Does the exterior sheathing on a double-stud wall accumulate worrisome quantities of moisture in late winter? Several researchers are now looking into this question, and Green Building Advisor has been sharing the researchers’ findings as the information becomes available.
The latest sheathing moisture measurements were made by Bill Hulstrunk, the technical manager at National Fiber in Belchertown, Massachusetts. I’d like to put an important piece of information on the table from the start: Hulstrunk is not a disinterested academic researcher; he is employed by a company that sells cellulose insulation. Clearly, his company is more inclined to share data showing that walls insulated with cellulose are performing well, and might not want to share any data that imply otherwise. That said, some of Hulstrunk’s data are thought-provoking.
Hulstrunk shared his moisture content readings in a presentation (“Hygrothermal Analysis of Superinsulated Assemblies”) at the Passive House conference in Portland, Maine, on September 22, 2014. His co-presenter was builder Chris Corson of EcoCor Construction.
Hulstrunk began his presentation by lauding the hygroscopic properties of cellulose. “Hygroscopic materials want to redistribute and equalize the moisture,” he said. “Hygroscopic materials like cellulose protect themselves and nearby materials. They can pull moisture out of nearby materials like sheathing and studs.”
Hulstrunk also provided a few cautionary statements about cellulose density. “Deep cavities require that cellulose be installed at higher densities,” he said. “I recommend 3.7 pounds per cubic foot for a 12-inch cavity or 4 pounds per cubic feet for an 18-inch cavity. Deeper cavities require more experienced installers, since multiple hose passes are required.” (For more information on this topic, see How to Install Cellulose Insulation.)
Walls without any exterior sheathing stay dry
Bill Hulstrunk set out to visit a number of homes with thick double-stud or I-joist walls insulated with dense-packed cellulose, including several homes built by Chris Corson. “WUFI predicts that the highest moisture levels are in March,” said Hulstrunk. “In my house, that seems to be the case. Most of these tests were done in March. I ran the tests with a Delmhorst moisture meter. The first problem that I ran into is that the standard 5-inch probes aren’t long enough. I wanted 2-foot probes. So I got some steel rods at Home Depot. I sharpened the rods and used them as probes.”
While the moisture content (MC) of the wall sheathing measured by Hulstrunk was usually a bit higher than the moisture content of the insulation, the moisture content of the insulation was fairly consistent. “When you get the probes into the cellulose, the moisture content is the same at every depth because of hygric redistribution,” said Hulstrunk. “The moisture content of the cellulose is consistent.”
Most of Hulstrunk’s data were unsurprising (see Image #2, below). After all, many of the thick cellulose-insulated walls that he checked with his Delmhorst meter have no exterior sheathing. (Instead of installing OSB or plywood on the exterior side of his walls, Chris Corson uses a vapor-permeable membrane called Solitex Mento Plus to hold the insulation in place.) Since this type of wall has no cold sheathing on the exterior side, and since Mento Plus is vapor-permeable, the walls stay dry. As I said, no surprise there.
Hulstrunk measured the moisture content of the interior finish materials, cellulose, and exterior sheathing (if any) at six New England houses. All of the readings were below 12.1% moisture content. These levels are low. (Mold or fungi won’t grow on wood unless its moisture content is above 20%, so that is the moisture content level that usually sets off alarm bells for researchers — especially if the 20% moisture content level occurs during the summer, when temperatures are warm. Decay won’t set in unless the wood has a moisture content greater than 28% and unless temperatures are above 23°F.)
Peel-and-stick on the outside of the walls
The most interesting data were collected at David Posluszny’s unusual house in Shirley, Massachusetts. Posluszny covered the exterior side of his wall and roof sheathing with Grace Ice & Water Shield — a peel-and-stick rubberized asphalt membrane that is impermeable to water vapor. This approach is typically used for a PERSIST house, where 100% of the insulation (usually rigid foam) is installed on the exterior side of the Ice & Water Shield.
Unlike PERSIST builders, however, Posluszny put all of his insulation on the interior side of the peel-and-stick. (His double stud walls are insulated with R-42 of dense-packed cellulose.) In other words, his house has a wrong-side vapor barrier — a classic building science no-no.
