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Monitoring Moisture Levels in Double-Stud Walls

Is there any evidence that double-stud walls have damp sheathing?

Posted on Nov 1 2013 by Martin Holladay

Most wood-framed buildings have no insulation on the exterior side of the wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . That means that the wall sheathing gets cold and wet during the winter.

Whether or not this common situation is a problem depends on who you talk to. Monitoring studies show that the moisture content of the OSB or plywood sheathing on some homes with 2x6 walls rises in February (especially on the north side of the house); fortunately, however, the sheathing dries out in March or April. As long as the sheathing stays dry for most of the year, it can usually endure a few weeks at an elevated moisture content without developing mold or rot. In most cases, sheathing won’t rot as long as the wall’s drying rate exceeds its wetting rate on an annual basis.

Not all builders are comfortable with this analysis, however. Some builders prefer to install rigid foam or mineral wool insulation on the exterior side of their wall sheathing, to keep the OSB or plywood above the dew point during the winter. Warm sheathing is dry and happy, so installing an adequate thickness of exterior rigid foam is one possible solution to cold sheathing worries. (The extreme version of this approach is called PERSIST. PERSIST homes put all of the wall insulation on the exterior side of the wall sheathing, and leave the stud bays empty.)

In theory, a very thick wall with lots of insulation on the interior side of the wall sheathing — for example, a double-stud wall — is riskier than an ordinary 2x6 wall, because the high-R insulation reduces heat flow through the wall, making the sheathing colder and wetter than ever. Some hygrothermalA term used to characterize the temperature (thermal) and moisture (hygro) conditions particularly with respect to climate, both indoors and out. modeling programs, including WUFI, show that wall sheathing on a 12-inch-thick double-stud wall insulated with cellulose can have an elevated moisture content under certain conditions. (The sheathing moisture content shown by WUFI varies, of course, depending on the inputs made by the software user; variables include inputs for climate, wall orientation, and the interior relative humidity.)

Cold sheathing warnings

These computer modeling results have made some builders and designers nervous. Here are samples of warnings about cold sheathing on double-stud walls:

  • The Building Science Corporation web site warns that a highly insulated double-stud wall “will work in extreme climates, but still has significant risks to moisture related durability issues and premature enclosure failure.”
  • Jesse Thompson, an architect in Portland, Maine, posted the following comment on a GBA forum: “A double-stud wall with any type of batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. is a high-risk wall system in a cold climate, due to the cold sheathing issue.”
  • A Building America study that used WUFI to model several high-R walls noted, “Previous hygrothermal modeling conducted by CARB and others of high R-value walls has indicated a serious potential for moisture damage to the wood sheathing if located on the exterior of the wall just beneath the claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. , especially in cold, moist climates.”

Having read these warnings for years, I set out to answer an important question: are these warnings based on actual measurements of the moisture content of installed sheathing, or are they based only on computer modeling?

Modeling studies show worrisome moisture levels

One modeling study that included somewhat worrisome findings, “Moisture Research — Optimizing Wall Assemblies,” was authored by Lois Arena and Pallavi Mantha. Arena and Mantha both work at Steven Winter Associates in Norwalk, Connecticut, where Arena is a senior building systems engineer and Mantha is a building systems analyst. Most of the research conducted by Arena and Mantha is funded by the U.S. Department of Energy, through its Building America program.

Some of the researchers’ findings sound alarming. Arena and Mantha used WUFI to calculate the predicted moisture content of several components of various wall assemblies, including 12-inch-thick cellulose insulated walls. The researchers wrote, “Results from WUFI indicate that condensation potential for the double [stud] cellulose walls is extremely high because the OSB in those wall assemblies is entirely outside of the insulation.”

“Realistically, we know that this is not true”

Delving deeper, I realized that some of the researchers’ warnings rested on a shaky foundation. The authors note, “WUFI offers several different methods for generating interior temperature and RH [relative humidity] levels. For this study, the interior conditions for all three wall types were generated using the ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. 160-2009 method.

“It should be noted that, in all climates, the interior RH levels predicted by this method reach 90% even though cooling was assumed. Using these interior conditions, WUFI predicts that there is the potential for mold growth on the interior surface of the drywall in all climates. Realistically, we know that this is not true.”

What is ASHRAE 160?

ASHRAE 160 is a published standard that provides moisture design loads for those who perform hygrothermal modeling. These design loads are similar in principle to design loads used by mechanical engineers. Among the design loads in the standard are a set of “design indoor conditions” which (for example) establish the amount of moisture that modelers should assume is generated by a building’s occupants.

For some reason, the ASHRAE 160 values used by Arena and Mantha produced unlikely WUFI results. Arena and Mantha wrote, “Further research into the appropriateness of the ASHRAE 160-2009 interior conditions in moist climates is needed. Interior relative humidity levels generated with this method are higher than recorded in actual studies (Arena et al. 2010) and result in overly pessimistic predictions for mold growth on the interior of the assembly. Considering that almost every wall in this study failed the ASHRAE 30-day criteria, it is recommended that this threshold be reevaluated by industry professionals.”

Here’s the translation: the modeling results don’t pass the sniff test. Arena and Mantha clearly recognize that fact, and they accurately deduced that the anomalies stem from their use of the ASHRAE 160 values.

To find out how these unexpected WUFI results may have occurred, I spoke with Anton TenWolde, the building scientist who helped develop ASHRAE 160. TenWolde explained that the indoor moisture values first published in ASHRAE 160 needed to be tweaked. “We already fixed this,” TenWolde told me. “We have passed and published three addenda to the standard since the original publication. The indoor relative humidity is now capped at 70%. The standard is under continuous maintenance. We felt it was the best possible standard at the time of publication, but it was by no means perfect.”

TenWolde pointed out that the developers of the standard lack good data on indoor moisture levels. “When you look for the numbers on moisture generation in residences, you find that they are not there. All of the numbers floating around for the last 40 or 50 years are probably too high. It’s also important to remember that this is a design standard, so conditions are not average conditions. They are extreme conditions, and they are supposed to be extreme. You can’t uses averages for design, because it’s not the average house that fails.”

Since the ASHRAE 160 conditions used in the WUFI modeling performed by Arena and Mantha were flawed, the WUFI results from that study shouldn’t be used to make design decisions. Instead, it’s worth looking at data from monitoring studies of real walls.

Fortunately, we’re beginning to get results from a few field studies of double-stud walls insulated with cellulose.

Monitoring results from field studies

From September 2007 to March 2009, Andy Shapiro, an energy consultant from Montpelier, Vermont, monitored the moisture content of the sheathing on a double-stud wall in Vermont. The walls were 12 inches thick and insulated with cellulose. The house had a single occupant and was equipped with an HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. .

One thing about the house was unusual: it had diagonal board sheathing rather than OSB or plywood sheathing. “The house was sheathed with rough-cut local lumber in random widths — a mix of hemlock, fir, and pine,” Shapiro told me. “The siding was white cedar shiplap over Tyvek DrainWrap, which is very vapor-open. The interior was finished with painted gypsum wallboard with no interior vapor barrier.”


WHAT MOISTURE LEVELS ARE CONCERNING?

Before reviewing the field study data, it’s worth discussing how wet sheathing can get before we should be concerned about the possibility of mold or rot.

Mold or fungi won’t grow on wood unless its moisture content is above 20% — a level that corresponds to 80% to 90% RH — 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. (Decay is quite slow when temperatures drop below 50°F.)


