In Minnesota, Jeff Fredrickson is planning a new house, and his research has included lots of reading on the design and construction of exterior walls. His goals are twofold: a wall that will stay mold-free for decades, and one that is “somewhat energy efficient.”
Starting at the inside, the wall would go like this: drywall, a polyethylene vapor barrier, JM Spider insulation in 2×6 stud walls, 1/2-inch plywood sheathing, a water-resistive barrier (Benjamin Obdyke HydroGap), fiber cement and stone veneer cladding.
“Is this a good wall?” he asks in a Q&A post at GreenBuildingAdvisor.
Specifically, Fredrickson worries about condensation in this Climate Zone 6 wall, the potential hazards of volatile organic compounds in the insulation, and whether the blown-in Johns Manvillle Spider fiberglass insulation is really any better than plain old fiberglass batts.
That’s the focus for this Q&A Spotlight.
First, skip the polyethylene
Although it was commonly used in exterior walls at one time, polyethylene is now regarded as a potential moisture trap in all but the coldest climates.
“The interior polyethylene is more likely to cause moisture problems than solve them, because it prevents your wall from drying to the interior during the summer,” writes GBA senior editor Martin Holladay. “If you don’t want mold, then don’t install the interior poly (especially if there is any chance that the house will ever have air conditioning).”
Poly may present a higher-than-usual risk, Nate G. adds, if Fredrickson puts stone veneer right over the water-resistive barrier.
Holladay has the same misgivings about stone veneer on the exterior, telling Fredrickson, “Stone veneer over wood framing and plywood or OSB wall sheathing is probably the most problematic cladding ever invented. This type of cladding has more moisture-entry problems than any other. Lots of siding types are most likely to be free of moisture problems — including vinyl siding and fiber-cement siding installed over a ventilated rainscreen gap.”
The wall doesn’t meet code minimum requirements
The plan has another fundamental weakness: It won’t meet the insulation requirements of the 2012 International Residential Code. In Climate Zone 6, the IRC calls for a minimum R-value in walls of either 20+5 or 13+10, the first number referring to the cavity insulation between the studs and the second number to the R-value of continuous insulation over the wall sheathing.
“Spider has about the same performance as perfectly installed R-21 fiberglass batts, slightly lower performance than perfectly installed R-23 rock wool,” Dana Dorsett adds. “But perfection is hard to get with batts — there are always some voids and compressions, whereas with Spider there are none.”
When thermal bridging is factored in, Dorsett says, the whole-wall R-value with Spider insulation would be about 15 — not especially high-performance.
To get the wall performance up to IRC minimums, he suggests adding rigid rock wool sheathing in the masonry cavity, which also would keep the sheathing warmer and drier. A layer of insulation 1 1/4 inch thick would be enough, although there’s no harm in going thicker.
When it comes to choosing an appropriate amount of insulation, Dorsett adds, there’s a good case for exceeding code minimums.
“Taking it higher than IRC minimums is still financially rational on a life-cycle basis in most cases,” he writes. “An inch of rigid rock wool on the exterior of the sheathing would improve both the mold resistance and the energy performance of that wall, and would also be pretty close to the IRC code-min. At 2 inches with the exterior rock wool you’d be in the ‘somewhat energy efficient’ range by almost anybody’s definition, though far from superinsulated, and well behind even what would it would take to be ‘Net Zero Ready’ in your climate.”
And whatever Fredrickson decides on the insulation front, Dorsett says, “the cheapest wall performance you can buy is to meticulously air seal the house.”
Making the wall more vapor-permeable
Dorsett has two other suggestions, both of which would enhance the wall’s ability to dry.
The first is to replace the plywood sheathing, with a perm rating of 0.5 to 1, with structural 3/4-inch asphalted fiberboard, which has a perm rating of 5 perms when absolutely dry but 15 or more when humidity levels are high.
“Fiberboard is less susceptible to moisture than plywood and lets moisture out, which makes the whole assembly more moisture-resilient,” he says. “It’s also about R-2, which means you can back off to 1 inch on the rigid rock wool and still hit IRC code-minimum performance.”
