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Open-Cell Spray Foam and Damp Roof Sheathing

When open-cell spray foam insulation is installed on the underside of OSB roof sheathing, the sheathing sometimes gets damp

Posted on Jan 3 2014 by Martin Holladay

UPDATED on July 8, 2015

Now that insulation contractors have been installing spray foam insulation on the underside of roof 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. for several years, we’re beginning to accumulate anecdotes and data on successful installations and failed installations. The anecdotes and data are enough to provide a few rules of thumb for designers and builders who want to install spray foam on the underside of roof sheathing.

Increasingly, building scientists are investigating why OSB roof sheathing on many spray-foam-insulated roofs stays damp for months at a time. Most of these damp-sheathing problems involve open-cell foam rather than closed-cell foam.

I’ve been reporting on wet-sheathing problems arising from the use of open-cell spray foam since 2005, when I wrote two articles on the topic for Energy Design Update (“Vapor Retarders and Icynene,” April 2005, and “Every Failure Holds a Lesson,” July 2005). As originally understood, the problem with open-cell foam was that it is vapor-permeable, and therefore allows moisture in the interior air to diffuse through the insulation and reach the cold roof sheathing during the winter.

Five years later, Mark Parlee, an Iowa builder, wrote a seminal article on an Icynene-insulated roof with rotten roof sheathing. His article, “Repairing a Rotting Roof,” was published in the June 2010 issue of the Journal of Light Construction. One of the factors that contributed to the failure described by Parlee was high indoor humidity.

Open-cell foam under roof sheathing can be risky

At a recent building science conference in Florida (Conference on Thermal Performance of the Exterior Envelopes of Whole Buildings XII, December 1-4, 2013), two academic papers were presented that shed light on questions surrounding the moisture content of roof sheathing that has been insulated on the underside with spray polyurethane foam.

One paper discussed a field study that found that even in a relatively warm climate (South Carolina), roof sheathing can accumulate moisture when open-cell spray foam is installed on the underside of the sheathing. Researchers speculate that exterior moisture (dew or rain) between the roof shingles is forced into the OSB roof sheathing by inward solar vapor drive.

The other paper reported on a computer modeling study that showed that when spray foam is installed on the underside of roof sheathing, open-cell foam is riskier than closed-cell foam in all U.S. climate zones.

Determining the best specs for an energy-efficient attic

The first paper, “Roof and Attic Design Guidelines for New and Retrofit Construction of Homes in Hot and Cold Climates,” was authored by William Miller, Andre Desjarlais, and Marc LaFrance. William Miller presented the paper.

The three researchers ran computer simulations to determine the best specifications for attics. Among the variables considered by the researchers were:

  • Adding insulation to the attic floor
  • Adding insulation to the roof assembly
  • Adding ventilation channels above the roof sheathing
  • Adding a radiant barrier
  • Specifying cool-color roofing
  • Air sealing the attic floor
  • Sealing duct seams
  • Transforming a vented attic into a sealed attic.

The researchers acknowledged that it’s hard to come up with recommendations that apply to every situation. They wrote, “The best retrofit option for your attic depends on climate, attic geometry, duct arrangement, amount of ceiling insulation, air leakage, and thermostat setting used to comfort condition the home. If the ceiling currently has less than code insulation, savings will be greater and payback period will be shorter.”

The researchers’ modeling exercise was based on HERS BESTEST and AtticSim software. They modeled a 1,550-square-foot house with an asphalt-shingle roof with an 18° slope. The researchers assumed that the attic included supply and return ducts. They considered four different cases for duct leakage: 4%, 10%, and 20% of supply airflow.

Thick insulation on the attic floor won't help if your attic includes leaky ducts

One of the researchers’ conclusions: When ducts are located in a vented, unconditioned attic, it makes little sense to install a deep layer of insulation on the attic floor. The duct leakage wastes so much energy that adding deep insulation doesn’t help much. (For a graphic representation of this principle, see Image #2 below.)

The authors wrote, “There are diminishing returns for adding floor insulation above about R-19 because losses from the ducts predominate. Therefore, if all one does is put more insulation down and ignore the HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. system, then hundreds of dollars’ worth of energy is still being lost from the HVAC in the attic. Frankly, adding only insulation still leaves a poorly performing home that has higher-than-necessary utility costs month after month.”

The researchers concluded, “The most important retrofit option, whether the duct system is in the attic or not, is to seal the attic floor.” They also advise, “For new construction the best option is to keep the ducts out of the attic, make sure the attic floor is sealed to limit whole-house air leakage, and add at least the code level of insulation to the ceiling.”

It rarely makes sense to consider retrofitting a radiant barrier. The authors wrote, “Simulations for hot climates indicate that a radiant barrier can pay for itself in 20-25 years. Without leaky ducts in the attic, the payback increases to about 38-50 years. In cold climates the radiant barrier is not an effective measure.”

Damp roof sheathing in South Carolina

What about creating a sealed, conditioned attic? The authors wrote, “Sealing the roof deck and gables of an attic with spray polyurethane foam insulation has gained popularity among many builders and code officials, especially in hot and very humid climates. …. The amount of applied foam should at least match the code requirements for the attic floor. Otherwise the system may actually increase heat flows into the conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. , especially if there are no ducts in the attic. In fact, the procedure should not be considered unless there is a leaky duct in the attic.”

During his presentation, Miller warned against the use of open-cell spray foam to create a conditioned attic. “The roof sheathing is humid when open-cell spray foam is used,” he said.

The authors reported on a field study that raise concerns about the use of open-cell spray foam to create a sealed, conditioned attic. They wrote, “Recent data collected from a test facility in Charleston, SC (Miller et al. 2013) revealed very interesting summertime trends in attic humidity for a sealed attic as compared to the conventionally ventilated attics. … [In a sealed attic insulated with open-cell spray foam,] peaks in measured relative humidity from two different sensors in the sealed attic showed values in excess of 80%-90%, and occasionally saturated air (i.e., 100%) occurred around solar noon. In other words, the moisture content in the sealed attic was consistently 80%-100% RH from solar noon to around 6 p.m. for the seven contiguous days.”

The authors continued, “Further, the trend was observed throughout the hot summer months. … Field measurements imply that some of the moisture from a previous rainstorm migrates to the underside of the shingles and underlayment. Irradiance drives moisture from an earlier rainstorm into and through the OSB deck and open-cell foam. As a result, the partial pressure of the attic air and the partial pressure of water at the foam-to-attic air interface occasionally exceed the saturation pressure of water vapor in the attic air.”

In a recent e-mail, William Miller elaborated on the probable mechanisms involved in this phenomenon. He wrote, “We still are working to determine the mode by which moisture crosses the envelope... I believe it occurs in two different paths:
1. The sealed attic is not airtight and outdoor moisture penetrates. There is no ventilation and night-sky radiation causes the trapped humid air in the attic to diffuse into the spray foam.
2. The roof deck becomes wet during the evenings because of night-sky radiation and subsequently the condensate is forced into the roof deck by irradiance the next day.
Both modes result in moisture storage in the spray foam, and the stored water begins to damage the wood deck.”

Continuing their account of the findings from South Carolina, the researchers wrote, “On these particularly hot days, relative humidity measurements showed super saturation (i.e., partial pressure of water vapor exceeded saturated pressure). At this condition, water vapor is in equilibrium with liquid water and therefore all interior attic surfaces are wet! Without a leaky duct, the sealed attic approach actually exacerbates humidity control in attics exposed in hot, humid climates. However, the presence of leaky ducts does not necessarily provide adequate protection from attic moisture. Colon (2011) field tested open-cell spray foam in a home in the hot muggy climate of south Florida. The roof deck was protected by an impermeable underlayment and the air-handler unit and ductwork were contained in the attic, yet moisture levels in the conditioned space increased above that measured a priori sealing the attic.”