The sheathing moisture content was higher during the winter months than Shapiro expected. In February and March of 2009, Shapiro recorded moisture content levels over 30% in two locations. (For perspective on moisture content readings, see the sidebar to the left, "What Moisture Levels Are Concerning?")

“The data isn’t complete,” Shapiro told me. “These measurements are really snapshots in time. But they are real moisture measurements. I chatted with John Straube about this, and given that this house has a moisture-open exterior, we’re not worried. But if we had the same readings with OSB sheathing, we’d be worried.”

Monitoring data collected by Lois Arena

In addition to co-authoring a paper on the use of WUFI modeling on high-R walls, Lois Arena has been monitoring the performance of some double-stud walls at a residential project in Devens, Massachusetts. Some of Arena’s data were reported in the September 2013 issue of Energy Design Update. Intrigued by the data, I called Arena and asked her to share up-to-date information from the monitoring study.

The monitored wall is an R-40 double-stud wall insulated with cellulose. The OSB-sheathed walls are clad with vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). siding, and the interior finish is gypsum wallboard painted with a 0.5-perm vapor-retarder primer.

The research team installed temperature, RH, and moisture content sensors in a variety of locations in two stud bays — one on the south side of the house, and one on the north. “The OSB on the north wall is now [October 2013] down to a moisture content under 10%,” Arena told me. “In February, it was up to around 20%, but it came down to 10% in mid-April. The time at 20% was brief, and it doesn’t sound too alarming to me. I like the fact that the walls are drying out.”

According to Arena, one reason that the sheathing dried quickly in the spring was the fact that the cladding (vinyl siding) is well ventilated. I asked her, “Would you agree that builders don’t have to worry about the integrity of the OSB sheathing on a double-stud wall, as long as the wall has a ventilated rainscreen gap?” She answered, “That’s exactly right.”

Kohta Ueno’s data

At a another house in the same residential development in Devens, Massachusetts — at a house built by Carter Scott — a different research team is conducting a similar monitoring study. Kohta Ueno of the Building Science Corporation now has two years of data on several high-R walls, including a 12-inch-thick double-stud wall insulated with cellulose. Like Arena's research, Ueno's research is funded by the Building America program.

Some of Ueno’s data were reported in a Fine Homebuilding article called “The Future of Housing in America.” The article noted, “Twelve-in.-thick double-stud walls with cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. and [walls] with low-density foam were tested. In the stud bay insulated with cellulose, the moisture content of the sheathing on the north side of the house spiked to 28% in the winter. That’s high enough to raise concerns about mold and rot. In the summer, though, it dried out to 8%, and given that cellulose is treated with borates, no one seems too concerned.”

Ueno’s data (like Arena’s data) show that south walls stay dryer than north walls. When I telephoned Ueno, he told me, “The first winter, the cellulose wall peaked at 20 to 25% moisture content. Those are the numbers for the north walls; the south walls were boring. For a few months over the winter the moisture content was high, then everything dried over the summer. Everything in all three monitored walls dropped over the summer to 10 to 12%.”

The moisture content of the OSB sheathing monitored by Ueno is somewhat higher than the values measured by Arena. Ueno told me, “One of the key differences between the walls we’re monitoring and the ones that Lois Arena is monitoring is that there is vapor retarder paint on her wall, while we’re using straight-up latex paint.”

I asked Ueno whether he thought the moisture spikes during the winter were cause for concern. “I don’t think it’s an issue,” he answered. “As Mark Bomberg says, ‘We measure the moisture content of wood during the winter but it rots during the summer.’ Everything was drying down by March. During the second winter of this study, the house was occupied by a family of four, and they didn’t have the ventilation system running. The system was not hooked up correctly. So the interior RH was 40% to 50% during the first half of the winter. Later, after the exhaust fan was fixed, the RH came down to 30% for the second half of the winter. All of the walls on the north side had a high moisture content for a few months. But by the end of September, were back down to 10 to 12%.”

For more information on the conditions of the Building Science Corporation's monitoring study in Devens, see Double-Stud Wall Field Monitoring and New England Net Zero New Construction Evaluations.

Ueno's data show that an important factor in moisture accumulation in sheathing on double-stud walls (in addition to the reduced heat flow through the wall assembly compared to walls with less insulation) is wintertime vapor diffusion through the assembly from the interior to the exterior. For more information on this factor, see The Return of the Vapor Diffusion Bogeyman.

John Straube's data

To complete my roundup of monitoring data, I telephoned John Straube, a professor of building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. science at the University of Waterloo in Ontario. Straube was happy to discuss his reasearch.

“We’re doing a study now," Straube told me. "We’ve got a test house here in Waterloo. The climate zone is similar to Burlington, Vermont. The interior humidity is kept between 30 and 40 percent in the wintertime. We’re testing a variety of walls, all with OSB sheathing: a typical 2x6 wall, a 2x6 wall with 3 inches of exterior Roxul, one with 2 1/2 inches of exterior XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation., one with 2 inches of foil-faced polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. , a 9 1/2-inch-thick I-joist wall, and a 12-inch-thick double-stud wall insulated with cellulose. All the walls have Tyvek, furring strips, and fiber-cement siding. The study is entering its third winter.”

Straube's measurements are similar to those made by other researchers. “The sheathing moisture content on the double-stud wall exceeds 20% in March, then hits a peak of 31% on the 26th of March," he told me. "It dips below 20% in the middle of April.”

The walls with insulation on the exterior side of the sheathing are faring better than the double-stud wall. Straube posed a rhetorical question: “Maybe you’re wondering, ‘What is the moisture content of the sheathing on the walls with exterior XPS in February and March?’ Well, it’s 8%.”

One of Straube's colleagues, Trevor Trainor, provided further information on the researchers' protocol. "The peak moisture content levels occurred during a period of intentional, controlled air exfiltrationAirflow outward through a wall or building envelope; the opposite of infiltration.," Trainor explained. "We intentionally injected interior air into the walls at a rate that we determined as a realistic natural air leakage rate for a building that meets Canadian Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. standards (0.2 cfm per square foot at 50 Pa). We used a rate of 40 CFH per 4 X 8 wall panel (the equivalent of 0.2 L/sec/m2). Depending on the geometry of the house, this roughly translates to 2.5 ach50."

Trainor added, "Obviously a house that is significantly tighter than this should perform better — depending on the distribution of the leakage."

What do these measurements mean?

To understand whether the moisture spikes seen in double-stud wall sheathing are worrisome, it's useful to compare these measurements with the moisture content of OSB sheathing on ordinary 2x6 walls.

“With an ordinary 2x6 wall, the moisture content of the OSB never gets as high as it does with these double-stud walls," Straube told me. "On a north wall in a humid climate, it might reach the high teens, but not the low 20s.”

When researchers interpret data from double-stud wall studies, they often frame the conversation using a “good news/bad news” format. “We know that actually, most of our wood products are surprisingly resilient,” said Straube. “But these monitoring results show that we are right on the edge between risky and where we would be safe. These elevated moisture contents are lasting for several weeks to two months, depending on the study, into April. And in April, the sheathing may be at 50 degrees. That is right in the zone where you might get a problem, though it might take years for the problem to manifest itself. It’s in the gray zone.”