In addition, Dorsett says, a smart vapor retarder such as MemBrain or Intello Plus is a class II vapor retarder when dry, with a perm rating of less than 1, but open to vapor when the relative humidity in the cavity becomes high enough.
“Alternatively, 1/2-inch plywood or OSB also behave as smart vapor retarders, and if you put it on the interior under the gypsum it becomes the structural sheathing, allowing you to use thinner (less-structural) fiberboard sheathing facing the masonry cavity,” Dorsett adds. “Most fiberboard is sufficiently structural to use as exterior sheathing on its own, but it’s not as strong as CDX or OSB.”
With fiberboard sheathing and a smart vapor retarder on the interior, the wall would be able to dry in either direction, he says, adding, “With 1-perm CDX and 6-mil poly (about 0.05 perm), the winter moisture drives are controlled, but it has very limited drying capacity, and has more moisture-susceptible sheathing in the colder/wetter side of the assembly.”
Don’t get stuck on a 2×6 wall
The builder Fredrickson has chosen uses 2×6 construction in all the houses he builds, but it’s by no means the best or only choice.
“I think that the reason that a couple of GBA readers are urging you to reconsider your fixation on 2×6 walls is that one good option for cold-climate builders is a double stud wall consisting of two parallel exterior walls framed with 2x4s,” Holladay says. “This gives your insulation contractor plenty of room for lots of insulation.”
If he is set on framing with 2x6s, Holladay adds, it would be better to use a continuous layer of insulation, typically rigid foam, on the outside of the sheathing.
Adds Nate G, “There are many, many ways to build good walls. If for some odd reason you have to use 2x6s, you need to make sure your walls’ drainage is as good as possible. That means 1/2-inch or deeper air channels behind the cladding, it means selecting a cladding that’s not a moisture reservoir (e.g. stucco, brick, or stone) unless it’s a freestanding wall physically separated from the rest of the wall such that bulk moisture can easily drain down through the bottom.”
Our expert’s opinion
Here’s what GBA technical director Peter Yost has to say:
If you want a “wall that stays mold-free for decades and one that is somewhat energy-efficient,” this is what I’d recommend:
To start, consider your site context. For durable building enclosures, matching design features to exposures — wind, surface water associated with slopes, ground water associated with water table depth — is key to durability.
Be anal about the continuity of your air barrier. You won’t get that with any of the materials or systems mentioned in your assembly so I just want to make sure that this is an element of your durable, energy-efficient building enclosure.
Install your stone veneer with a system like LathNet. Free-draining, vented systems for adhered masonry veneers are not inexpensive, but they are critical for “mold-free walls lasting for decades.”
The JM Spider insulation system works just fine and is a standout for its overall environmental profile. See this blog by my colleague Alex Wilson.
Given that I am not exactly sure what “somewhat energy-efficient” means, I am going to assume that you don’t have the appetite for a truly high-performance system like the double-stud wall. But in your climate, honoring the next point below on drying potential, you should strongly consider continuous rigid exterior insulation (for how much you should use, see Calculating the Minimum Thickness of Rigid Foam Sheathing).
Design your wall assembly to dry in at least one direction. You certainly don’t need a polyethylene Class I vapor retarder. And what type of vapor retarder you need depends on your insulation strategy (see above) and just what type of wintertime interior relative humidity you expect to maintain in your home (which also will depend on what type of mechanical ventilation system you use).
Locate your HydroGap WRB to the interior of your exterior rigid insulation. Warming this complex organic material (the WRB) will improve its durability. Also, consider the location/plane of your window installation as it lines up — or not — with the location of your WRB.
Consider the moisture-sensitivity of your structural sheathing in the overall context of your assemblies. There are a lot of variables here — whether or not you do indeed install all of your claddings with a ventilated rainscreen; what type if any vapor retarder you use; whether or not your dedicated water control layer — WRB and flashings — is truly continuous. Frankly, I consider plywood’s variable vapor permeability compared to any OSB as a plus, but I also think that higher-quality OSB (such as Huber’s Advantech), with drying potential to the interior, will remain “mold-free for decades.”