ZIP System R Sheathing vs Roxul Comfortboard
Building in the Canadian Rockies, so climate zone 5-6 ish.
Up until now I have been considering building a 2×6 exterior wall (with Roxul Comfortbatt in the stud bays) covered in 1/2″ exterior plywood, installing a air/moisture barrier from Stogard over the exterior of the plywood sheathing and then fastening1 1/2″ Roxul Comfortboards over the Stogard.
The addition of the Comfortboard is primarily to raise the interior surface temperature of the sheathing so any moisture within the wall cavity doesn’t condense on the plywood. Needless to say, the Comfortboard complicates the window, door, siding and trim details.
Now I see the ZIP System R-Sheathing.
Using it would eliminate several steps, but is there any disadvantage to having the rigid insulation on the interior of the sheathing rather than on the exterior? Are there added dangers for moisture becoming trapped inside the wall cavity?
I’ve read that there have been issues in the past of difficulty sealing the seams between panels. Does anyone have experience with this ZIP R system or would recommend it over the Stogard/Comfortboard approach?
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I have been going back and forth on Zip R-sheathing (or Hunter Xci NB, among others) for a large addition in climate zone 6 for two years. I have also been considering using Comfortboard on the exterior of a conventional wall assembly with regular Zip sheathing instead of foam. So we are in the same boat. The more I study GBA, FHB and other sources the more I feel like it is an endless rabbit hole and any decision is full of compromises.
Zip R has benefits: It provides a good thermal break and a great WRB. It simplifies the construction process and it solves a lot of problems for the window/door installation. It will save time compared to exterior insulation. And you can end up with a well-sealed box in the end.
Zip R has drawbacks: The big negative pointed out on GBA is it creates cold sheathing that could lead to condensation and result in mold or the OSB ultimately failing. I know this is based on science, but I don’t see how moisture is going to penetrate the polyiso that has a very low perm rating and it certainly is not going to come through the WRB if it is properly sealed.
Builders comment that they are concerned about structural integrity with the Zip R foam between the studs and the sheathing. This seems valid, but Zip R’s insulation layer is a lot stiffer than conventional polyiso. I took a sample of the thickest Zip R and was able to crush the insulation layer with some heavy duty screws and a lot of torque. I can imagine wind and racking pressures working the foam over time. The thicker the product the worse this would be. Huber addresses this with an aggressive nailing pattern and recommending cross-bracing with the thicker panels. Hunter recommends heavy duty cap screws on furring strips for their thicker products. I would be tempted to use a similar approach for racking at the corners even with a thinner nail board or use Simpson's WB wall bracing straps.
So what are the alternatives? Put the insulation on the outside of the sheathing. GBA, Building Science Corporation and a lot of experts say that is the proper location for insulation – some even say to put it all out there. Doesn’t that insulation end up with some of the same issues as Zip R? It is going to get punctured by lots of nails for furring strips for the rain screen and the siding. Those nail holes are going to get worked by a wind and racking pressures. And now you have the weight of the siding leveraging the insulation. In a residing project I saw what 1” of exterior foil-faced foam looks like after 20 years. The expensive seam tape had failed in many places and there were lots of signs of damage around the cap nails.
Comfortboard is an interesting exterior option. It is a fibrous product so the nails aren’t damaging it. It won’t absorb moisture. The negative is it isn’t solid. The Comfortboard 80 product is pretty soft. It is going to be very difficult to have furring strips in a plane on a 3 or 4 inch layer. So windows are going to be even more of a nightmare and you would likely not be able to put as much insulation there. Maybe a commercial construction exterior mineral wool product would work better if you can find it.
So where does that leave all of us wrestling with this decision that we will have to live with for decades? Choosing the compromise that works for our build. As for me, I am still looking for the rabbit. Perhaps he has the best solution – build underground.
Well said Ron Rosen #1
Scott, your question is discussed a lot here on GBA, in various forms. The hoops that Huber had to jump through to get their ZIP-R sheathing approved for structural use leave me comfortable with it structurally--as long as the proper nailing schedule is followed. Their thicker panels require longer-than-standard nails to develop the proper shear strength. The foam is stiff but with the way loads are imposed on offset nails I would be shocked if the foam's strength factored into the equation. The foam also has to be thick enough to keep the interior face of the foam above the dewpoint temperature.
