducing the thermal bridging of the studs as the continuous exterior rigid, so close enough.

      If you do go with 2x3, keep in mind that the AFB is designed for metal studs so it is wider. You would have to space the cross strapping at 17.5" OC.

      As Malcolm has pointed out, you don't need to worry about wind washing, especially behind cladding where there is no real air velocity. You can also always put a layer of cheap housewrap or felt over them as well.

      You can also use the same material to build your window bucks if you want outie windows (or 3/4" wider if you want to come out to the siding). Just make sure you lap your WRB layers properly over it. There are details on this site of WRB behind foam with outie windows.

  11. Don_Christensen | | #14

    Malcolm, Akos, Thank you for the additional details. It sounds like a workable assembly if you want to avoid the high cost of mineral wool board + difficulties getting the strapping in plane, or if you're leery of foam for some reason (wildfire risk, insects?).

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