Should builders worry? “The monitoring studies say that these double-stud walls are on the edge, not obvious failures," said Straube. "After all, where are the bodies? The failures happen only when we have a leaky window or an air leak or badly managed construction moisture.”

The message for builders of double-stud walls is simple: don't screw up the details. “These walls are on the edge," said Straube. "What that means is you don’t have a lot of room to move when it comes to making mistakes. When I compare a double-stud wall to a wall with 3 inches of exterior rock wool or 2 inches of extruded polystyrene, I find more risk.”

Straube advised, “Build a simple box with ventilated cladding, and do a good job with airtightness. Those are strategies to reduce the risk. Choose a sheathing that is more vapor-permeable than OSB — plywood, fiberboard, or DensGlass Gold.”

Martin Holladay’s previous blog: “Low-Road Buildings Are Homeowner-Friendly.”

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Tags: , , , , ,

Image Credits:

  1. Kohta Ueno - Building Science Corporation
  2. Kohta Ueno
  3. Lois Arena - Building America
  4. Andy Shapiro
  5. Building Science Corporation

1.
Nov 1, 2013 8:23 AM ET

helpful summary, a couple of questions
by Leigha Dickens

This is something that's been nagging at me too, the difference between models and experimental data. It is great to see an summary of what we've seen with some real structures so far.

I take Ueno's experiment to mean he looked at one 12 inch double stud wall with cellulose and one double-stud wall with open cell spray foam. That foam is vapor open too, so I'm very curious what the sheathing moisture content was in the case of that stud bay.

Also, what was the thickness of the exterior XPS and Roxul on the 2x6 walls in Straube's study?


2.
Nov 1, 2013 8:58 AM ET

Waterloo Study
by Trevor Trainor

Leigha -

I can speak to your last question. The walls that we are studying have 3 inches of Rockwool (Density of 8 PSF), 2 1/2 inches of XPS and 2 inches of foil faced polyiso. All were installed in two layers. The idea was to create 3 walls with the same nominal R-value to get a direct comparison.


3.
Nov 1, 2013 9:02 AM ET

Edited Nov 1, 2013 11:38 AM ET.

Response to Leigha Dickens
by Martin Holladay

Leigha,
Q. "I take Ueno's experiment to mean he looked at one 12-inch double stud wall with cellulose and one double-stud wall with open-cell spray foam. That foam is vapor open too, so I'm very curious what the sheathing moisture content was in the case of that stud bay."

A. Kohta Ueno has two winters of data. During the first winter, the moisture content of the sheathing on the spray-foam wall was higher; during the second winter, the moisture content of the sheathing on the cellulose was was higher. He told me, “We had two different walls that we were monitoring, one with cellulose and one with open-cell foam. The cellulose-insulated wall was 12 inches thick. The other wall had foam that was 5 1/2 inches thick. The first winter, the moisture content in the sheathing on the wall with open-cell foam wall was a little higher than it was on the wall with cellulose. That result confused me at first, but then I realized that the foam provides a little bit of vapor resistance. The second winter was more interesting. All of the walls were high. In the wall insulated with cellulose, the peak moisture content was in the range of 25%, maybe up as high as the low 30s, on the north side. The spray foam wall was in the 18 to 20 percent range. All of the walls, by the end of September, were back down to 10 to 12%.”

Q. "What was the thickness of the exterior XPS and Roxul on the 2x6 walls in Straube's study?"

A. It looks like Trevor Trainor just answered your question. Thanks, Trevor.

I have edited the article to include the exterior insulation thicknesses -- information which John Straube provided in our phone conversation, but which I wasn't typing fast enough to catch.


4.
Nov 1, 2013 9:19 AM ET

The Cold Sheathing Problem
by Ron Keagle

This “cold sheathing problem” is one of the most interesting issues that I have encountered regarding superinsulated construction. I welcome the elaboration on the topic, and I have a few fundamental questions and observations to consider:

What is the source of moisture that accumulates in the sheathing?

When measuring moisture content inside of walls or in sheathing during the winter; and finding an elevated moisture content; is it known whether the source of that moisture is the interior or the exterior?

If the source of moisture is the interior, why would it get past the proper air sealing of the interior side of the wall?

If the source of moisture is the exterior, why would it not continue drying inward and be absorbed, retained, or buffered by the cellulose? Indeed, why would that inward moving moisture not just continue drying inward all the way to the interior, and not accumulate in the sheathing?

If the wintertime moisture rise in sheathing is from exterior moisture, I assume that it is a one-time adjustment because it is only induced by a lowering of average temperature. However, if the wintertime moisture rise in sheathing is from interior moisture moving outward, the source will be self-sustaining with a continuous feed of moisture generated in the interior living space.

Therefore, if the source of moisture is the exterior, it is supplied as a higher moisture content of air, and will be more readily absorbed by the sheathing simply because of the sheathing’s lower wintertime temperature.

However, if the source of moisture is the interior, it reaches the sheathing as a continuous supply of condensed water. In that case, the lower average temperature of the sheathing would not be necessary for it to absorb a continuous supply of actual contacting water resulting from an outward moving supply of vapor from the interior living space.


5.
Nov 1, 2013 10:01 AM ET

An important note about the Waterloo Data
by Trevor Trainor

Martin -

When comparing our Waterloo data to the other studies it should be noted that the peak moisture content levels occurred during a period of intentional, controlled air exfilatration.

We intentionally injected interior air into the walls at a rate that we determined as a realistic natural air leakage rate for a building that meets Canadian Energy Star standards (0.2 CFM per sq. ft (or 1.02 L/s/m2) at 50 Pa). We used a rate of 40 CFH per 4 X 8 wall panel (the equivalent of 0.2 L/sec/m2). Depending on the geometry of the house, this roughly translates to 2.5 ACH50.

Obviously a house that is significantly tighter than this should perform better - depending on the distribution of the leakage. I think that is what Dr. Straube means when he states that these type of walls are on the edge and that you have to get the details right. I would say that this type of wall is sensitive to construction quality, where as the exterior insulated walls that we studied are more robust - just my 2 cents.


6.
Nov 1, 2013 10:10 AM ET

Edited Nov 1, 2013 10:11 AM ET.

Response to Ron Keagle
by Martin Holladay

Ron,
Moisture flows through walls are a dynamic phenomenon. The interior surface of the sheathing will take on moisture when the nearby air or insulation materials are wet enough, and will lose moisture when the nearby air or insulation materials are dry enough. Simultaneously, the exterior surface of the sheathing will take on moisture when liquid water gets past the siding and dribbles down the sheathing, or when nighttime radiational cooling lowers the temperature of the siding below the temperature of the outdoor air, and will lose moisture when the sun comes out. Moisture flows in both directions from both surfaces of the sheathing, and the direction of the moisture flows can reverse several times a day. That's what we mean by a dynamic phenomenon.

Winter conditions will cause the moisture content of wood exposed to exterior conditions to rise, up to a point. John Straube said, "As things get cold, water vapor begins to store in the material. However, it will not rise to 25% MC unless you do something else -- unless, for example, there is condensation or rain, frost or dew."

Other factors that can increase the moisture content of sheathing are air leaks in the wall assembly and elevated indoor humidity levels. In some cases, even diffusion can make a difference -- that is, diffusion of interior water vapor through the drywall and insulation. That's why Kohta Ueno speculates that the 0.5-perm vapor-retarder paint on Lois Arena's wall might account for some fraction of her lower readings.