Physics dictates that, if the interior wall finishes of Posluszny’s house are vapor-permeable, normal winter conditions (a warm, humid interior and a cold, dry exterior) will guarantee that water vapor will be driven through the vapor-permeable insulation until it encounters a cold surface — in this case, the plywood wall sheathing. This outward vapor drive will cause the moisture content of the sheathing to rise over the winter, peaking in March. If the sheathing is covered with a vapor-permeable housewrap, the sheathing can dry to the exterior; however, this drying path is stopped at Posluszny’s house by the Ice & Water Shield.
Builders who want to establish an exterior air barrier don’t have to use a vapor-impermeable product like Ice & Water Shield, of course. Standard recommendations for exterior air barriers include the Zip System (using OSB and a proprietary tape), plywood taped with Siga Wigluv (or some other type of high-quality tape), a vapor-permeable European air barrier membrane like Solitex Mento, or even a vapor-permeable peel-and-stick product like Henry Blueskin VP.
At the presentation in Maine, Hulstrunk said, “My first reaction when I saw the walls was to advise David to remove the Ice & Water Shield. He told me that he wasn’t going to do that.”
“I’ve never lost any sleep about it”
When I telephoned Posluszny and asked him why he installed Ice & Water Shield on the exterior side of his wall sheathing, he responded, “I knew exactly what I was doing, even though I was warned not to by my U. Mass. professors and Bill Hulstrunk.” (Posluszny has a degree in building construction technology from the University of Massachusetts in Amherst.)
“I’ve never lost any sleep about it,” Posluszny continued. “I came up with the design when I was doing my thesis at U. Mass., after spending years reading case studies, going to people’s homes, and talking to contractors who have torn apart roofs that have failed. I didn’t come across any problems in buildings where the cellulose was properly installed, or in a building without recessed lights. In every situation with a failure, I found that the failure was due to something other than the cellulose.”
I asked Posluszny whether he had seen Kohta Ueno’s sheathing moisture readings from a house built by Carter Scott. Posluszny responded, “Were Carter Scott’s walls insulated to the proper density? I have serious doubts.”
I sent Posluszny a few follow-up questions by e-mail. First, I asked whether his walls had a “smart” vapor retarder on the interior side — something like MemBrain. Posluszny responded, “No. There is no need when you are using properly installed cellulose. My interior is just 1/2-inch gypsum board with standard latex paint.”
My next question concerned whether he had monitored the interior relative humidity during the winter. He responded, “I keep a temp and RH sensor in the crawl space, first floor, and loft. However, I only control the first floor. The other two are just fun to watch. I keep the first floor at 68 degrees and 45% RH, allowing for 5% tolerance, or 40%-50% RH range. This is controlled by an automatic humidistat on my HRV. I turn off this control during the summer, and use the timer function on the HRV. The humidistat automation is only used during winter months.”
“I broke the rules”
Bill Hulstrunk’s visit to Posluszny’s house was recorded on video, and the video has been posted on YouTube.
The moisture readings were made on January 29, 2013. In the video, Posluszny says, “I broke the rules, and I put a vapor barrier on the outside of the house. … We took moisture readings in the north, south, west, and east walls. We checked the moisture gradients in the loft and crawl space. We are pleased to find all the moisture levels came back well within the comfort zone. This wall assembly works.”
Hulstrunk tells the camera that the moisture content of the sheathing on the south side of the house is 7.5%. On the north side of the house, it’s 7.8%.
At the conference in Portland, Maine, Hulstrunk reported two sheathing MC readings from Posluszny’s house in Shirley: one was 7.6% MC, and the other was 11.6% MC.
All of these readings are well below the level of concern.
Other researchers have gotten higher readings
Hulstrunk’s moisture readings raise several questions. It’s worth noting that Hulstrunk took his measurements on a single day in late January. Even if Hulstrunk’s measurements were accurate — and I have no reason to doubt that they were — it’s impossible to know whether the moisture content of Posluszny’s wall sheathing stayed low or rose sharply in February and early March.
