Jeff Smith is building a house in North Dakota and is far enough along to have installed insulated sheathing on the exterior. He’s chosen Zip R6 sheathing, a product made by Huber Engineered Woods that combines OSB structural sheathing and rigid foam.
“Our intent was to create a tight, energy-efficient home, and to deaden sound,” Smith writes in a post at the Q&A forum. “However, now that we have the sheathing on and are siding, I have read people stating that R-sheathing is not well suited for cold climates and that we should have used a thicker foam and placed it on the exterior side of the sheathing.”
His concern is moisture. When a continuous layer of foam on the exterior of a building is too thin, the risk is that moisture making its way through exterior walls will condense and freeze on the inside face of the sheathing during the winter.
“So my question is this: What should we do, at this point, to ensure our walls do not have moisture issues or that they dry appropriately when moisture does enter the wall?” Smith asks. “For what it is worth, the siding is LP Smartside.”
That’s the topic for this Q& A Spotlight.
Right church, wrong pew
Zip-R can work in Climate Zone 6, but it would have been better to choose Huber’s R-12 version rather than the R-6, Michael Maines tells Smith. However, because the inside of the walls is still open, a solution is still very much within reach.
“You can add closed-cell spray foam to the interior to create a flash-and-batt detail. Just make sure at least 35% of the R-value is on the exterior,” Maines writes. “A riskier solution that could work (no guarantees) is to install a variable-permeance membrane on the interior, such as Siga Majrex or Pro Clima Intello, and to keep a close eye on your interior humidity levels.”
Maines normally wouldn’t recommend spray foam, but this situation seems well suited to it. An inch or two of closed-cell foam will create an effective air barrier, and Smith can fill the rest of the wall cavity with less expensive fluffy insulation — either fiberglass or mineral wool would be typical choices.
Does the placement of the foam layer matter?
Smith points out that Huber’s R-sheathing places the foam layer on the inside. So, the condensing surface for moisture exiting the house would be on a layer of foam, not OSB.
“I assume the concern here is that the interior face of the foam might act as a condensing plane. Is that correct?” Smith asks. “So, by adding spray foam against the foam, there would no longer be a condensing plane within the wall cavity. With or without the spray foam, the OSB portion of the sheathing will remain cold , so I assume we are not concerned about moisture accumulating on the OSB in either scenario. Is that correct?”
That’s right, Maines replies. That’s the idea behind making sure that 35% of the insulation is outside the sheathing in Smith’s climate zone. With the layer of closed-cell foam in place, the inner face of the sheathing may be cold, but it won’t be getting much moisture from the interior, he says. A rainscreen would make the assembly more resilient.
He suggests that Smith specify closed-cell foam (2 lb. per cubic foot), and preferably one that uses an HFO (hydrofluoroolefin) blowing agent rather than the more environmentally destructive HFC.
“In theory, you shouldn’t need the interior vapor retarder,” Maines says. “Latex paint on drywall will meet the code requirement of a Class 3 vapor retarder. But a variable-permeance membrane won’t hurt anything either.”
Is racking a potential problem?
In conventional wood-frame construction, sheathing (either panels or boards) is nailed directly to the studs. This provides good resistance to racking — the risk that the wall will collapse like a cardboard box under a load.
Racking, says John Clark, is a possible risk with the use of Zip-R sheathing.
“Unlike sheathing attached directly against the framing, Zip-R panels with thicker foam cannot be relied upon to provide racking resistance,” Clark claims. “Typically, this is remedied with the addition of diagonal bracing. You may want to double check local codes and Huber recommendations.”
Winter condensation on the back of the panel isn’t an issue, he adds, because the foam layer is vapor impermeable.
Wrong on both counts, Maines replies.
First, Huber has tested its Zip-R products for shear strength and developed appropriate nailing schedules. The R-12 sheathing would require 4-inch-long nails (Bostich makes a nail gun that will handle nails of that length).
“I answer questions from builders regularly about Zip-R thickness,” Maine says. “One last night from a builder I work with a lot, who got a design from someone else that shows Zip-R too thin for our climate zone, just like Jeff’s situation. I predict an eventual class action lawsuit against Huber, not because they have a bad product, but because they have not successfully conveyed the critical thickness issue.”
Is spray foam the only option?
The potential for off-gassing from the spray foam is a factor for Smith. He cites some specific health concerns in this family that makes him want to avoid spray foam.
“That being said,” he adds, “in your opinion, is spray foam the only safe step we can make at this point to not risk failure of our walls?”
No, replies Dana Dorsett, Smith does not have to use spray foam.
With the addition of an inch of foam, exterior walls can remain relatively vapor-open without the risk of moisture accumulation on the back side of the sheathing, he said. But the wall can still be safe without foam if other means are used to keep moisture out.
“There are thousands, even tens of thousands of existence proofs in Canada that a 2×6 fluff-filled wall with R-5 [rigid foam] on the exterior and a 6 mil polyethylene vapor barrier on the interior isn’t a moisture problem,” he writes, “but the air sealing of the vapor barrier and wall gypsum needs to be done well.”
Installers in Canada are well trained, he says, but here in the U.S., Smith may have to be his own inspector. Either polyethylene sheeting or a smart vapor retarder can be used successfully — just don’t be too sloppy about the air-sealing details, Dorsett recommends.