7.
Nov 1, 2013 10:21 AM ET

Response to Trevor Trainor
by Martin Holladay

Trevor,
Thanks very much for the information you provided. I have edited the article to reflect that information.


8.
Nov 1, 2013 10:23 AM ET

excellent article
by michael pi

Thank you for this fantastic article, which comprehensively addresses a hot (cold?) issue with a very popular wall assembly. I contacted a prominent building science practitioner/author who said that virtually every wall they model fails ASHRAE criteria, that it is generally felt to be far too strict, and that they don't rely on that metric. I have two questions related to this article:

1. I'm not entirely understanding the science behind why that walls with exterior foam (or other exterior insulation) appear to be staying more dry - as they seemingly can't dry to the outside as effectively should they get wet. Is it the retention of heat keeping the sheathing warm, or the lack of penetration of exterior moisture, or some other factor?
2. I'm interested in thoughts about Zip system exterior sheathing, due to the fact that it is seemingly very popular and less permeable in various ways than OSB. I'm particularly interested in this in the context that the Bensonwood OBPlusWall seems to use this as exterior sheathing, with interior OSB sheathing as an air barrier and 9.5" dense packed cellulose. I suppose I don't understand how a wall system that uses exterior sheathing even less permeable than OSB could effectively prevent moisture issues in this context.


9.
Nov 1, 2013 10:38 AM ET

Edited Nov 1, 2013 10:41 AM ET.

Response to Michael Pi
by Martin Holladay

Micheal,
Q. "I'm not entirely understanding the science behind why that walls with exterior foam (or other exterior insulation) appear to be staying more dry - as they seemingly can't dry to the outside as effectively should they get wet. Is it the retention of heat keeping the sheathing warm, or the lack of penetration of exterior moisture, or some other factor?"

A. Physics (and the psychrometric chart) tell us that cold materials tend to be damp and warm materials tend to be dry, which is why our clothes dryers use heat to remove moisture from clothes, and why we put damp crackers in the oven to revive them and restore their crispness.

There is a temperature gradient through a wall assembly during the winter. The temperature of the siding is close to the outdoor air temperature, while the temperature of the gypsum drywall is close to the indoor air temperature. The temperature of the sheathing will be somewhere in between.

If you install foam insulation on the exterior side of the sheathing, the sheathing moves closer in temperature to the interior conditions. In winter, the interior conditions are warm and dry.

If you take the extreme case -- a PERSIST wall, with all of the insulation on the exterior side of the sheathing, and no insulation between the studs -- then the OSB sheathing will be at interior conditions -- as warm and dry as your coffee table.

If your wall has enough exterior foam to keep the sheathing above the dew point during the coldest weather of winter, then there is no need for the wall to dry to the exterior. It can dry to the interior if necessary. The sheathing really doesn't have to dry out in any case -- unless there is a catastrophic problem like a flashing defect that dumps rain into the wall assembly -- because the sheathing is always warm and dry. While the sheathing on an ordinary wall gets wet every February, and therefore needs to dry out seasonally, the sheathing on a wall with exterior foam never gets wet in the first place.


10.
Nov 1, 2013 10:51 AM ET

Edited Nov 1, 2013 10:55 AM ET.

Second response to Michael Pi
by Martin Holladay

Michael,
Q. "I'm interested in thoughts about Zip system exterior sheathing."

A. Zip System sheathing is a type of OSB. The manufacturer uses better resins (adhesives) to bond the wood flakes together than are used on ordinary OSB, so Zip System sheathing isn't as vulnerable to rot as ordinary OSB.

The vapor permeance of Zip System sheathing is about the same as other brands of OSB -- in other words, not very permeable. The numbers are a little loose, because the vapor permeance of ordinary OSB will vary depending on its moisture content.

Zip System sheathing has negligible R-value, so it doesn't solve the cold-OSB problem. However, it is probably more likely to stand up to repeated cycles of wetting than ordinary OSB.

Here's what John Straube wrote: "The vapor permeance of Huber Zip is in the same range as commodity OSBs. OSBs that we test have quite a range of wet-cup vapor permeances, and both roof and wall ZIP are essentially the same. But Zip, like all OSB, does not have the high permeability of a building paper, so one needs to design carefully. For best practice, that means insulation on the exterior to warm it up and to blunt thermal bridges at mudsills and floor joists. If you use a double-stud wall, then I am concerned, but I have a low risk threshold since I am a forensic consultant too. :)
The back of the Zip has a different texture because of the way the product is pressed, and you will notice that other OSB is like this too. There is no difference in performance as far as we have been able to see. Finally, sealing [tape] to plywood is very difficult without mastic; sealing to OSB works OK with primers and some care; and sealing tape to Zip is pretty darn easy. The smooth surface of Zip is its huge virtue."


11.
Nov 1, 2013 11:13 AM ET

Leigha's Question; Additional Writeups
by Kohta Ueno

I take Ueno's experiment to mean he looked at one 12 inch double stud wall with cellulose and one double-stud wall with open cell spray foam. That foam is vapor open too, so I'm very curious what the sheathing moisture content was in the case of that stud bay.

I think Martin answered this question pretty well, but if you wanted to see the graphs yourself, you can download them from BSC's website. Unfortunately, we have only completed writeup reports for that first winter (when the interior RH was very low)--the second and third winters will be in an upcoming Building America Report! But the first winter definitely demonstrates that the 12" cellulose wall was the wettest, and the 12" and 5-1/2" open cell foam walls stayed drier in the winter.

Day 1.2b: Double-Stud Wall Field Monitoring (http://www.buildingscienceconsulting.com/presentations/documents/0102b_D...)--presentation I gave in summer 2012; might be a bit cryptic without explanatory text (see report below).

BA-1303: New England Net Zero New Construction Evaluations (http://www.buildingscience.com/documents/bareports/ba-1303-new-england-n...) Big brick of a report on our Building America work with that builder--see Chapter 5 Moisture Monitoring of Twelve-Inch Double-Stud Walls. Again, only covers the first winter.


12.
Nov 1, 2013 11:27 AM ET

Reply to Martin Holladay
by Ron Keagle

Martin,

I understand your points about moisture flow being a dynamic phenomenon. Here is what I am getting at. The cold sheathing problem seems to be based on the observation and scientific expectation that moisture content of sheathing rises during the winter.

So, if that is all there is to it, then the only remedy is to prevent the sheathing temperature from dropping to the extent that moisture gain in sheathing can cause damage. This requires placing insulation outside of the sheathing to keep it warmer.

However, if the extra wintertime moisture observed in sheathing is also being caused by other factors such as poor air sealing, flashing problems, or outward vapor diffusion, in addition to just the lower temperature of the sheathing; then it opens the possibility that the problem may be adequately remedied by fixing those other defects without adding exterior insulation.

Therefore, it seems important to find out where the excess wintertime sheathing moisture is coming from. It is important because if we just assume the entire problem is based on the falling sheathing temperature, it says that the thickening the wall insulation in a double stud system is basically a flawed concept once a certain thickness is reached. This is a fairly dramatic conclusion that calls for a sea change in the approach to superinsulated wall design. So, I think it is important to make sure the conclusion is accurate.

I would like to see moisture testing done on the sheathing of thickened double stud walls with a test house that is 100% air sealed on the interior living space side. Then further testing could be done after making the interior living space side 100% diffusion sealed, so that diffusion alone could be quantified.