With ZIP-R, having the OSB on the exterior is not ideal, but if you use a rainscreen system, if it gets wet it can easily dry to the exterior. If you add a cheap WRB over it, that is added insurance.
Rigid exterior mineral wool can also work fine. Many people prefer it, as it's vapor open and has low embodied energy. We used it on Fine Homebuilding's 2016 ProHome. It is squishy, and takes a bit more effort than rigid foam, but from what I hear it's not that hard to get a flat plane, especially with thinner layers. I know people applying 8" layers; that starts to get tricky, but it's still do-able. An advantage of mineral wool is that it allows drying to the exterior at all times of the year. Another is that if you skimp on the R-value when it comes to dewpoint control, it's not a major issue, because the sheathing can dry to the exterior.
Thanks for your replies. Yes, a main concern with the ZIP R system is the fact that the rigid insulation is in between the OSB sheathing and the stud, requiring more and longer nails to secure the panels to the wall. You would also need to be very vigilant with the nailing patterns. Another concern is the permeability of the rigid insulation (All moisture within the wall cavity would have no choice but to dissipate to the interior).
With the Comfortboard approach, I was planning on adding 2x2 material around each door and window opening and butting the Comfortboard up against that. The exterior wall sheathing and the built out door and window openings would also be protected by the Stogard Gold line of products before attaching the Comfortboard..
http://www.stocorp.com/sto_products/sto-gold-coat/
Unless anyone else has other suggestions, this is probably the best system I can think of for exterior walls.
Ronald,
That's a really helpful summary of the issue. One small comment: The concern some of us face with the structural performance of Zip R is that the sheathing is designed to provide shear-strength for the walls. The foam interferes this this, although as you and Mike say, Huber has compensated with their fastener schedule, so it shouldn't be a problem outside high seismic regions. With exterior foam the problem is with securing the cladding. Similar, but structurally, much less central to the integrity of the building's structure.
One simple technique if you like everything about Zip-R but you don't trust it for shear strength: install diagonal bracing on the studs before installing the insulating sheathing. There are Simpson straps made for that purpose, or old-fashioned, let-in 1x4s meet code in many situations.
R602 has a lot of interesting (to some of us) options and requirements for wall bracing: https://codes.iccsafe.org/public/document/IRC2015/chapter-6-wall-construction.
Scott, keep in mind that any 2x battens you add will bypass the Comfortboard. If condensation on the inside of your sheething is a concern, that concern would still exist around your penetrations due to the thermal bridge.
Lance, I thought about that but I'm not sure if it would be a concern. Applying a 2x2 batten around all four sides of a window or door opening would result in the batten being on one side of the 1/2" plywood sheathing and a 2x6 stud (or header) on the other side. I think the structural material pressed against the inside of the plywood would prevent condensation, whereas in the cavity bay the interior face of the sheathing only has the insulation pressed up against it. I suppose I could apply a bead of caulk between the wall studs and the sheathing around each opening to prevent moisture from getting in there.
Scott, there is a product called Thermalbuck made specifically for wrapping window openings in applications like yours.
Thanks, Michael. I didn't know these existed.
I know this is an old thread, but on our house we used the 2-1/2" Zip R-Sheathing. Our Structural Engineer specified using screws to adhere it because it is virtually impossible to to get the length of nail required or to find a nail gun that can nail it. Also, a wimpy nail vs a stocky screw, the screw won out. The only technical problem was that the screw heads stood proud of the sheathing and made it so we needed several passes of the Zip tape to seal it. I would defiantly recommend using the Zip liquid seal instead of the tape on the thicker sheathing.
Lawrence,
You might want to check with Huber. I was under the impression that Huber required the use of nails, not screws. See the table below (from the Zip R Installation Guide).
Hi Martin,
I recall that we did confirm the use of screws with Huber. We had to use the really big Headlok screws. They basically said that they defer to our Structural Engineer. The problem we had was that we used the R-12 product, and we were unable to find the nails that had the length they required. We even had trouble finding the HeadLok screws at one point.