After suggesting a number of specific steps Smith might take to reduce air leaks, Dorsett says that if the siding hasn’t been installed yet, there’s still time to add more foam on the exterior, specifically 1 1/2 inches or more of unfaced Type II EPS.
Our expert weighs in
Peter Yost, the technical director at GBA, adds this:
There are building science/moisture management relationships between rainscreen claddings, types of cavity and exterior insulation, and the class of interior vapor retarders (particularly for cold climates). The 2015 codes do try and weave these together, but it is not an easy task.
Since this blog is focused on Huber’s Zip system R-sheathing, let’s start by recognizing the web resource the company has, as well as its well-staffed tech support system. Huber technical representatives can also be reached by phone (800-933-9220, ext. 2716).
Zip-R sheathing systems are well-suited for cold climates. It is important that the full wall assembly be configured based on climate; hence, this online resource: “Designing with continuous insulation – Learn how to determine the right amount of exterior insulation to ensure your project meets code” (a CEU webinar course).
I see three options for Jeff Smith:
- Flash the appropriate depth of closed-cell spray foam and complete the cavity fill with air-permeable insulation (the layer of closed-cell foam plus the rigid foam in the sheathing should equal at least 35% of the total wall insulation R-value). This moves the first condensing surface to the interior face of the spray foam, moving it far away from dew point in this assembly and enabling a Class III interior vapor retarder. So the cavity has good drying potential to the interior should moisture get into the framing cavity.
- Fill the cavity with air-permeable insulation with a smart vapor retarder (SVR) on the interior. SVRs move from borderline II – III (1 perm) when dry to vapor open (greater than 10 perms) when wet, so this meets code and can work well, so long as this approach is coupled with reasonable interior relative humidity in the home during the winter.
- Add rigid insulation to the exterior of the Zip-R sheathing. This will shift the proportion of R-value sheathing-to-cavity fill, elevating the temperature of the first condensing surface (the interior surface of the polyisocyanurate rigid insulation in the ZIP-R). This approach will accommodate a Class III interior vapor retarder, promoting cavity drying to the interior. The type of exterior rigid insulation will affect the drying potential of the OSB sheathing in the Zip-R. A vapor-open rigid mineral wool would promote the most drying, while additional polyiso rigid would sandwich the OSB between two layers of a Class II material. While the code addresses many of the factors determining moisture performance, the codes do not address the type of exterior rigid insulation.
What about venting the backside of the cladding? The code recognizes this moisture management approach as well by permitting a Class III interior vapor retarder with vented claddings in cold climates. It’s interesting that three very different configurations — vented cladding, exterior insulation, and different interior vapor retarders — all address managing wintertime moisture and drying potential.
What about the structural performance of the Zip-R, given that there is non-structural rigid insulation between the framing and the OSB? Huber analyzed the structural performance of Zip-R when the product was developed. As with any structural wall system, the connection of the sheathing to the framing is driven by the type, number, and spacing of fasteners. All the information needed is easily obtained here.
Is it easy to keep all of the building science, the codes, and their divergence/alignment straight? Unfortunately, no, particularly when even the various codes do not align. Here is how Allen Sealock, Zip System product director, summarizes the code issues in cold climates:
“The problem is the disconnect between the Energy Code [IECC] requirements and the vapor retarder requirements in the IRC for Climate Zones 6 – 8. This first table below is from the energy code. In Climate Zones 6 – 8, the energy code prescriptively calls for 20+5 for 2×6 walls or 13+10 for 2×4 walls. (This means R-20 cavity plus R-5 continuous insulation, or R-13 plus R-10 in continuous insulation.)
“The table below is from the 2015 IRC. Notice that the R-values of exterior insulation required to enable the use of a Class III vapor retarder are completely different than the ones above from the energy code. I often remind people that the energy code is based solely on energy usage. A 20+5 wall will conduct the same amount of heat as a 13+10 wall.
“The R-values specified in the IRC for vapor retarders has nothing to do with energy usage. It is how much insulation is needed to keep the first condensing surface above the dew point. The confusing part to most people is that the exterior insulation R-values between these two codes don’t match up for Climate Zones 6 – 8.
“However, it is completely allowed by code to build a 20+5 wall in Climate Zone 6 as long as it has a Class I or II vapor retarder [See Figure 3 below].
“The table below is from the 2015 IBC. The IBC changed the language in an attempt to prevent the double vapor barrier scenario.”
Sometimes, the code — and the options within the codes — can make our lives as building professionals easier. Unfortunately — and maybe more often than not — the reverse is true. I firmly believe that knowing the building science behind the code is the only way to really understand and implement the building code.
Note: Steve Baczek and I are conducting a series of workshops across the country entitled Homebuilding Crossroads. The theme is the relationship between high-performance building driven by building science and the 2015 codes. It’s not a simple topic. Following the code does not mean that architects and builders are freed from mastering building science-driven moisture management. Huber Zip-R system is one of the sponsors of the Homebuilding Crossroad workshops.
Full disclosure: Do I have a special relationship with Huber? To the extent that they pay me to develop and conduct these workshops with Steve, the answer is yes. Do they exert any influence or direction over the content and the conduct of the workshops? No, they hired Steve and me only under the condition that we develop the content independently. Huber’s only stipulation was this: Get the code and the building science right.