Testing with zero air leaks and zero diffusion would show to what extent the cold sheathing problem is actually caused by cold sheathing alone.

This experiment could also be done with a variety of air-permeable insulation types for further study of the effect. Then, finally, I would like to see the experiment accompanied by a sample of the sheathing placed alone in the open near the wall system being tested, so the moisture gain in the sheathing from season change alone could be measured and compared to what is happening to sheathing built into the wall system.

So while the moisture flow is a complex, dynamic phenomenon, I would like to see testing that can break down the complex dynamics so we can see what is happening with the cause and effect of the individual components of the complex phenomenon.


13.
Nov 1, 2013 11:37 AM ET

Yeti
by Dan Kolbert

I think of cold sheathing as the Big Foot (or perhaps Abominable Snowman is the more apt metaphor) of building science - much discussed, rarely seen. I've been asking for years for someone to show me sheathing rotting away from this problem and it seems like every issue is ultimately caused by something else- water intrusion, lousy flashing, poor window or insulation installation, etc.

Obviously the more insulation we put in an assembly the less our heat loss can cover for sloppiness elsewhere. But if anyone has direct field evidence of a problem purely caused by cold sheathing I would love to see it.


14.
Nov 1, 2013 11:47 AM ET

Edited Nov 2, 2013 6:51 AM ET.

Second response to Ron Keagle
by Martin Holladay

Ron,
Just as "house as a system" thinking forces us to pay attention to the ways that a range hood fan can affect the operation of a water heater, so hydrothermal thinking forces us to pay attention to all of the factors that affect moisture flows through building assemblies.

The list of factors is very long, which is why WUFI is such an amazing accomplishment (as well as why it is, for the most part, a useless tool from a builder's perspective). Here are some of the factors that affect moisture flows through walls:

Interior relative humidity
Interior temperature
Air flow rates through the envelope
Location and size of the air leaks through the envelope
Vapor permeance of all layers of the assembly, including paint, gypsum wallboard, smart retarders, insulation, sheathing, and siding
Presence or absence of a rainscreen gap
Depth of the rainscreen gap
Presence or absence of ventilation openings at the bottom and top of the rainscreen gap
Orientation of the wall
Width of the roof overhangs
Exterior climate (temperature, relative humidity, rainfall, and insolation data)
Exterior wind speed and exposure
Exterior shading

Trust me, Ron -- I left a few factors out.

So of course the MC of the exterior sheathing is not a simple function of the outdoor temperature. No building scientist in the world believes that it is.

And of course all building scientists would love to see more monitoring studies that measure the effects of all of the factors on my list. Building scientists love data, and always advocate for more studies and more funding.

In the meantime, we all muddle along with the best available information. Hats off to the scientists working to research these issues.


15.
Nov 1, 2013 11:54 AM ET

Response to Dan Kolbert
by Martin Holladay

Dan,
Thanks for your comments. I remain committed to the hunt for the elusive yeti, as you do, and will be happy to announce that the yeti is a mythical creature, if the search is eventually fruitless.

Even John Straube admits, "These double-stud walls are on the edge, not obvious failures. After all, where are the bodies?"


16.
Nov 1, 2013 11:58 AM ET

location matter?
by aj builder, Upstate NY Zone 6a

2 story frame... Sheathed... 18' or so ... is the moisture the same or is it high or low on the wall?

A thought I have had is that using pressure treated plates for exterior walls might be worth doing. Moisture and rot loves wood joints. If half the joint was pressure treated the joint just might be quite well protected even with standard studs and sheathing. I have the same idea for tiled showers and baths. Use PT framing the under water safe sheathings. PT plywood under toilets...

Anyway... Is wall moisture evenly distributed?


17.
Nov 1, 2013 12:21 PM ET

Accounting for the moisture
by Trevor Trainor

Ron -

Maybe I can shed a little light on the origin of the moisture.

In our experiment, with no diffusion and no intentional air leakage, the double stud sheathing moisture content reached 17% when it plateaued- midway through the first winter. The standard 2 X 6 wall and the exterior insulated walls were about half of that (7-9%). I believe that the difference here was due to the built-in moisture in the cellulose migrating to the exterior sheathing during cold weather (ie. the ping pong effect).

We then started injecting air into the walls. The double stud increased to 32% M.C. during air injection, while the standard wall maxed out at 29% M.C. The difference here was that the standard wall seems to have plateaued, while the double stud was still climbing at the end of the air injection phase. The exterior insulated walls stayed at 7-9%. The changes we see here are due to air leakage condensation. The standard wall and the double stud wall both had cold sheathing and reacted in a similar way to air leakage.

Later in our experiment (in late spring) we wetted the OSB from the outside using wetting mats (to simulate rain leakage). The double stud wall reacted exactly like the standard wall, reaching 14-15% M.C., but drying quickly when the wetting phase ended. (more on the exterior insulated walls at a later date)

My take away from all of this is that the sheathing of double stud walls will get wet in the winter - how wet they get will depend mostly on built in moisture and air leakage. They will also dry in the summer. Whether they dry down to safe moisture contents depends on how wet they got in the winter (along with solar exposure, climatic considerations, cladding ventilation and material choices)

Hopefully this helps a little - there is still much to find out!

Trevor


18.
Nov 1, 2013 12:21 PM ET

Response to A.J. Builder
by Martin Holladay

A.J.
Q. "Is the moisture the same, or is it high or low on the wall?... Is wall moisture evenly distributed?"

A. The answer can be determined by studying the graphs (Images 4 and 5). In general, orientation matters more than height. In other words, all of the sensors on the south wall recorded lower MC readings than any of the sensors on the north wall.

The graph reporting Kohta Ueno's data shows that the upper sensor on the north wall stayed dryer than the mid-level sensor or the lower sensor.


19.
Nov 1, 2013 12:23 PM ET

"the modeling results don’t pass the sniff test"
by Katy Hollbacher

What a valuable in-depth analysis... thank you. I especially appreciate your calling out issues w/ hygrothermal modeling that's sometimes based on flawed reference standards or other assumptions. Too often, WUFI amateurs blindly rely on misleading results to direct their design decisions: whether those decisions end up being overly conservative or unconservative, this is very worrisome to me. I really appreciate posts like this that collate data from real experts and interpret WUFI analyses in the context of understanding how these things perform in the real world. Kudos!


20.
Nov 1, 2013 12:23 PM ET

Response to Kohta Ueno
by Martin Holladay

Kohta,
Thanks very much for your comments, and for the useful links. I have edited the article to include links to the two published BSC documents.


21.
Nov 1, 2013 12:28 PM ET

Response to Katy Hollbacher
by Martin Holladay

Katy,
You wrote, "Too often, WUFI amateurs blindly rely on misleading results to direct their design decisions."

I couldn't agree more, which is why we should all see WUFI as a reseracher's tool, not a builder's tool or designer's tool.

I've been worried about the problem of yahoo WUFI users for years, and my irritation is coming to a boil. I'm planning to write a blog on the topic titled "WUFI is Driving Me Crazy."


22.
Nov 1, 2013 12:33 PM ET

Where are all the bodies?
by John Semmelhack

From what I've seen and read, it seems that it usually takes a "perfect storm" of building science screw-ups to actually generate "bodies"...(think Vancouver condos or EIFS in Wilmington, NC, for example). But then again, what if EVERY builder in zones 5-8 started building double-stud walls with osb? Would we see bodies then?


