Wall assemblies that are both well insulated and free of long-term moisture problems are a frequent topic for the Q&A Spotlight. Hasn’t this subject been raked over enough in the past to answer just about every possible question?
Not according to Nat, a GBA reader and structural engineer who is planning his own house in Climate Zone 5. Even after poring through GBA and other websites, Nat finds himself uncertain about the planned wall assembly on his budget-conscious new home.
The house design is simple. A plain box with a gable roof and a full basement, roughly 1,100 square feet per floor. Nat wants to keep detailing as simple as possible. His plans for exterior walls include clapboard siding over a rainscreen, then Zip R-9 sheathing—a combination of OSB and polyisocyanurate insulation—and a 2×6 wall insulated with mineral wool batts in the stud cavities.
“Despite a lot of reading I’m finding a hard time getting full comfort with the Zip R-9,” Nat writes in a Q&A post. While the ratio of batt insulation to exterior rigid insulation appears safe, the insulation rather than the OSB becomes the condensing surface should moisture move through the assembly.
“I see plenty of drawbacks with the Zip-R and I know it’s probably not the best R for my money—but I really like how it seems to involve very little atypical detailing and very easy to make sure it’s well executed,” Nat says. “Is this a safe and reasonable assembly for my zone?”
That’s the key question here, but Nat’s initial post opens the door to other options, such as the pros and cons of an exterior layer of insulation over the sheathing. For more on those topics, use the link above to check the full text of the post.
Let’s start with the building code
Peter Engle starts with the requirements of the International Residential Code for cavity plus exterior insulation in Climate Zone 5: R-13 cavity insulation and R-5 of continuous exterior insulation.
“That is a ratio of fluffy insulation to foam of 13/5=2.6,” Engle says. “Your proposed wall system is R-19 + R-9 (significantly better than code). Your ratio is 19/9 or 2.1. Lower is better, so you should be good.”
Even if the polyiso insulation used in Huber’s Zip-R sheathing is “derated” somewhat to account for changes in its thermal properties in cold weather, the insulated panels should work as planned.
Not so, writes Jon R: “The Zip-R design doesn’t have any ‘continuous exterior’ or ‘over the wall’ insulation and you shouldn’t pretend it does. The OSB will be cold and any air leaking out will condense/sorb on it.”
Although the assembly may work nonetheless, Jon R adds, Nat should read two articles he cites (they may be found here and here) for a better understanding of the building science involved in his plans.
Jon R’s preference would be a wall built without OSB, with a perm rating of greater than 1 to the exterior, and a Class II vapor retarder on the interior.
Opinions are worlds apart
Nat finds this advice typical of what he’s found so far—that is, two widely divergent points of view.
“The Zip-R was recommended as a starting point by [a] peer/colleague architect that is knowledgeable and uses this setup on some of their projects,” Nat says. “After doing my own research it seemed reasonable based on my above points. I recently decided to engage an architect to do an envelope review and his first comments were along the lines of Zip R in this setup will have problems and is not OK.”
Still, Nat finds the distance between the two points of view a little surprising.
“As a structural engineer I often have disagreements with other engineers but I’d be hard pressed to think of a situation where I am at polar opposites on the basics of a fundamental structural system,” he says. “With wall assembly it seems like the building ‘science’ is flavored by a lot of opinion. Surely there is a way for me to determine that this is a safe or not safe assembly?”
A closer look at the IRC
The IRC prescription for insulation levels in Nat’s climate zone is an energy requirement, but it doesn’t guarantee that the wall assembly will be free of moisture accumulation, says Michael Maines.
IRC table R702.7.1 lists what is needed to change from a Class I or class II interior vapor retarder to a Class III interior vapor retarder. “It’s not explicit, but the science says you can extrapolate the ratios of interior vs. exterior R-value in this table for other R-values,” Maines says.
What that means in Climate Zone 5A is that about 28% of the total R-value of the wall assembly should be on the exterior. This should keep the condensing surface above 45°F, he says, but it’s also cutting it close. “It’s always safe to increase the exterior-to-interior or exterior-to-total ratios,” Maines adds.
With R-23 worth of cavity insulation, the minimum safe impermeable exterior insulation is R-8.9. “When this ratio matters is when it’s cold outside, which is also when polyiso’s R-value drops,” Maine says. “You may be fine with their R-9 product but I would err on the side of caution.”