Sealed attics that include ductwork are hard to model

The second paper, “A Hygrothermal Risk Analysis Applied to Residential Unvented Attics,” was authored by Simon Pallin, Manfred Kehrer, and William Miller. Pallin presented the paper.

The researchers’ aim was to develop a computer model for sealed, conditioned attics which contain forced-air ductwork. This type of attic is complicated to model, because the temperature and humidity in such a space are affected by so many factors. (See Image #3, below.)

The authors list the following factors:

  • Indoor heat and moisture production
  • Hygrothermal material properties
  • Natural and forced unintended air leakage
  • Features of the HVAC system, i.e., dehumidifying/humidifying effect, airflow rate, etc.
  • Geometrical variations of the building components
  • Outdoor climate
  • Orientation and location of the building and slope of the roof
  • Workmanship
  • User behavior, i.e., HVAC setpoint temperatures, airing, maintenance, etc.

As the authors note, “An unvented attic hosting an HVAC system must be considered as a very complex hygrothermal system.” They decided to use two existing computer models, MATLAB and WUFI, for their study. MATLAB (a software program for mathematical computations) was used to create a numerical model, and MATLAB data were used as inputs for WUFI.

The simulated attic had OSB roof sheathing and asphalt shingles. The OSB was insulated on the underside with a layer of spray foam; both open-cell foam and closed-cell foam were modeled. The attic was modeled for seven cities chosen to represent U.S. climate zones 1 through 7: Miami, Florida; Austin, Texas; Atlanta, Georgia; Baltimore, Maryland; Chicago, Illinois; Minneapolis, Minnesota; and Fargo, North Dakota.

The R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the roof foam was assumed to match the minimum prescriptive requirements of the 2011 IECC International Energy Conservation Code.; depending on the climate zone, this R-value ranged from R-30 to R-49.

In addition to the variables already mentioned, the researchers changed several other parameters to determine their effect on conditions in the attic. These parameters included:

  • The thermostat setpoint
  • The air leakage rate from supply and return ducts (ranging from a low of 4% leakage to a high of 20% leakage)
  • The airtightness of the attic floor (ranging from a low of 0.4 cfm50/CFA to a high of 2.0 cfm50/CFA)
  • Indoor moisture production.

In all, the researchers ran 224 software iterations. For each iteration, they looked at the moisture content (MC) of the OSB roof sheathing as well as the amount of energy required for space heating and cooling. (See Image #4, below.)

For each climate zone, they identified the best-performing combination of parameters — in other words, the parameters that resulted in dry OSB — and the worst-performing combination of parameters — in other words, the parameters that resulted in damp OSB.

“Open-cell foam is risky in all climate zones”

The authors wrote, “Every best-performing unvented attic roof is constructed with a closed[-cell] SPF and with a low indoor moisture supply. The opposite is true for the most risky roof. In all cases, except for climate zone 4, a high duct leakage has a positive effect on the MC of the OSB, most likely due to the dehumidifying effect of the HVAC cooling coils, which, by a higher rate of air leakage, will have a higher influence on the vapor content of the attic air during the operating cooling mode.”

As an example, the worst-performing OSB in climate zone 4 is the OSB on a north-facing roof insulated with open-cell foam, over an attic with a duct system that is fairly well sealed (4% duct leakage), an attic floor which is fairly leaky (2.0 cfm50/CFA), and a house with high indoor moisture production. After the software was run for one simulated year, the net result of these factors was OSB with a moisture content of 54%.

At his presentation in Florida, Simon Pallin summed up the researchers’ findings this way: “Open-cell foam is risky in all climate zones.”

The authors wrote, “The MC in the OSB sheathing varies mostly due to whether the spray polyurethane foam (SPF) is vapor closed or open. Having an open SPF is actually a risk in all the investigated U.S. climate zones, 1 to 7, depending on the chosen values of the other varying parameters. Naturally, the outdoor climate will influence the MC of the OSB, but also the indoor moisture production rate has a significant impact. … A high air leakage rate from the air distribution duct system has a positive impact on the MC of the OSB sheathings due to the dehumidifying effect of the HVAC unit, though it has a negative influence in terms of the HVAC energy demand.”

It's not inward solar vapor drive

When interpreting the results of computer modeling studies, it’s important to remember that modeling results sometimes differ from field study results. That said, the work of the researchers quoted in this article consistently shows that the use of open-cell spray foam to create a sealed, conditioned attic is riskier than the use of closed-cell spray foam, in all climate zones.

Paradoxically, leaky ductwork helps protect OSB from getting too damp. If a builder chooses to install leaky ductwork in a sealed attic, the roof sheathing will probably be dryer than it would be if the ducts were well sealed.

Here at, we have been receiving an increasing number of questions from homeowners who are complaining that their unvented conditioned attics (most of which were created by installing open-cell spray foam on the underside of the roof sheathing) have problems with high indoor humidity. Building scientist Joe Lstiburek has investigated a number of attics with similar problems. “How come the moisture ends up in the attic space and not in the main part of the building?” Lstiburek wrote. “Moisture-laden air is lighter and less dense than dry air. Moisture-laden air ends up in the attic due to this ‘hygric buoyancy.’” [Editor's note: For another perspective on Lstiburek's "hygric bouyancy" theory, see the Comment #90 by Bill Rose, below.]

Although Lstibuerk used to think that the moisture in these attics was morning dew that was being driven through asphalt shingles by inward solar vapor drive, he now reports that the inward-solar-vapor-drive theory has been disproved.

If you have an unvented conditioned attic with high humidity problems, the best solution is to install a supply-air register connected to your forced-air HVAC system in the attic (and in some cases, a return air grille as well). If the attic is heated in the winter and air conditioned during the summer, humidity problems should disappear.

Rules of thumb for builders

Here are my recommendations for builders who use spray foam to create a sealed, conditioned attic:

  • Recognize that this type of roof assembly carries more risk than a vented, unconditioned attic. Keeping ducts within the conditioned space of a building (not in the attic) is preferable to installing spray foam on the underside of roof sheathing.
  • If you want to lower the risk of damp OSB, choose closed-cell spray foam, not open-cell spray foam, to insulate the underside of the roof sheathing.
  • To further limit your risk, consider installing ventilation channels above or directly below your roof sheathing.
  • If you choose to install open-cell spray foam against the underside of roof sheathing in Climate Zone 5 or colder zones, make sure that you include an interior vapor retarder.
  • If you choose to install open-cell spray foam against the underside of roof sheathing in a humid climate, your HVAC system be designed to condition the attic air and lower humidity levels in the attic.

Problems with “breathable” assemblies

Some green builders are convinced that wall and roof assemblies have to “breathe.” While “breathability” has no technical definition, many builders assume that “breathable” means “vapor-permeable.”

The argument in favor of breathable assemblies is undermined by the increasing evidence that vapor-impermeable spray foam usually performs better than breathable spray foam. A major explanation for the damp and rotting roof sheathing described in this article is the fact that the designer or builder specified “breathable” foam instead of vapor-impermeable foam.

Martin Holladay’s previous blog: “Stupid Energy-Saving Tips.”

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Tags: , , , , , , , ,

Image Credits:

  1. Mark Parlee
  2. William Miller, Andre Desjarlais, and Marc LaFrance
  3. Simon Pallin, Manfred Kehrer, and William Miller

Jan 3, 2014 2:26 PM ET

Could a coat of primer...
by Chase Counts

...on the sheathing prior to the installation of open-cell spray foam of the roof deck reduce the risk of saturating the OSB? We have enclosed several attics over the past two years with open-cell in our mixed climate and have not observed these issues...yet, at least.