23.
Nov 1, 2013 12:33 PM ET

Response to Martin Holladay
by Ron Keagle

Martin,

I understand your points about all the complexity of vapor movement, but to say that “a double stud wall is on the edge [of failure]” seems like it is speaking fundamentally about double stud walls. Specifically, it refers to such walls causing the coldest sheathing. But if we are talking about the whole variety of moisture threats, then any wall is on the edge of failure if it has a big hole where the rain comes in. The sheathing temperature may not be causing any problem at all.

You said: “So of course the MC of the exterior sheathing is not a simple function of the outdoor temperature. No building scientist in the world believes that it is.”

Yet the conclusion that a double stud wall is on the edge of failure rests only on the premise that the sheathing takes on more moisture solely because it is exposed to outdoor temperature.

In the most fundamental sense, I do not see how one can draw any conclusions about sheathing temperature causing a moisture problem simply by measuring the moisture content of sheathing.


24.
Nov 1, 2013 12:50 PM ET

Yeti as myth...
by Dana Dorsett

http://www.slate.com/articles/technology/future_tense/2013/10/yeti_dna_i...

While rot & structural failure on the OSB from vapor-diffusion alone in real-world assemblies may hit Yeti status, mold on the OSB sheathing in from vapor diffusion through higher-permeance interiors in cold climates isn't as rare as some might believe, and mold in stud-bays with known interior side air-leaks such as unsealed electrical outlets could be considered common (though not as common as mold in assemblies with air-leaky poly vapor barriers.)

Of course every house has mold- it's only a matter of how much, and where (and in some instances, what type.)

I'm not sure anyone is really concerned about structural failures from cold sheathing, but anything that increases mold hazard developing over decades of use can't be simply shrugged off. Rome wasn't built in a day, but it didn't burn in a day either. Houses built with an air-tight interior today isn't likely to be fully air-tight in every stud bay 100 years hence.

Designing and building to be resilient to imperfections, either baked-in-the cake on day-1 or developed over time is a worthy goal, given that the perfect house has yet to be built. Building with less susceptible sheathing or with sufficient exterior insulation are worthwhile, to improve the resilience of the assembly.

Robert Riversong's approach of skipping the sheathing and nailing ship-lap siding directly to the studs and dense-packing deep trusses with cellulose feels pretty dubious over a century long time frame (and certainly unsuitable for designs that lack deep roof overhangs.) It would be interesting to see one of THOSE houses instrumented & tracked, given that there are now probably dozens of examples out there in New England.


25.
Nov 1, 2013 12:51 PM ET

Edited Nov 1, 2013 12:57 PM ET.

Response to Ron Keagle
by Martin Holladay

Ron,
Here's how science works: scientists formulate a hypothesis, and test their hypothesis by conducting an experiment or gathering data. If the data don't fit the hypothesis, they refine their hypothesis so that their new hypothesis fits the data. Lather, rinse, repeat, as it said on the old shampoo bottles.

No hypothesis is definitive, and every hypothesis can be overturned at any point by new data.

That said, the correlation between the temperature of exterior sheathing and its moisture content is very well established. I invite you to report data strong enough to overturn it.

Other factors matter too, as I pointed out and as you acknowledge. If you want to build a wall with cold sheathing, clearly the sheathing temperature is out of your control. So you have to manipulate the other factors to bring your wall assembly a little bit back from the edge of the cliff. You do this by choosing a moisture-tolerant sheathing, by including a ventilated rainscreen gap, and by paying strict attention to airtightness. In that way, the wall assembly becomes less risky.

Or, alternatively, you install an adequate thickness of exterior rigid foam or mineral wool, thereby making the sheathing warm and dry. Your choice.


26.
Nov 1, 2013 1:03 PM ET

Edited Nov 1, 2013 1:05 PM ET.

Response to John Semmelhack
by Martin Holladay

John,
"It takes a perfect storm," you wrote. But if the leaky condo crisis in Vancouver taught us anything, it's that ordinary everyday construction practices can lead to catastrophic failure on a massive scale. The Vancouver experience wasn't a perfect storm -- it was just business as usual (no sill pans under windows, no flashing where deck rails are attached to walls, and stucco over OSB without an air gap).

Sometimes it takes a few years before we recognize the problems caused by business as usual. And then POW -- the problems hit us all at once, like a lightning bolt.


27.
Nov 1, 2013 1:07 PM ET

Response to Dana Dorsett
by Martin Holladay

Dana,
You wrote, "Mold on the OSB sheathing from vapor diffusion through higher-permeance interiors in cold climates isn't as rare as some might believe, and mold in stud-bays with known interior side air-leaks such as unsealed electrical outlets could be considered common."

True enough. We've all seen 2x6 walls with moldy OSB. The question we are all wondering is, "Are today's double-stud walls riskier or less risky than the (somewhat leaky) 2x6 walls we're all used to?"

Time will tell.


28.
Nov 1, 2013 1:40 PM ET

Vancouver...
by John Semmelhack

Martin,

My understanding of the Vancouver situation is that the combination of using interior polyethylene and more insulation in the 1980's and early 90's were the two additional ingredients (the "special sauce", if you will) to the existing recipe ('no sill pans under windows, no flashing where deck rails are attached to walls, and stucco over OSB without an air gap') that likely pushed these buildings over the cliff.


29.
Nov 1, 2013 1:50 PM ET

Quotes
by Jesse Thompson

Martin,

While it can be nice to see your name in print, I have to think my quote above doesn't really add much to the discussion.

We're certainly not building scientists, forensic experts or even builders here at our office. I was just repeating the warnings you've quoted above from actual experts like the folks you've been talking to in detail here like Building Science, Building America and Stephen Winter Associates. Standing on the shoulders of giants and all that…

It's great to hear actual technical detail on the issue. We haven't been seeing any bodies in our area either, just the usual rotted out windows sills. Did have a good one recently with rotted wood sheathing and studs behind a brick veneer wall right on the coast with no weeps or drainage plane, but that's an obvious one.

Jesse Thompson


30.
Nov 1, 2013 1:51 PM ET

Reply to Trevor Trainor
by Ron Keagle

Trevor,

Thanks for that explanation of your testing method. As I understand it, you eliminated air leaks or outward diffusion, so that no extra moisture taken on by the sheathing could have come from the interior living space. You conclude that extra moisture came from the original moisture content of the cellulose when it was installed. Do you have any idea what result you would have gotten had there not been excess moisture in the cellulose?

If there is no air leakage or diffusion from the interior living space; and if the wall starts out with dry insulation; and if there are no exterior water leaks; then how much increase in moisture would you expect to see in the sheathing as it moves from summer into winter?


31.
Nov 1, 2013 2:01 PM ET

thanks, and a couple more thoughts
by Leigha Dickens

Kohta: thanks for the additional links! I will look for the upcoming Building America report. Since the open cell foam walls in the study did stay somewhat drier...does that mean we able to see the distinct fingerprints of vapor diffusion vs air infiltration in comparing the spray foam walls to the cellulose walls? All other things being equal, the cellulose walls allow both air movement across the cavity and vapor diffusion, while open cell spray foam walls would only allow vapor diffusion. I'm not sure how the vapor permeability of cellulose and open cell foam compare, though.