Does it matter where the insulation is?
GBA Editor Brian Pontolilo is curious whether Maines feels comfortable following the IRC requirements when using Zip-R sheathing, rather than having the insulation layer on the exterior of the sheathing (With Zip-R, the polyiso is on the interior side of the OSB).
“Will, as Jon suggests above, the cold OSB create the possibility of absorbing moisture?” Pontolilo asks. “Or is the inside surface of the foam layer of the Zip-R the only condensing surface to be concerned about?”
Maines thinks the Zip-R can be used safely, even if the R-value of the foam is low enough to allow moisture accumulation on the interior. As long as there are no air leaks, essentially no moisture should find its way through the assembly.
But, he adds this: “But just because the OSB is safe doesn’t mean the assembly is safe—you can still get moisture accumulation on the face of the foam facing the stud cavity, at least in theory. I’ve seen condensation in walls insulated with a flash-and-batt system, which is nearly the same thing as the Zip-R approach.”
Be conservative in your approach
Given the dramatically different points of view, Nat has a hard time imagining an average homeowner delving into these issues. But Pontolilo, while acknowledging the differences of opinion, suggests that by building conservatively Nat should be fine.
“Two things I would suggest,” Pontolilo says. “First is that you are not so laid back about the air sealing. If you keep air out of the walls, you greatly minimize the potential for any moisture related issues. And air sealing is relatively inexpensive work. You just need to plan ahead.
“Second, I agree with you that a smart vapor retarder seems like some worthwhile insurance (and can help with the aforementioned air sealing if detailed to do so).”
Along those lines, Maines adds that membranes while variable permeance are “affordable insurance,” they don’t solve all problems. He suggests that Nat consider making the exterior insulation layer thicker.
“Or,” he says, “you may find, as I have every time I’ve tried spec’ing or using Zip-R, that it’s just not the best solution. Exterior wood fiber insulation is vapor-open so you can use any thickness you want, though more is better. Or recycled foam is easy enough to apply over conventionally installed sheathing, without the worries about embodied carbon.”
The structural considerations
With Zip-R sheathing, the interior surface of the insulation is what’s in contact with the framing of the house. The structural layer of sheathing is separated from the studs by the thickness of the polyiso.
“Some builder have concerns about the structural integrity of ZIP R and the difficulty of installing it right, basically getting nails to drive properly,” Pontolilo writes. “But you are an engineer, so I’m sure you can make an educated assessment of the structural aspects of it and a decent framer should be able to figure out how to nail the stuff right.
“Steve Baczek has emerged as one of the leading architects in high-performance home building and building science and he uses ZIP R regularly with no concerns.”
Engle is one of those who questions the structural side of using Zip-R. “Personally, I’m not a big fan of Zip-R,” he says. “From a structural standpoint, I don’t like decoupling the sheathing from the framing. I know that Huber has the engineering data to show it works, but I’m skeptical. I live in high-wind country, so I worry.
“I also share Jon R’s distrust of the polyiso’s ability to prevent moisture from reaching the OSB sheathing,” Engle adds. “OSB is just so intolerant of moisture that makes me worry too.”
Our expert’s opinion
A few thoughts from Peter Yost, GBA’s technical director:
I hear Nat’s frustration with the apparent inexactness of hygrothermics. It is important to note that how moisture moves in building assemblies, particularly given film theory, is a relatively young science, certainly compared to the physics of structures. That said, below are my efforts to clear some hygrothermal muddiness.
First condensing surface: With air-permeable cavity insulation, the first condensing surface is not the cavity insulation, but the first surface, which is the interior face of the polyisocyanurate rigid insulation.
For an extended and really helpful backgrounder on why moisture accumulates on the interior surface of the innermost exterior sheathing rather than in the cavity insulation, see this Building Science Corp. article by Joe Lstiburek.
I am quoting an early paragraph of this article by Lstiburek to defend some of our hygrothermal muddiness:
“Diffusion is supposed to be easy because all you need to know is one equation—Fick’s Law—and you are done. Ha. Trouble is that the equation is only an approximation, and we can’t really measure the material properties to make the equation work anyway because the properties change with both temperature and vapor pressure. And there are many different materials. Too bad for us that there are temperature differences and vapor pressure differences across assemblies. Too bad for us that there are many materials making up assemblies. Too bad for us that everything varies all over the place all of the time.”