Closed cell sounds like it performs much better in practice but the cost is significantly higher and diminishes ROI but of course we don't want to jeopardize the lifespan of construction materials either. Has anyone else experienced this in a mixed climate or tried another method?

Jan 3, 2014 2:42 PM ET

Response to Chase Counts
by Martin Holladay

Frankly, I'm skeptical of whether a coat of primer would make much difference. The best way to answer the question, of course, would be to experiment with primer in two adjacent test huts.

Q. "Has anyone else experienced this in a mixed climate?"

A. We have reliable reports of saturated roof sheathing adjacent to open-cell spray foam from Montana, Iowa, Vermont, and Massachusetts. My article reports on evidence of damp sheathing in a building in South Carolina.

Jan 3, 2014 7:13 PM ET

by Richard Beyer

Martin did you or a third party inspector inspect the spray foam to determine if it was installed in accordance with the manufacturer's written standard? I'm betting industry will defend this issue and blame it on the installation of the spray foam insulation or the roofing contractor.

Jan 4, 2014 1:19 AM ET

All climates?
by Mark Johnson

This warning against open cell foam in all climates, sure seems to clash with what Joe Lstiburek has promoted for a decade. Do I understand correctly?

There have been some trials in very dry climates, such as Phoenix AZ. Is it a mystery how this foam design can be risky in such a dry climate? It is to me, and would be really nice if someone could explain in elementary detail how it would work in a hot-dry climate.

I do know about 15 years ago, there was a series of house experimentally built in the Cinco Ranch neighborhood of Katy TX, just west of Houston. It was part of the Building America program, and Pulte was the builder. Definitely in the hot-humid climate category. Apart from a paper or two, the results have been mysterious and little publicized. Some other builders who might be in a position to know, hint darkly about roof decking rotting, and lawsuits resulting -- I get the impression news is being withheld on this matter. If anyone can shed any light on lessons learned here, please share what you know!

Jan 4, 2014 7:42 AM ET

Response to Richard Beyer
by Martin Holladay

The bulk of this article reports on two scientific papers, each written by a team of three researchers. The papers were presented at a conference I attended in Florida.

One paper referred to moisture accumulation in the roof sheathing of a test facility in South Carolina; I don't know if the spray foam in that building was checked by "a third party inspector," but it's reasonable to assume that the researchers at the test facility did their best to assure that the spray foam was well installed.

The other paper reported on a computer modeling study. The computer model assumed, as far as I know, that the foam was installed well.

The phenomenon of vapor diffusion through open-cell foam (even well-installed open-cell foam) in cold climates has been well documented. Problems generally don't occur unless there is high indoor relative humidity; however, such houses exist.

The modeling study listed many factors that affect the moisture content of the roof sheathing on this type of roof. As I reported, here are the factors that raise the risk: north roofs are riskier than south roofs; attics with tight ductwork are riskier than attics with leaky ductwork; attics with leaky ceilings are riskier than attics with tight ceilings; and homes with high indoor RH are riskier than homes with low indoor RH.

In all climate zones, open-cell foam was riskier than closed-cell foam.

Jan 4, 2014 7:50 AM ET

Edited Jan 4, 2014 7:56 AM ET.

Response to Mark Johnson
by Martin Holladay

Q. "This warning against open-cell foam in all climates, sure seems to clash with what Joe Lstiburek has promoted for a decade. Do I understand correctly?"

A. Yes.

Concerning your question about "mysterious and little publicized" failures in Texas, I'm not sure what you are referring to. I know that the Building Science Corporation helped a builder solve high indoor humidity problems in some Building America homes in Texas by installing stand-alone dehumidifiers (in addition to the split system air conditioners).

I also know that there have been clusters of problems with wet roof sheathing in low-slope roofs in the desert Southwest. The main cause of these failures was night sky radiational cooling that make the roof sheathing colder than the outdoor air temperature. The source of the humidity that rotted those roofs was interior moisture. These roofs were insulated with fiberglass batts -- once again, a "breathable" insulation that contributed to the failure.

Jan 4, 2014 8:40 AM ET

smart vapor retarder can help
by Ken Levenson

An inboard smart vapor retarder and airbarrier membrane like INTELLO or DB+ can help. As the roofing is vapor impermeable and insulation is vapor permeable (breathable), a control layer inboard with the property of these membranes, providing a vapor retarder in winter to prevent wetting but be vapor open in summer to allow drying inward, while also providing airtightness, can make them robust assemblies. I might suggest Alex Wilson's GBA blog post regarding them:
And full disclosure, my company, 475 High Performance Building Supply, provides these materials to the US market.

Jan 4, 2014 10:39 AM ET

Question for Martin
by Richard Beyer

Martin, Thank you for your response. I think you just spelled out what the manufacturer's claim of fault will be. "It's the building owner and/or designer's problem", not the product, when thing's go south as this article illustrates. I'm currently working on a multi-million dollar home situated on a major waterway in Connecticut where open cell spray foam is installed in the walls/roof and closed cell was installed in the floor above the crawl space. Note:The floor is above a conditioned crawl space with flood gates. Here's the kicker... the room is @ 12 feet wide X @ 22 feet long with a custom spa / hot tub which holds 1400 gallons of heated water which is Ozone treated. The room does not have windows which open but does have a door to a patio and two interior doorways @ 60 inches wide with double doors. The roof assembly is cedar shake. sheathing, 2" air space, cleats on the sides of the rafters and luon attached to the cleats. Open cell is attached to the luon. What's your opinion? Do you think this home owner may have future trouble?

Jan 4, 2014 11:33 AM ET

Regarding the South Carolina testing...
by John Semmelhack

Martin, regarding the South Carolina test facility, you reference William Miller as believing that the moisture accumulation was perhaps due to summertime air leakage depositing moisture inside the attic and then subsequent night-time vapor diffusion from inside to outside. If it was an air-tightness problem, then this would seem to be a problem with the application….not the spray foam itself. Did they not test/inspect the air-tightness of the attic?

Regarding Miller's other hypothesis of post-rain, solar-driven moisture accumulation in the South Carolina study - IF this were the main driving force, wouldn't closed cell foam be even worse, since the moisture would be driven into the OSB, but stopped (more or less) from drying to the inside?

Jan 4, 2014 2:13 PM ET

Edited Jan 4, 2014 2:24 PM ET.

The Cinco Ranch Houston project
by Mark Johnson

I am attempting to attach a copy of a 2003 BSC paper titled "Residential Dehumidification Systems Research for Hot- Humid Climates". Authors are A.F. Rudd, J.W. Lstiburek, P. Eng, and K. Ueno. This is exactly the project I was wanting to describe. It documents trying to help a builder (Pulte) with dehumidifiers and several other methods. When they refer to the 3 homes with cathedralized attic insulation, those very same researchers designed the original construction, so they were addressing their own problems. To be fair any home in this climate could easily have high indoor humidity, often due to high infiltration and feeble air conditioner setup.

Of course they do not document their own built homes had moisture problems, I think they may have had a clue at the time of this paper. But I don't know with any certainty, it is sad when hearsay is the best news source one has.

It seems to me generic good advice is not to be an early adopter, to wait until a new material or technique has proven itself a decade in the field before embracing it. Everybody tells me ducts in conditioned space is the only intelligent way to go, but evident to me is there are multiple ways to do that and only some ways don't turn around and bite you.