Trevor: Your detailed explanation of the methodology you used is really enlightening, thanks. I'm curious about about one thing--"with no diffusion and no intentional air leakage..." how did you create an initial situation with no vapor diffusion? Just make sure the humidity conditions on both sides of the wall were identical?


32.
Nov 1, 2013 2:08 PM ET

Response to John Semmelhack
by Martin Holladay

John,
Yes, the Vancouver builders were using interior polyethylene -- but so were a great many builders all over North America. As I said, it was business as usual.

And the disasters weren't happening just in superinsulated buildings, so the disasters weren't a function of high levels of insulation. Just ordinary 2x6 walls with fiberglass batts, for the most part. As I said, business as usual.

One thing Vancouver has that Las Vegas doesn't is wind-driven rain, and so climate was one factor that made the city's disaster world-famous.


33.
Nov 1, 2013 4:44 PM ET

Replies to Ron and Leigha
by Trevor Trainor

Ron - the built-in moisture that I am referring to is just what the cellulose would pick up from atmospheric humidity. Since cellulose is very hygroscopic it will never be dry - unless the atmosphere surrounding it is completely dry. The 'ping pong' effect results in a concentration of what ever moisture is in the cellulose at the sheathing side when it is cold out. The thicker the cellulose wall, the greater the effect.

Liegha - We used 6 mil poly sheeting to stop any vapour diffusion from the interior


34.
Nov 1, 2013 7:31 PM ET

great topic
by Bill Rose

The discussion is interesting. My take, at least for starters, is simpler. The average RH in January in central Illinois is between 85% and 90%. So any material outdoors and protected—say a garage wall—will come to equilibrium at the high MC that corresponds to that RH. Boards and plywood, we’ve discovered, will do fine and dry out with the spring. My jury is still out with regard to OSB, whether it will last 100 years. It should relax over time, and weaken as it relaxes. We can discuss rain wetting and diffusion and drying rates and drying directions and WUFI runs, but I just want to know if the sheathing product will survive in equilibrium with winter conditions, protected and not subject to excessive loads.

That’s all we can do with good building science—protect it, keep it from excess water loads from outdoors and in, and make sure we are using our energy dollar for purposes other than artificial drying of exterior structural building materials. I used to think I could make a building last forever. I was saddened with the appearance of OSB to think that, perhaps, despite bringing the best of my art to a project, the sheathing may still become unserviceable 100 years later.


35.
Nov 1, 2013 8:56 PM ET

Response to Dana Dorsett
by Malcolm Taylor

Dana, rather than using Robert Riversong's preferred wall section which still has the cellulose in contact with the wood siding, what do you think of the more recent alternatives which substitute a WRB for the exterior sheathing and separate that from the siding by including rain screen strapping?


36.
Nov 1, 2013 9:13 PM ET

What is it about OSB?
by Dan Kolbert

The processing of the fibers? The glues holding them together?


37.
Nov 1, 2013 9:39 PM ET

Vapour Permeable Sheathing Panels
by Andrew Robinson

Just curious - would these "vapour permeable sheathing panels" that are apparently used in Europe solve these issues? I'm thinking that the sheathing would still get cold, but the vapour would pass through more easily ... or at least dry out faster?


38.
Nov 1, 2013 9:56 PM ET

Safe(r) moisture (M%) of OSB/plywood and wood sheathing
by floris keverling buisman

It is good to hear that expert comment that these double studs walls with exterior OSB/plywood are on the tipping point, but it might be worrisome that most find being on the edge an acceptable practice. I certainly agree these walls are risky, when seeing OSB M% that exceed 20% and even go into the low 30%'s (just like WUFIs run with 160p set up right, EN15026 or a simple sine curve predict).

In my opinion this means that all the safety reserves of the exterior sheathing moisture storage capacity are (more than) exhausted. The question is: do we want to build on the edge - or have a drying reserve/M% reserve that can give some play for some small amount of interior air and exterior water leaks...? I would argue one would, especially in these well insulated assemblies.

If you look at the European norm EN 353/EN636 (see APA-Europe's EN publications It has been defined there that solid lumber (EN 335 in (see Materials for Architects page 122) is regarded free from damage/decay below 20 M%, when not PT. The limit for Plywood/OSB is defined in EN335-2/EN636-2 at an equivalent of 18M% (see also page 130 of Materials for architects").

Consequently, using retarding sheathing (OSB, Plywood) or smart vapor retarders on the exterior of the interior stud wall, makes much more sense for well insulated assemblies. It allows the sheathing to be warm and when combined with fibrous insulation to it's exterior, gives much more drying reserves and creates a more forgiving enclosure (See image). A variety of these assemblies can also be found on our 475 blog:Construction details for Foam free construction

AttachmentSize
wall 3cI double stud - window penetration.pdf 90.18 KB


39.
Nov 1, 2013 10:02 PM ET

Edited Nov 1, 2013 10:17 PM ET.

I volunteer my 12" cellulose/exterior roxul wall for study
by Patrick Walshe

If anyone wants to measure moisture content in our walls they are welcome. We are at low elevation on Vancouver Island, BC and have recently completed house and suite with staggered stud walls 9.25" thick blown with dense packed cellulose, plywood sheathing, typar, 2" roxul comfortboard IS, 1x4 strapping, and fiber cement board siding. On the inside of the main house is airtight drywall with latex paint and on the suite there is Certainteed Vapour retarder Membrain and then regular drywall with latex paint. The entryway is 2x6 wall with roxul batts and 2" exterior roxul. Both units have HRV's and an interior humidity of around 45% to 48% though I might increase the ventilation a bit to bring down to 35-40% - at least while construction moisture dissipates. The Venmar EKO 1.5 I have on 30 minutes out of every hour to get 40 CFM which is about right for the new Building Science ventilation rate (1700 square feet, 3 bedroom, well mixed) and the suite is 950 sqft with a Venmar Constructo 1.5 set on low (60 CFM which about twice what I calculate is needed, but will likely get the Lite Touch Controller which can be set for 20min out of every hour, or maybe just a hygrometer and fiddle with the outside temperature dial on the main controller, until the humidity is just right ). You can learn more and reach me at www.agreenhearth.com


40.
Nov 2, 2013 6:14 AM ET

Response to Bill Rose
by Martin Holladay

Bill,
Thanks for your comments. Anyone who has spent much time at construction sites has seen OSB that has decided to "relax." Relaxed OSB is returning to its origin as a pile of wafers.

1x8 boards eventually relax as well, of course, becoming organic matter and topsoil -- but the OSB seems to be in more of a rush to relax than the boards. OSB is like young New York City office workers taking the train to the beaches of Southampton on Friday night, eager to have a few drinks on the way to the beach house. Can't wait to start relaxing.


41.
Nov 2, 2013 6:20 AM ET

Edited Nov 2, 2013 6:43 AM ET.

Response to Dana Dorsett and Malcolm Taylor
by Martin Holladay

Dana and Malcolm,
Plenty of builders (not just Robert Riversong) have built homes without exterior OSB or plywood sheathing, either using board sheathing, skip sheathing, fiberboard sheathing, or one of the European membranes (a strong WRB) to retain the insulation. As long as you can keep the membrane from bellying too much when the cellulose is installed, and as long as you have a way to brace the walls (for example, with interior plywood), all of these approaches are worth considering.

The bellying problem can be real, however. I have visited a job site with fiberboard sheathing with serious bellying problems.