Derating polyiso insulation because of temperature dependency: The best reference on this issue that I could find is BSC’s Info-502: Temperature Dependencies of R-values in Polyisocyanurate Roof Insulation. Some key points:
- Both BSC and the Polyisocyanurate Insulation Manufacturers Association (PIMA) make a strong point of not extrapolating the testing and results of polyiso roof insulation to walls.
- There is quite a bit of variation in this phenomenon from manufacturer to manufacturer; we just don’t know which is which. Huber ZIP-R uses Atlas polyiso board, and more than once technical folks at Atlas, off the record, have reassured me that their board works fine, and all of the manufacturers have been working on their formulations to ameliorate this issue.
- One of the BSC recommendations is to use R5 for any polyiso roof insulation used in a cold climate (PIMA’s “Thermal Resistance and Mean Temperature: A Report for Building Design Professionals” reports a value of R5.5 for Climate Zone 5 winter value). That means for CZ 5A, 1.5 inches would be R7.5, still meeting code (remembering that we are not supposed to use this info for walls, only roof insulation).
- This BSC info document is now quite dated (2013).
Yes the OSB in any ZIP-R will be “cold:” This is because it is exterior to any insulation. But:
- It is not the first condensing surface.
- It is facing a dedicated air space with both free drainage and ventilation (if the rainscreen is open top and bottom).
- Any water condensing on the interior surface of the polyiso insulation needs to make its way as a vapor to the OSB. The vapor permeance of the polyiso insulation (according to the ZIP System R-Sheathing ESR-3373) is less than 1.0, making the polyiso a Class II vapor retarder. I would not be worried about interior vapor making its way through the insulation to the OSB. As pointed out earlier, worry more about airtightness, but not with this assembly: the taped ZIP-R creates a formidable continuous air control layer.
Vapor permeance of OSB: Per the APA graph below (resulting from testing by the National Institute for Science and Technology and Oak Ridge National Lab), even generic 7/16-inch OSB has a dry cup vapor permeability of slightly greater than 1.0 perms, more than 2.0 perms at 40% relative humidity and as high as 12 perms at 85% relative humidity. This classifies any OSB as a solid Class III vapor retarder, not Class II. I have seen the results of internal testing of vapor permeance (as expressed graphically as sorption isotherms) by Huber on its panels and in the lower range of moisture content it’s very similar to the APA graph but the curve steepens much more than the APA curve at high RH (85% and up).
Class II vapor retarder on the wall assembly interior: With the right ratio of continuous exterior rigid and cavity R-value, a Class III interior vapor retarder is fine.
Let’s not forget interior wintertime relative humidity: The variables regarding hygrothermal performance of building assemblies in the winter in cold climates includes interior relative humidity. If the winter interior RH is well-managed and/or occupant density is low, there is a lot less risk of interstitial condensation than if RH is not well-managed and/or there are a lot of occupants. A change of 5% in interior RH—say from 40% to 45% results in a change in dew point (at 68°F) of 3.1°F, from 42.8°F to 45.9°F).
Smart vapor retarder insurance: While it can’t hurt to include this in this assembly, I don’t think that it is needed, even as insurance. Having said that, if this interior smart vapor retarder can be economically detailed as also an interior air control layer, that can’t hurt!
Structural comfort with the non-structural polyiso insulation separating the structural sheathing from the framing: Huber did not separate these two components lightly. They chose this configuration so that builders did not need to fuss with exterior rigid insulation details at penetrations like windows and doors. There always are trade-offs. But as with any structural sheathing system, if you get the fasteners spaced right and flush with the sheathing, it’s not hard to get the shear values the lab gets. And they built in the same safety factor on the developed shear that any structural engineer would.
One last thought on this topic comes from Steve Baczek. He notes that in high wind zones, his structural engineer insists on a site inspection of Zip-R panels. In fact, an Evaluation Service Report from ICC-ES (ESR-3373) requires such an inspection for these Huber panels. Steve believes that any shear panel in a high wind zone should be inspected after installation.
Note from Peter Yost: In the interest of full disclosure: Huber is the lead sponsor of the Homebuilding Crossroads educational series that Steve Baczek and I do. Huber does not provide me or Building-Wright with any other financial support nor does it make any attempt to influence my work at either Homebuilding Crossroads or GBA.
-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.