Locally I have been hearing that exposed foam in the attic could be considered a fire hazard unless kept behind another layer of sheet rock. Allegedly this has the status of code violation (not forgetting code enforcement is very lax in TX). If this bears out, the added cost may tip the economic balance away from this concept. In its place I have heard emphasis on duct sealing and ceiling-to-attic air tightness, while tolerating HVAC equipment in an unconditioned attic. Of course ducts and even equipment could be brought inside, at a cost.

Interesting topic. Hope this helps.

Jan 4, 2014 5:46 PM ET

Response to Mark Johnson
by Malcolm Taylor

It appears that being an early adopter of both double wall or foam insulation techniques proffered by many as being solutions to increasing building envelope efficiency would have left you with sheathing moisture problems. Those who stood on the sidelines and watched things shake out may have the last laugh.

Jan 4, 2014 9:17 PM ET

Edited Jan 4, 2014 9:18 PM ET.

OSB is not a quality material. So don't expect it to be!!!
by aj builder, Upstate NY Zone 6a

OSB is low cost low quality material. It rots very easily. It gets too wet very easily. Customers today use a home with out thinking for one second about such things as moisture. I have customers telling me pasta can only be cooked without a lid. I have customers running humidifiers 24/7 in every room. I have customers with fifty plants along a wall of windows in a room just big enough for the plants. I have customers that don't go in their cellar ever that never knew their sump system stopped working a few years back.

I also have homes that have open cell with not even close to a problem in the sheathing. Just an average home with less than out of control inhabitants, no huge plant set up, no humidifiers gone wild..... well... you get the idea.

Careless house use and crappy materials.... are the problem.... I will conduct the study for a dollar. Save your money.

Build a crappy house... live in it out of control... let's call the lawyers and start some million dollar studies. That's the 21st century selfie pic.


Jan 5, 2014 7:11 AM ET

Response to Ken Levenson (Comment #7)
by Martin Holladay

You are right: an interior vapor retarder is recommended when open-cell spray foam is installed on the underside of roof sheathing in cold climates, and GBA has consistently recommended the use of interior vapor retarders in this type of application for any builder who chooses open-cell spray foam.

Nevertheless, it's still true that open-cell spray foam is riskier than closed-cell spray foam in all climate zones.

Jan 5, 2014 7:17 AM ET

Edited Jan 5, 2014 7:19 AM ET.

Response to Richard Beyer (Comment #8)
by Martin Holladay

It certainly sounds as if the builder or designer of the project you describe is taking on a lot of risk.

It makes no sense to insulate the floor above the crawl space; it would have been better to insulate the crawl space walls (and to seal the crawl space vents).

Using open-cell spray foam for the roof assembly above a spa / hot tub room is extremely risky. It sounds like the builder has included a ventilation channel above the open-cell spray foam, which helps a little, but depending on the vent channel to keep the roof out of trouble is risky.

The best way to insulate this type of wet room is to put all of the insulation above the roof sheathing. This is usually done by following the PERSIST method: covering the top of the roof sheathing with peel-and-stick (rubberized asphalt), followed by several layers of rigid foam with staggered seams.

Jan 5, 2014 7:26 AM ET

Response to John Semmelhack (Comment #9)
by Martin Holladay

Q. "Regarding the South Carolina test facility: ... If it was an airtightness problem, then this would seem to be a problem with the application, not the spray foam itself. Did they not test/inspect the airtightness of the attic?"

A. Good question; I don't know. However, it's fair to ask another question: if sealing an attic with open-cell spray foam can leave enough air leakage paths to cause this type of problem (and I know that the mechanism is, at this point, hypothetical), and if the situation was unnoticed by researchers until problems developed, then couldn't the same situation happen at hundreds or thousands of other homes?

Q. "Regarding Miller's other hypothesis of post-rain, solar-driven moisture accumulation in the South Carolina study - IF this were the main driving force, wouldn't closed cell foam be even worse, since the moisture would be driven into the OSB, but stopped (more or less) from drying to the inside?"

A. Yes. The other researcher who has looked into this phenomenon is Armin Rudd, who sometimes calls this "ping-pong" moisture (because it comes and goes as the sun comes out). Rudd blamed inward solar vapor drive for the phenomenon a decade ago, and then began to have his doubts about what was going on when some data didn't fit the theory. At that point he announced, "It would be great to have a million dollars for a study to figure out what is going on in these roofs," but the money didn't materialize. The last time I spoke to Rudd about the issue, he said that it was still a mystery.

Jan 5, 2014 7:40 AM ET

Edited Jan 5, 2014 7:43 AM ET.

Response to Mark Johnson (Comment #10)
by Martin Holladay

I am very familiar with the study you refer to -- "Residential Dehumidification Systems Research for Hot- Humid Climates" -- because I wrote an article on the study for the January 2003 issue of Energy Design Update. My article was called, "Houston Study Looks At Dehumidification Options."

Frankly, I'm not sure what the relevance of that study is to the issue discussed here. The researchers note that it's important to control interior humidity in a tight, well-insulated home with high-performance windows. The researchers were surprised to find high indoor humidity at some Building America homes they were involved with in Houston. They developed several solutions to the problem and recommended implementing the best performing option, which was also the least expensive (installing a stand-alone dehumidifier).

What's the point? Interior humidity levels can sometimes be surprisingly high. That fact should only make installers of open-cell spray foam more nervous, not less nervous.

You are certainly right that spray foam needs to be covered with an intumescent coating or, even better, with drywall to reduce the fire hazard. You are also correct that many building inspectors have been looking the other way rather than requiring compliance with these requirements.

Jan 5, 2014 7:54 AM ET

Response to Malcolm Taylor (Comment #11)
by Martin Holladay

On the "early adopter" problem: it's certainly true that building scientists learn more every year, and are able to refine their recommendations based on better knowledge. It's also true that prominent building experts have made recommendations that needed to be revised once research refined our understanding.

But we don't really have the luxury of building poorly insulated, leaky houses as we wait for more research results. Fortunately, most homes with double-stud walls and most homes with spray-foam-insulated roofs are performing quite well. The ones with problems are usually the ones with high indoor humidity -- a factor that builders can't really control. So there is reason for concern, but not panic.

Jan 5, 2014 8:02 AM ET

Response to A.J. Builder (Comment #12)
by Martin Holladay

You are right that plywood stands up to moisture better than OSB. But if researchers are finding very high moisture content readings in roof sheathing -- and they are, in some cases -- then those high levels will be enough to rot plywood eventually. I don't think that switching from OSB to plywood is the entire answer to this issue.

You're a builder. You say that some of your clients keep their indoor humidity very low, and some keep their indoor humidity very high. Does that fact worry you? Do you ever have sleepless nights worrying about your clients with humidifiers and 50 house plants?

Jan 5, 2014 1:34 PM ET

Response to Martin,
by Malcolm Taylor

The way building scientists or proponents of new technologies report their recommendations seem to share certain characteristics with the way medical discoveries or health advice are made public - that is as facts as opposed to working hypothesis. At any given point they seem sure they know the whole picture with all the attendant consequences, and this almost always turns out to be untrue. I'm just suggesting a bit less enthusiasm and a bit more humility might save us all some future grief.

Jan 6, 2014 11:20 AM ET

Edited Jan 6, 2014 11:21 AM ET.

Response to Malcolm Taylor
by Martin Holladay

As I'm sure you know, one of the strengths of the scientific method is that our body of knowledge is always being refined in light of new data. This Renaissance approach was disturbing to the medieval mindset, which assumed that knowledge was fixed and unchanging.

