As builders and designers, we have so many ways we can improve the performance of a wall assembly. Deeper wall cavities will allow for more insulation, double wall construction gains you the ability to move the air and/or vapor control to the middle of the wall leaving the inner framing cavity free for mechanical/electrical/plumbing (MEP) equipment. Then there’s Building Science Corporation and Dr. Joe Lstiburek’s perfect wall, with the right ratio of continuous exterior insulation to interior cavity insulation that lessens the worries of vapor control. Continuous exterior insulation can be designed in several ways, foam-based products, ridged mineral wool, wood fiber insulation, or nail base products. And then there’s Huber’s Zip System R-sheathing, what we call on the jobsite, Zip-R.
Zip-R isn’t new, I first became aware of the product back in 2014. What makes this product unique is that the continuous insulation layer is located between the exterior wall sheathing and framing. This location simplifies the installation of CI and wall sheathing compared to the traditional location of the insulation, which is exterior of the wall sheathing. And to add to the advantages of the product, in most applications, it is still a structural sheathing product when installed per Huber’s manufacturer’s instructions. (Consult a structural engineer if there are any questions or concerns on a specific design or building location.)
Other benefits are the product has a factory adhered water resistive barrier and can be installed as an air barrier. The installation of the product only requires one trip around the home as opposed to two or three trips when using other CI options. And Zip-R is one part of the total Zip Systems package. There are a wide variety of tapes and sealants that are part of the “system”.
Choices for wall assemblies on the Winnie Pretty Good House
When I first became involved with the Winnie Pretty Good House Project, the homeowners, builder, MEP contractor (my brother), and I had several discussions on different products and assemblies that could be used for the customer’s home. We discussed building code minimum (which, in my home state of Minnesota, is currently an R-21 cavity only insulation in a 2×6 wall) with simply improving the air sealing and water management details. We also looked at building a thicker wall using locally milled 2×10’s that would nearly double the cavity insulation value. Still another option we considered was continuous exterior insulation using a rigid mineral wool product, but this method includes more complicated bulk water management details that some builders do not want to deal with. What we finally settled on was Huber’s Zip-R product line. The builder on this project had used standard Zip panels in the past and was familiar with the product, though he had never used Zip-R before.
The homeowner decided on the Zip R-9 panel, 1.5 inches of polyisocyanurate insulation bonded to a 7/16″ OSB panel with a fully adhered (factory applied) water resistive barrier. The wall framing is 2×6 insulated with an R-21 Rockwool batt, resulting in a center of wall cavity insulation level of R-30. The total wall thickness from the Zip wall sheathing to interior face of the drywall is just under 8 inches.
Working with Zip R-9
Having the insulation layer between the wall framing and sheathing adds a layer of complexity, but still offers a familiar installation to standard wall sheathing methods. The same tools are used, fastening is still performed with a pneumatic or cordless nail gun (or you can hand nail if you so desire). Here in Northern Minnesota, we have very low risks of seismic activity, simplifying engineering.
We used pneumatic nail guns to fasten the sheathing directly to the framing using Huber’s recommended size nails and fastening pattern. Nail sizes were 0.131 shank by 3.5-inch-long nails. Our local lumber yard did not stock that size nail, so they were special ordered for this project. The nailing pattern was 3-inch spacing along the edges and 12-inch spacing in the field. It took running a few nails through the gun to figure out the correct air pressure so that over-driven fasteners where few and far between.
We had a few tubes of liquid flash to cover the heads of any nail that did end up overdriven.
Another detail we had to figure out was how we wanted to handle the window installation. The windows had a nail fin and were always planned to be “outies” or installed at the sheathing layer opposed to “middie” or “innie” windows that typically do not have a nail flange and are installed towards the inner area of the window rough opening. Because the foam layer is between the sheathing and framing, I wanted to cover the exposed foam so that it was less likely to be damaged as the home was constructed.
We ended up choosing standard Zip Sheathing in the 7/16 thickness option to fill in the exposed foam around the opening. We enlarged our windows rough openings by 1-inch to accommodate the additional thickness of the Zip Sheathing. We also planned on using a rainscreen to promote drainage and drying between the cladding and sheathing (the plywood buck framing the window opening). Once all this was in place, we simply treated the rough opening like every other window opening we prep. A flashing tape was installed across the sill (we used Huber’s Stretch Tape) and the sides and top of the window were sealed using standard Zip Flashing Tape. Taping this location was mostly for air control, though there is also a water management component in the decision to use this detail.
After windows, the next detail to figure out were the outside corners. Huber’s instructions would have allowed us to simply butt one outside panel against the other and seal using a wide piece of flashing tape. Much like the window openings, this would have left an area of foam that could have been damaged. We instead chose to remove a 2-inch piece of foam to allow for a better protected and easily sealed corner.
We had a lot of planning that went into this detail. Framing layout had to be adjusted to accommodate the offset from the insulation and sheathing. This adjustment would have been needed in one direction off the corner regardless, but choosing to use this detail resulted in adjustments to both wall framing layouts off the corner. The framing crew nailed the detail. Once the Zip-R was in place, we simply taped the corners.
Speaking of taping, Zip Tape has printing all over every roll stating to “Roll the Tape”. The process is usually done by using a J-roller. The one used on this project had small Z’s imprinted in the roller head, allowing someone to see if the tape was indeed rolled.
As I discussed earlier, we decided to install a vented rainscreen to promote draining and drying between the cladding and sheathing. The product we used was simply ripped pieces of 3/8″ CDX plywood. A piece of window screen was installed at the top and bottom of wall to keep the bugs from making a home.
A couple additional details we implemented in the design, we used Siga’s Fentrim tape to air seal the sheathing to the slab on grade foundation and we used a 12-inch wide piece of 3M 8067 All Weather Tape to tie the primary air control layer (the Zip-R sheathing) to the interior ceiling, which is reinforced polyethylene acting as both the ceiling air barrier and the required vapor retarder. These details resulted in a mid-build blower door test of 0.39 ACH50. Proof that Zip Systems makes an effective air control layer.
Additional things to know.
Something to be aware of when choosing to use Zip-R products. The polyisocyanurate insulation used in the product is a Class I vapor retarder, water vapor cannot dry outward though the insulation. Because of this limitation, you will not want to use a Class I vapor retarder on the interior (DO NOT USE STANDARD POLYETHYLENE SHEETING AS A VAPOR RETARDER ON THE WARM IN WINTER SIDE OF THE WALL ASSEMBLY!) You will want to move to a Class II vapor retarder such as kraft faced insulations or one of the “smart” or responsive vapor retarding products (we used Certainteed’s Membrain product). There is an option to move to a Class III vapor retarder, painted drywall, if you can maintain the proper ratio of exterior insulation to interior cavity insulation. (You can find more about this ratio requirement here: BSI-100: Hybrid Assemblies | buildingscience.com)
Will I use the Zip-R product again? Absolutely, as a matter of fact, I have three different projects I am involved in that are planning on using the Zip R-6 product. Stay tuned for more on those projects.
You can find Huber’s current Zip-R installation manual here: https://www.huberwood.com/zip-system/insulated-r-sheathingInstallation-Manual-ZIP-System-R-sheathing-Wall.pdf (huberwood.com), a link to the Huber Zip-R Systems website: ZIP System® Insulated R-Sheathing | Huber Engineered Woods (huberwood.com), and, 14 things to know about Zip-R: https://www.huberwood.com/blog/14-things-about-zip-system-r-sheathing-you-need-to-know