42.
Nov 2, 2013 6:28 AM ET

Response to Dan Kolbert
by Martin Holladay

Dan,
You asked, "What is it about OSB?"

Let's see. You take a perfectly good tree, with long fibers held together by lignin, and you chop it up into a pile of wafers. Then you say, "Hmm. I wonder if I can try to glue these wafers back together to make something strong enough to build a building out of?"

Engineers and scientists will eventually make a final ruling on whether OSB is durable enough to use as a building material. But farmers with old-fashioned Yankee skepticism just raise their eyebrows.

I built a garage this summer, using a method common in northern Vermont. My nearest neighbor cut down some spruce trees, all within 1/4 mile of the building site. A farmer in Sheffield with a bandsaw mill cut the trees into 2x4s, 2x6s, 2x10s, and boards. That approach makes more sense to me than trying to glue a pile of wafers together.


43.
Nov 2, 2013 6:32 AM ET

Response to Andrew Robinson
by Martin Holladay

Andrew,
Q. "Would these vapour-permeable sheathing panels that are apparently used in Europe solve these issues?"

A. To build a more resilient wall, we need to take as many steps as possible away from the edge of the cliff. Choosing vapor-premeable sheathing (as John Straube has advised) is certainly one way to take a step away from the cliff.

Don't forget the other recommendations as well: paying attention to airtightness and including a ventilated rainscreen gap.


44.
Nov 2, 2013 6:37 AM ET

Response to Floris Keverling Buisman
by Martin Holladay

Floris,
The approach advocated by your company -- interior sheathing panels that provide bracing and act as a vapor retarder, and vapor-permeable exterior sheathing or exterior WRB membranes -- is indeed less risky than a typical OSB-sheathed double-stud wall.

However, proponents of this typical European approach need to acknowledge that it is just one way to build walls. Installing insulation on the exterior side of the wall sheathing (rigid foam or mineral wool) also works well.


45.
Nov 2, 2013 6:41 AM ET

Response to Patrick Walshe
by Martin Holladay

Patrick,
Your wall sounds resilient. If you have plywood sheathing covered by Roxul mineral wool and a ventilated rainscreen gap, you're good to go. While it's always interesting to monitor the MC of sheathing, I'm guessing that your plywood is in the safe zone.


46.
Nov 2, 2013 10:12 AM ET

Stuff Happens
by John Brooks

Of Course "Stuff" happens with all assemblies ... even "Outsulated" walls.
Even with the "A-team" contractors and supervision by a well known Building Scientist.

All it takes is something like "reverse flashing".


47.
Nov 2, 2013 10:24 AM ET

Response to John Brooks
by Martin Holladay

John,
You're right. The only reason we are talking about resilience and the need to step back from the cliff is because "stuff happens."

It's possible to get flashing errors on a double-stud wall or on a foam-sheathed wall. So we want to start with a robust wall assembly -- one that isn't near the cliff -- so that the wall is able to handle "stuff" without catastrophic failure.

Whether you have a double-stud wall or a foam-sheathed wall, including a ventilated rainscreen gap goes a long way towards lessening the impact of flashing errors.


48.
Nov 2, 2013 11:02 AM ET

Regarding the "ping pong" effect
by Dick Russell

One first gets the impression that the studies are arguing against the double stud wall as being more risky, from the "cold sheathing" problem. Just as adding insulation outside of the sheathing puts the sheathing temperature closer to the interior temperature (warmer), adding instead some insulation inside the sheathing puts its temperature closer to the outside temperature. Comparing a single stud wall with the double stud wall, but with no exterior insulation in either case, the sheathing temperature already will be close to the outside temperature. Going from an R20 wall to R40, and assuming that the sheathing and siding provide R1, the double wall sheathing gets colder by perhaps a degree and a half, not a whopping drop.

It's already been noted that without artificially introduced "air leakage" from the inside into the wall cavity the sheathing moisture rise was substantially lower, as Trevor noted in his post #17. From Trevor's post#33: "Since cellulose is very hygroscopic it will never be dry - unless the atmosphere surrounding it is completely dry. The 'ping pong' effect results in a concentration of what ever moisture is in the cellulose at the sheathing side when it is cold out. The thicker the cellulose wall, the greater the effect."

It would seem, then, that the"problem" with the double stud wall might not be so much the thickness of the wall, vs a standard wall with the same fill, as it is the nature of the insulation. As I've argued before (in an earlier but similar thread on "cold sheathing"), much of the concern seems greatly lessened if there is a well-detailed interior air barrier. Perhaps also we could add to the list of details for good wall construction (rain screen, etc.) a substitution of a non-water absorbing cavity fill, such as blown fiberglass, for cellulose, to reduce or eliminate the "ping pong" effect, at least in climates where exterior humidity is of more concern.


49.
Nov 2, 2013 11:18 AM ET

Edited Nov 2, 2013 11:21 AM ET.

Breaking Down the Cold Sheathing Problem
by Ron Keagle

In thinking about the cold sheathing problem, I have developed the following observations and line of reasoning:

The cold sheathing problem is a result of too much insulation on the warm side of the sheathing. It results in the sheathing taking on a higher moisture content from any one of three different mechanisms:

1) If sheathing is chilled, it becomes thirstier, so it will take on additional moisture from the ambient air on either side of the sheathing.

2) If sheathing is chilled, it becomes thirstier, so if it is exposed to bulk water leakage, it will take on and hold a higher moisture content.

3) If sheathing is chilled below the dewpoint of air on either side of it, moisture in that air will condense on the sheathing and be drawn into the sheathing.

I would consider mechanism #1 to be the core of the problem because its only remedy is to prevent the sheathing from getting cold by insulating outside of it. Because of that requirement, only mechanism #1 fundamentally requires a change in double stud wall design. Basically, that change moves toward the obsolescence of double stud wall design because the more insulation placed outside of the sheathing, the less is needed inside. So, moving in this direction leads to seizing the advantage of reverting to a single stud system by moving sufficient insulation outside of the sheathing.

Mechanism #2 is not part of the core of the problem because it can be prevented by proper workmanship and maintenance.

Mechanism #3 is not part of the core of the problem because it can be prevented by proper air sealing.

So that leaves only mechanism #1 raising the question of whether it increases the moisture content of sheathing during the winter high enough to grow mold. If it raises the moisture content that high, then a conventional double stud wall is a flawed concept.

Therefore, it is important to isolate mechanism #1 and find out what its affect is.


50.
Nov 2, 2013 11:38 AM ET

Response to Ron Keagle
by Martin Holladay

Ron,
You concluded, "So that leaves only mechanism #1 raising the question of whether it increases the moisture content of sheathing during the winter high enough to grow mold. If it raises the moisture content that high, then a conventional double stud wall is a flawed concept. Therefore, it is important to isolate mechanism #1 and find out what its effect is."

That's exactly the topic of this article, and the subject of the monitoring studies I cite. All of the studies I cite measured moisture contents above 20%, and some above 30%. These walls were built with above average attention to airtightness, and represent real-world double-stud walls. Only Straube's study in Waterloo involves a deliberate air leak.

If you want to explain away the results by assuming the the walls measured by Andy Shapiro, Kohta Ueno, and Lois Arena weren't airtight enough, I think that conclusion shows an unrealistic expectation for airtightening that is unlikely to be achieved on most job sites.


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