Ideally, American students would get a good grounding in the scientific method in the 7th and 8th grades, so that citizens would have a firm basis for understanding reports of scientific findings. Sadly, that grounding is lacking.

Building science is a relatively new branch of physics, and we learn more every year. As a journalist, I don't think it would be responsible to withhold data or research results just because I know that current recommendations may be overthrown in 10 years. Readers of this website deserve to know the most recent data available. I have to trust our readers to understand that, as in any scientific field, new data may trump current understandings in the future.

Jan 6, 2014 11:40 AM ET

Response to Martin Comment #5
by Jason Hoerter

In my opinion, some of the inputs that raise the risk of high moisture content are dependent on one another so they do not hold up independently. Specifically, you say attics with leaky ceilings are riskier than attics with tight ceilings. Building science suggest that when you create an unvented attic, you must remove the vapor retarder and insulation from the attic floor to ensure there is plenty of air and humidity exchange between the living space and the attic. Assuming you keep your home at a reasonable humidity level then the humidity in the attic is controlled. Now, if you have high humidity levels in your home, then the humidity in your attic will also be high. In this case, possibly having a tight ceiling is better, but not when the humidity levels in your home are controlled. I would like to say that this research further emphasizes the importance of removing the insulation and vapor retarder from the floor when creating an unvented attic, a step that is sometimes ignored due to added labor. Your comment saying a leaky ceiling increase the risk suggests keeping the insulation and vapor retarder on the floor is a benefit.

Jan 7, 2014 10:40 AM ET

Respose to Jason Hoerter
by Martin Holladay

You are right that the various factors that affect the moisture content of OSB above a sealed attic are interrelated. The researchers I cite are aware of these interrelations, and WUFI software attempts to take these interrelations into account.

The major mechanism for moisture transfer from the conditioned space below an attic into the attic above is air leakage. Vapor diffusion plays a relatively small role -- something to keep in mind when considering whether the presence or absence of a vapor retarder on the attic floor matters.

Jan 7, 2014 4:07 PM ET

Yes Martin as to risky business thoughts
by aj builder, Upstate NY Zone 6a

Anyone's risks and most risks cross my thoughts daily. That is me. Calculated educated risk taking is something of a passion and part of my work and play. Whether flying a hang glider off a thousand foot cliff edge, surfing with sharks somewhere near or far, or trying to talk a client into adding antifreeze to a hydronic setup so that there is no surprise to find out setting back a tstat too far or running a woodstove so much that a back room freezes... floods...

Anyway... we all in the business... know water water water... from where to there... because of (you name it).. creates lots of economic activity.

Jan 7, 2014 4:17 PM ET

Share what you know
by aj builder, Upstate NY Zone 6a

As to the problems homeowners cause... or I cause... that is a main reason why many of us come to sites like GBA. Very rewarding to actually solve water problems... insulation problems... and or relay some thoughts as what someone might be doing that maybe they should not do... like be the cause of too much water in a structure doing damage. Ultimately how a home is managed and maintained is out of my hands.

Some people just need to boil a few gallons of water daily. After I share my thoughts they are free to boil gallons or not... reap and they shall sow... someone jump in and explain

I sleep like a rock by the way.

Jan 8, 2014 9:23 AM ET

Last paragraph
by Dan Kolbert

I'm not sure I draw such a broad conclusion from your story, Martin. I've always been nervous about open cell SF for the reasons it seems to be causing trouble - it seems to accept moisture faster than it distributes or dissipates it. I'm not sure it's reasonable to tar cellulose or fiberglass with the same brush.

Jan 8, 2014 10:09 AM ET

Response to Dan Kolbert
by Martin Holladay

There is nothing in my last paragraph to indicate that cellulose and fiberglass behave the same way as open-cell spray foam. Every material has different properties.

I think it's fair to say that if a builder tried to install fiberglass batts or blown-in fiberglass directly against the underside of roof sheathing -- without any rigid foam on top of the roof sheathing -- then the results would be even worse (more risky) than is the case with open-cell spray foam.

So, if we want to discuss tarring with the same brush, I'll go right ahead and tar fiberglass with the same brush. Here's my warning: don't install fiberglass insulation directly against the underside of roof sheathing unless you have installed a significant layer of rigid foam above the roof sheathing.

The situation with cellulose is a little more complicated, and I address the ins-and-outs of the debate in my article, How to Install Cellulose Insulation.

The main point I was trying to make in my concluding sentences is that "breathability" is not an unalloyed virtue. Whenever you are designing a floor assembly, wall assembly, or roof assembly, you have to use your brain. Advocates of "breathable" assemblies often fail to use their brains. Sometimes it's good to choose materials that are vapor-permeable; other times, you definitely want to choose materials that are vapor-impermable.

So my message is: when choosing materials, you have to think.

Jan 8, 2014 11:48 AM ET

Peer review should have addressed the limitations of the papers
by Joe Lstiburek

The studies mentioned from Clearwater are misleading. The Des Moines and cold climate failures are a separate issue.

Open-cell foam does not work without a vapor retarder in Climates Zones 5 and higher. That is why the code prohibits this. This is because the vapor openness of the open-cell spray foam is overwhelmed by high interior levels of moisture. The fact that many installers and manufacturers promote this and that the code is not enforced is troubling but it is not a building science issue. Des Moines should not have happened because open-cell spray foam should not have been used. People "get away" with open-cell foam in cold climates if the interior moisture levels are low. "Getting away" with things is not a sound business or building science strategy.

The Clearwater papers are more troubling because peer review should have addressed the limitations of the work.

Unvented attics like unvented crawlspaces are unfortunate terms but for reasons of history and popular usage we are stuck with them. The correct terms should be conditioned attics and conditioned crawlspaces. The misunderstanding arising from this terminology confusion is painfully obvious in the Clearwater work.

Both conditioned attics and conditioned crawlspaces must have a mechanism of moisture removal just as conditioned houses must have a mechanism of moisture removal. In houses we do this with either dilution ventilation or mechanical means - either an air conditioner or a dehumidifier depending on load. During the winter dilution ventilation is used. During the summer in humid climates the air conditioner does the job - except in ultra-low-load buildings where supplemental dehumidification is necessary.

Conditioned attics must be coupled to the conditioned part of the house in humid climates in order to control the moisture levels in the attic. This is particularly necessary with open-cell foam due to its moisture storage characteristics.

The moisture in the unvented attic with open-cell foam studied by Oak Ridge was interior moisture. It did not come through the shingles and roofing paper and roof sheathing through solar driven moisture. We showed this in our Houston work a decade ago. We told them this. This should have been picked up by peer review.

There was no mechanism for moisture removal in this attic - a simple approach is the use of a supply air duct coupled with a return path. This was not done. In many cases it has proven to be unnecessary when supply ducts are leaky and ceiling planes are leaky - typical construction - the air change provided between the attic space and the house with leaky ducts and a leaky ceiling serves to remove the moisture in the attic. Where supply ducts are tight and where the ceiling plane is tight this mechanism does not work and an active supply duct and deliberate return path is necessary. This should not be news to experienced researchers. It is pretty well understood by many installers using spray foam. But it is apparently not well understood by everyone and clearly a code change would be helpful. No air change measurements via tracer gas were done to provide information about the lack of communication between the attic and the house. Modeling was used by individuals with little or no experience with this type of construction. The modeling was reviewed by people with even less. The comments on moisture sources and energy impacts was also misleading.

Unvented attics do not generate moisture (note the change in term). Moisture is not passing through the attic roof deck into the attic. Unvented attics do not add an additional moisture load to the house. There is no difference in the moisture load in a house with a vented attic and an unvented attic. The moisture load in houses with vented attics and unvented attics is clearly the same in houses with equal airtightness and equal foundation construction and equal interior occupancy and therefore interior moisture loads. There is therefore no difference in energy load to control this moisture. So what is going on? Unvented attics provide a complication that can be a modeling and measurement dog's breakfast.