As I reported in a November 2013 article called “Monitoring Moisture Levels in Double-Stud Walls,” Kohta Ueno, a researcher at Building Science Corporation, has measured wall sheathing with moisture contents that range from 20% to 25% on the north wall of a double-stud cellulose-insulated wall in Massachusetts — and even higher readings when the interior relative humidity was high.
I called Ueno up recently to get an update of his ongoing monitoring study. “We opened up both of the walls on the Carter Scott house — the north wall and the south wall — and we found shockingly little damage in spite of two winters of high moisture content readings at the sheathing,” Ueno told me. “When we opened up the north and south walls, everything looked fine. When we pushed on the cellulose, there were no signs of major voids. It looked like a classic dense-pack installation. There was just some surface rust on the staples and fasteners, and a little grain raised on the sheathing, but no mold or problems like that. I don’t think these walls are showing a problem.”
I asked him if he cared to speculate about what is going on at the Posluszny house. “With exterior peel-and-stick, there is no moisture coming from the outside, and there will be a fair amount of drying to the interior if the sun hits a wall,” Ueno said. “But on the north side of a building, the side that rarely gets sun, I would have expected the sheathing moisture content to be in the 15% to 20% range. At the Carter Scott house I was monitoring, we saw sheathing moisture contents that were well into the 30% plus range when the occupants were running their house with high interior humidity, when there was no ventilation.”
Temperature corrections and readings that go haywire
Obtaining accurate moisture content readings is notoriously tricky. In a follow-up e-mail to me after our conversation, Ueno asked, “When Bill Hulstrunk measured 7% MC, was he also doing the temperature correction? It’s probably not a huge effect (from 7% to 10-11% at cold conditions), but it’s another thing to add to the list.”
I forwarded the question to Hulstrunk, who responded, “I have not temperature corrected the readings, although I did record both the indoor and outdoor temperature readings for each of the moisture tests. One reason for this is that I put some plywood in the freezer and took several readings as the plywood warmed back up to room temperature. Surprisingly, I did not notice a linear decrease in moisture content readings as one would have expected, which makes me wonder if the glues in the plywood are somehow interfering with the readings. Chris Corson has speculated that the some condensation may be forming on the end of the warm probes as they are being inserted into the plywood, skewing the results. I will repeat this test once again with both solid wood and plywood when my wife is not looking, since she was not excited with the wood that she found in the freezer the last time around. Thing is one disadvantage of not having a lab.”
After reading Hulstrunk’s response, Ueno wrote, “In Bill’s experiment, if you pull wood out of the freezer and bring it into room conditions to warm up, you will tend to develop a high surface RH, or possibly adsorption of water and/or condensation. This could be totally negligible (e.g., pretty dry conditions inside), or it could actually be liquid water condensation. When I’ve played with temperature/MC effects, we had the luxury of doing it in a climate chamber with temperature controls, with the samples sealed inside vapor impermeable packaging (to keep them at a constant MC). What I’ve found in my monitoring is that drier wood isn’t strongly affected by freezing conditions, but that really wet (e.g., 30% MC+) wood measurements go totally haywire — I think it’s freezing of water inside the structure of the wood, changing the electric resistance properties. … I am not arguing, ‘Bill’s measurements are wrong’ — all I’m saying is, ‘Huh. These aren’t consistent with measurements I’ve taken, or other folks I know. Is there an explanation why?’”
When I asked Ueno whether he thought that Posluszny’s wall assembly was robust, Ueno answered, “This is not a wall I would ever recommend.”
The jury is probably still out
Hulstrunk’s data are intriguing. There is no doubt that the hygroscopic nature of cellulose permits hygric redistribution, and that this hygric redistribution can sometimes keep otherwise risky wall and roof assemblies out of trouble. So far, there is no reason to suspect that Posluszny’s wall and roof sheathing have any problems at all. Will the walls last 50 years? Time will tell.
In the meantime, Posluszny has remarkably low energy bills. The stellar thermal performance of his house is undeniable.
Martin Holladay’s previous blog: “It’s Not About Space Heating.”