An unvented attic does not affect the amount of moisture in a house except in an indirect way. Unvented attics often lead to lower air change in building enclosures because they typically result in tighter construction. During the winter that means that an increase in dilution ventilation is required via mechanical means to bring the air change up to the air change equal to that of a more leaky house. During the summer the unvented attic leads to a reduced air change which reduces the quantity of exterior moisture entering the building and therefore it typically reduces the interior moisture load - which is a good thing. But the unvented attic also often reduces sensible heat gain because ducts are within the conditioned space and this can lead to a part load humidity issue from oversized air conditioning systems. In any event in buildings with equal air change an unvented attic does not increase interior moisture loads but can lead to an increase in interior relative humidity if the air conditioning system does not remove this moisture. This is therefore a sizing and equipment issue not a vented unvented attic issue.

The ping-pong effect has been mentioned and discussed by myself for over a decade. It is of no significance if a moisture removal mechanism is in place. It is not an issue with closed cell foam for obvious reasons - closed cell foam does not store moisture. Moisture laden air is less dense and more buoyant than dry air. Heated air is more buoyant than unheated air. That moisture migrates upwards into attic spaces should not be a mystery. In materials such as open-cell foam (and cellulose) some of this moisture is stored particularly at night and during the winter (so we have both diurnal and seasonal effects). During the day the sun drives this moisture back into the attic. During the summer the sun drives this moisture back into the attic. If this moisture is not removed the quantity increases. Note that this moisture comes from the inside. In sloping roofs the moisture marches up the slope with each "ping" due to buoyancy and tends to accumulate at ridge areas first. We noticed this in cellulose insulated unvented roofs in Houston. The issue in Houston was resolved by removing the insulation at the ridge areas to allow the assembly to dry and to provide a mechanism of moisture removal in the attic spaces - air change or dehumidification. With open-cell foam the quantity of moisture stored is less with each cycle or ping than cellulose but over time if this moisture is not removed it can be quite substantial.

The second study is a modeling study that does not address the moisture removal mechanism nor buoyancy. It is fundamentally flawed for these reasons.

To condem open-cell foam based on ignorance, hearsay, poorly designed experiments and modeling approaches that are beyond their range of applicability in the face of several decades of successful field experience is a disservice to the industry. I have no particular love for open-cell foam or for closed cell foam or for cellulose or for fiberglass. I think they all have a place and I recommend the use of all of them. But each has its limitations and when used their limitations must be accounted for.

Respectfully yours,
Joseph Lstiburek

Jan 8, 2014 12:12 PM ET

Edited Jan 8, 2014 8:16 PM ET.

Response to Joe Lstiburek
by Martin Holladay

Thanks very much for your knowledgeable and useful comments.

You and I are in agreement on most of the points you raise, although my emphasis probably differs from yours. When it comes to disagreements between prominent researchers who challenge whether published ASHRAE papers have been properly peer-reviewed, I'm going to have to step back a bit and wait for the academic controversy to be settled over the next few months. Clearly, your perspective differs from that of the six authors cited. Such disagreements are healthy and usually fruitful.

In “Roof and Attic Design Guidelines for New and Retrofit Construction of Homes in Hot and Cold Climates,” the three authors (William Miller, Andre Desjarlais, and Marc LaFrance) wrote, "Field measurements imply that some of the moisture from a previous rainstorm migrates to the underside of the shingles and underlayment. Irradiance drives moisture from an earlier rainstorm into and through the OSB deck and open-cell foam." Note the use of the verb "imply." The researchers posited that the data implied that inward solar vapor drive was responsible for the elevated moisture levels. They did not state that the matter was settled.

After the paper was published, William Miller sent me an e-mail in which he stated, “We still are working to determine the mode by which moisture crosses the envelope... I believe it occurs in two different paths:
1. The sealed attic is not airtight and outdoor moisture penetrates. There is no ventilation and night-sky radiation causes the trapped humid air in the attic to diffuse into the spray foam.
2. The roof deck becomes wet during the evenings because of night-sky radiation and subsequently the condensate is forced into the roof deck by irradiance the next day.
Both modes result in moisture storage in the spray foam, and the stored water begins to damage the wood deck.”

I believe that I accurately reported that the three researchers had not definitively pinned down the source of the high moisture content in the roof sheathing.

You wrote, "Open-cell foam does not work without a vapor retarder in Climates Zones 5 and higher." I believe that I have stressed that fact for many years, and repeated that fact in this article. GBA recommendations have reflected that fact for years.

You wrote, "Conditioned attics must be coupled to the conditioned part of the house in humid climates in order to control the moisture levels in the attic. This is particularly necessary with open-cell foam due to its moisture storage characteristics." It could be argued that your recommendation flows naturally from the warnings voiced in my article. When open-cell spray foam is installed on the underside of roof sheathing, designers and builders need to pay attention to the RH of the air in the attic, or the roof sheathing can get damp. We're on the same page here, Joe. If we don't pay attention to these issues, builders can get into trouble.

You wrote, "The ping-pong effect has been mentioned and discussed by myself for over a decade. It is of no significance if a moisture removal mechanism is in place." Fair enough. One way to stay out of trouble is to include a moisture-removal system in the attic. Another way to stay out of trouble is to use closed-cell spray foam.

To lower the RH in attic air, you recommend a "mechanism for moisture removal in this attic - a simple approach is the use of a supply air duct coupled with a return path." In essence, your recommendation mirrors an observation made by researchers Simon Pallin, Manfred Kehrer, and William Miller, who noted that "a high duct leakage has a positive effect on the MC of the OSB, most likely due to the dehumidifying effect of the HVAC cooling coils, which, by a higher rate of air leakage, will have a higher influence on the vapor content of the attic air during the operating cooling mode.” Your recommendation to include a small supply air register in the attic, along with a small return-air grille, has the same drying effect as leaky ductwork.

You wrote, "To condemn open-cell foam based on ignorance, hearsay, poorly designed experiments and modeling approaches that are beyond their range of applicability in the face of several decades of successful field experience is a disservice to the industry." I certainly agree with your warning about modeling studies that are beyond their range of applicability, which is why I wrote, "When interpreting the results of computer modeling studies, it’s important to remember that modeling results sometimes differ from field study results."

But I stand by my statement that installing open-cell spray foam on the underside of roof sheathing is riskier than installing closed-cell spray foam. You have explained that the risk can be lowered by conditioning the attic air (using the HVAC system to lower the RH of the attic air). That's certainly a helpful recommendation, and I will certainly add your recommendation to my concluding remarks.

But it's worth emphasizing that the need to condition the attic air is related to the riskiness of open-cell foam. And it's also worth emphasizing that if three Oak Ridge National Lab researchers can end up with a test facility with damp roof sheathing because they failed to properly condition the attic air, then the same error could easily be made by Bob the builder.

Jan 8, 2014 12:18 PM ET

Now you tell me
by Dan Kolbert

"So my message is: when choosing materials, you have to think."

Jan 8, 2014 12:30 PM ET

Response to Dan Kolbert
by Martin Holladay

You'd be surprised how often builders and designers forget to follow that simple, useful principle.

Jan 8, 2014 12:43 PM ET

by Dan Kolbert

No, I wouldn't. I do reno, thus playing clean-up.

Jan 8, 2014 8:35 PM ET

smart vapor retarder can help by Ken Levenson
by Mark Parlee

Ken, The difficulty in installing a smart vapor barrier in an attic would be very great. If it is not installed to perfection then it would not work because the vapors will migrate to the leak points. If there is a spray applied smart barrier then that changes things.
This discussion is a good one and leads me to believe it would be a good idea for me to get back over and take some moisture readings at various places on this home.

Jan 9, 2014 7:35 AM ET

Response to Mark Parlee
by Martin Holladay

While air barriers have to be nearly perfect to work well -- a small hole can defeat the purpose of an air barrier -- vapor retarders don't have to be perfect. Even when a vapor retarder has holes covering 5% of the material, it is still 95% effective at controlling vapor diffusion.

I am wary of any spray-applied vapor retarders when these are applied to cured spray foam. Researchers at the Building Science Corporation have tested vapor-retarder paints on cured spray foam, and found that the paints are ineffective at limiting vapor diffusion. For more information on this issue, see Joe Lstiburek Discusses Basement Insulation and Vapor Retarders.

Jan 9, 2014 8:14 AM ET

by Allan Edwards

Here in Houston I've used open cell foam in attics and walls in my new homes for about 5 years, spraying the foam between rafters from the top plates up to the ridge. I've not had any callbacks or complaints regarding moisture. My AC contractor has always insisted in having a supply duct (with return) in the attic "dumping" some conditioned air in the attic. Honestly I always wondered why he did this, I guess in retrospect he is doing the right thing.

Jan 9, 2014 9:20 AM ET

How does this relate to cathedral ceilings?
by Eric Kronberg

I'm trying to get my head around how this would relate to a cathedral ceiling situation. We are in Atlanta (CZ2), do some second story additions/attic build outs, and use open cell foam for this on a regular basis. It would seem to me that this fully conditioned attic situation should remove the extra moisture from the attic air. Thus, it would seem to leave the main source of any moisture at the roof deck to be coming from being driven through the asphalt shingles, through the underlayment, into the decking. I know this wasn't necessarily the main focus of the studies, but looking at the situation I described would seem to be able to isolate how much moisture is coming through from above.

Jan 9, 2014 9:38 AM ET

Response to Eric Kronberg
by Martin Holladay

The three researchers (William Miller, Andre Desjarlais, and Marc LaFrance) aren't sure where the moisture is coming from; they are hypothesizing.

In Comment #27, Joe Lstiburek expresses his view that the source of any moisture is the interior air, so that if you are able to control the indoor relative humidity, you shouldn't have any problems (at least in warm climates). That advice is consistent with field experience.

If you are a builder who is worried that there is no way to control occupant behavior (and after all, the occupants are the ones who are in control of the indoor relative humidity), then you may feel that it's worth switching to closed-cell spray foam.

Jan 9, 2014 10:19 AM ET

Edited Jan 10, 2014 9:42 AM ET.

Response to Joe Lstiburek, Comment #27
by Simon Pallin


My comments were intended to add value to the discussion and clarify misunderstandings regarding the presentation and paper that was presented in Clearwater, FL. However, it is better that if further questions remain, please feel free to contact me directly.


Jan 9, 2014 10:47 AM ET

Manufacturers, Installers & Building Officials
by Armando Cobo

I've been teaching and preaching about this practice of 5.5" open-cell foam (R20) sprayed under the roof decking without any rigid foam above the roof decking for years. It's a common practice in TX, NM and the whole SW. I've talked to Manufacturers and Retailers, and their response is that their open-cell foam becomes air-impermeable at 2.5" or 3.5", so 5.5" (R20) is OK to use under the performance code (R806.4.5.1), even though the Builder may not be following the performance code.

Just last year at Summer Camp, I spoke with two manufacturer's reps about the same issue, and both told me that since their O.C. was air-impermeable at 2.5" or so, and as long as they can "get away" with it, they are in the business to sell foam.

Building Officials are buying into this theories and allowing Builders to follow those practices. It's all a shame. I do hope more studies and reports bring to light how bad this practice can be!

Jan 9, 2014 12:24 PM ET

Edited Jan 9, 2014 12:33 PM ET.

My builder is just completing

My builder is just completing the dry wall installation and taping of my new Energy Star to be qualified home in Andover, CT. The home has engineered 2 X 6 wall stud construction with plywood sheathing on both the unvented cathedral roof and the walls. Fortunately, the cathedral ceiling has 4" of closed cell spray home directly under the roof sheathing followed by open cell spray foam to fill the remainder of the rafter cavities covered by 1 1/12 " of XPS and finally dry wall. I hope this configuration prevents the rotting of the roof sheathing which is covered by #30 felt and lifetime high reflectivity asphalt shingles.

After reading the discussions about "Open-Cell Spray Foam and Damp Roof Sheathing", I am wondering if the same priniciples of drying apply to my walls. My walls have the following configuration; vinyl siding, 3/4" foil faced polyiso carefully taped at seams, Tyvek, 1/2" plywood sheathing carefully taped at seams. open cell spray foam filling the entire 2 X 6 cavity, followed by drywall. I had intended to coat the interior side of the wall sheahing with 2" of closed cell spray foam followed by open cell spray foam. I did not do this because it was advised in much of the latest home building literature, "not to apply impermeable insulation on both sides of wall sheathing." which would prevent drying in both directions Although we are in the process of finishing the taping of the drywall, which itself introduces high mositure content,, my builder and I have taken relative humditiy readings and found high readings of 55 - 60 % in this unfinished home. This home is heated by two minisplits and will be ventilated by and ERV for which I am trying to also obtain an HRV conversion core.. My windows are fiberglass triple pane with extensive glass patio sliders and french doors onto a deck facing my west lake view.

My worry is that the relative humidity of the home will stay at too high a level and penetrate the permeable open cell spray foam in the walls causing rotting of the wall sheathing? Although it is too late now, I wonder if I should have used closed cell spray foam on the interior side of the wall sheathing to prevent this rotting? Fortunately, the simple solution is to install a dehumidifier in the cathedral living room. This may be a solution to the potential rotting of the wall sheathing but I am disappointed that I did not get the wall insulation configuration correct from the beginning. I believed as expert recommendations did also, that my existing configuration would allow the wall sheathing to dry to the interior!!! I am interested in any comments. Thank You.

Jan 9, 2014 1:08 PM ET

Edited Jan 9, 2014 1:10 PM ET.

Response to Joseph Poland
by Martin Holladay

In your climate zone (Climate Zone 5), the minimum R-value for rigid foam installed on the exterior of a 2x6 wall is R-7.5. (To learn why, read Calculating the Minimum Thickness of Rigid Foam Sheathing.)

You have chosen to install 3/4-inch-thick polyiso, which has an R-value of R-4.5 or R-3.8 (depending on whether you want to de-rate the polyiso for its reduced performance in cold temperatures). That's not enough to keep your plywood sheathing above the dew point during the winter.

So you made a design error. You can probably stay out of trouble, though, if you control your indoor relative humidity during the winter. Keep your house dry, and your walls will probably be OK.

If it's not too late, you might consider doubling the thickness of your exterior rigid foam.

Jan 9, 2014 1:21 PM ET

Edited Jan 9, 2014 1:37 PM ET.

by aj builder, Upstate NY Zone 6a

Dehumidify your home starting now. Winter air is very dry, in the past we kept windows cracked during the taping painting phase. Today with very tight construction use a dehumidifier. Also do not heat with moisture creating portable heaters. My choice is to use a standard furnace either a temporary one or the one to be installed in the home. Just don't run air through the ducts when sanding and creating dust.

Your walls should be fine. I have two homes with open cell only that are fine. They have ventilation via always on with bathroom lights bath fans (Panasonic) and they have air conditioning and they do not have humidifiers or large sources of moisture creating sources like 50 house plants for example.

All the past info on GBA says that walls have less problems than cathedral ceilings.

So, Joseph, dehumidify to get your new building moisture out now and maybe for the next year or two keep an eye on it, Run the bath fans 24/7 right now.... get your numbers down toward 30% somewhere far away from 60%.

New home moisture takes time to remove.... be patient, it will lower.

Edit; Martin's advice is the prudent advice. But, my experience has been with open cell and no exterior foam and all is perfectly fine. On the other hand I have been involved with a few rigid foam installations that were so rotted that all wood framing had to be replaced where water accumulated in materials adjacent to the rigid foam. Granted the homes had water intrusion but the rigid foam (foil faced) made those area (like closed cell foam) vapor barriers and so once wet they didn't dry and they them rotted to point of becoming potting soil. I should post the pics someday.

The one thing I know for where I live is a highly insulated cathedral ceiling using rigid foam needs to be vented above the rigid.

Jan 9, 2014 1:49 PM ET

Edited Jan 9, 2014 1:58 PM ET.

Thank You Martin,EPA Energy

Thank You Martin,

EPA Energy Star Version 3 (Rev.06) Thermal Enclosure Rater Checklist states in "item 4.4: Reduced thermal bridging at above-grade walls separating conditioned from unconditioned space, (rim / band joists exmpted) using one of the following options: 4.4.1 Continuous rigid insulation, insulated siding, or combination of the two; >= R-3 in Climate Zones 1 to 4, >= R-5 in Climate Zones 5 to 8." The manufacturer of my foil face polyiso rates it at R-5 at 3/4".

Although manufacturers claim that open cell spray foam becomes air impermeable at 3-4 inches, is this an industry proven fact? If this is the case, then is it true that mositure will never come in contact with my wall sheathing despite the relative humidity of the interior rooms? Thank You.

Jan 9, 2014 2:00 PM ET

One word; Buoyancy
by aj builder, Upstate NY Zone 6a

"Buoyancy"... like in 1967... "plastic"

Thank you again Joe L. for another insight worthy of unlimited gratitude. I will pass it on. Went to one of your seminars way back when at West Point sponsored by NYS. If any readers get a chance make sure you get to see Joe speak. The only problem I had Joe is you are way too entertaining to remember any of the information instead over the humor!


I love learning about building science.... green is not the reason I still come here... building science is.

We unfortunately lost one of the best advocates of green at this site. Wish he enjoyed his green world only and not smacking around anyone who did not worship at his feet. I miss his very insightful semi natural green advocating thoughts. Many here do.

Simon and Martin... Joe ended his post.... respectfully yours... I wish the tone of both of your posts did too especially you Simon. You did not need to exclamation bomb Joe in your post. You and I have to check our egos sometimes or maybe with me all the time. Discuss with Joe, I love it... but be nice... Joe is almost a God... to me anyways... I have to meet you sometime and find out if you too are worthy of God status... not yet Simon. Point me to your seminars... I'll see if I can raise your status for me. Take this well...get a laugh from it... life is best with several chuckles per hour... have a few on me.

Jan 9, 2014 2:00 PM ET

Open Cell is Zone 3 is Imperative
by Todd Witt

We are located in Alabama and we spray Icynene open cell as well as closed cell foam. We size and design the HVAC systems where we install the spray foam. I completely disagree with your article. I think there is no question as to open cell foam being the superior product in Zone 3 where we are located. There are multiple reasons that I don't have the time or care to list. However, I would like to mention that when you write misguided and misinformed articles like this you create a disservice to the entire building industry. I will spend hours upon hours explaining to customers that happen to Goggle your article why it is completely wrong.

Jan 9, 2014 2:07 PM ET

Response to Joseph Poland
by Martin Holladay

The building code (and Energy Star checklists) provide minimum guidelines. They do not state best practices. Notice that the Energy Star checklists says that the rigid foam will be >= the stated values (in other words, greater than or equal to). It's your choice. (The correct answer is "greater than," not "equal to.")

The polyiso manufacturer labels their product as R-5. That's true during the summer, but not true during the winter. For more information, see In Cold Climates, R-5 Foam Beats R-6.

Even if the polyiso were a true R-5, it still wouldn't have a high enough R-value to keep your wall sheathing above the dew point during the winter.

Q. "Although manufacturers claim that open cell spray foam becomes air impermeable at 3-4 inches, is this an industry proven fact?"

A. Yes, it is a fact.

Q. "If this is the case, then is it true that moisture will never come in contact with my wall sheathing despite the relative humidity of the interior rooms?"

A. There are two mechanisms for vapor transport from your interior environment to your wall sheathing: exfiltration (escaping air) and vapor diffusion. Your open-cell foam stops the first mechanism but not the second (because it is vapor permeable).

The fact that you won't have any air movement is a fact in your favor, which is one reason why I think your wall will probably be OK if you can keep your interior RH at a reasonable level. However, you will still have vapor diffusion through your spray foam, and the moisture will still accumulate in your sheathing. Unfortunately, the 3/4-inch polyiso reduces the ability of the sheathing to dry to the exterior without being thick enough to keep it warm, so the polyiso is working against you.

Jan 9, 2014 2:17 PM ET

Edited Jan 9, 2014 2:22 PM ET.

Response to Todd Witt
by Martin Holladay

You wrote, "I completely disagree with your article."

This article is a report on two academic papers that were presented at a conference in Florida. Many GBA readers are interested in the latest research, and we do our best to report it here.

The conclusions of the two research teams, which I believe was accurately reported here, are open to debate, as are the conclusions of any scientist. Joe Lstiburek has raised a few objections to the researchers' conclusions; and one of the authors of one of the papers (Simon Pallin) has responded to Lstiburek in a rebuttal.

I'm not sure whether you disagree with the fact that I reported on these academic papers, or whether (like Lstiburek) you want to dispute the conclusions of the researchers. If you think that the researchers have made errors, you need to point them out.

Jan 9, 2014 2:28 PM ET

Martin, " Thank You Again"

Martin, " Thank You Again" for a very detailed, scientific, and understandable explaination. Unfortunately, part of the reason we did not use thicker foil faced polyiso on the exterior of the sheathing is the large additional labor involved with building out the window fames to accommodate the larger thickness of insulation.

Would an internal polyethylene vapor barrier installed just before putting up the drywall have solved my potential vapor diffusion problem? Also would using an impermeable paint to cover my drywall solve this same potential problem?

Jan 9, 2014 3:04 PM ET

Response to Joseph Poland
by Martin Holladay

Either MemBrain (a smart vapor retarder) or vapor-retarder paint would probably be a good idea. If your drywall is already up, it's probably too late for the MemBrain. But you might want to use the vapor-retarder paint as a primer.

Jan 10, 2014 12:18 AM ET

Overkill Martin, and
by aj builder, Upstate NY Zone 6a


Martin, and Joseph... good is good enough... more than good is not gooder at least with all the open cell walls sprayed in my neck of the woods.

It's like one bolt in a wing strut doing the job... but two bolts weakening the whole assembly. I actually have dealt with this situation with a customer at one time.


Jan 10, 2014 12:36 AM ET

Question to Joe L's post #27---Removal of ridge insulation
by john walls

Joe, I was with you until you got to the part about solving the Houston house problem by eliminating attic insulation along the ridges. Are you speaking of "bare" ridges---like looking at the bottom of exposed sheathing? I can see how this would dry due to total exposure, but are there other issues to worry about with a bare deck?
I'm a big fan, so please take this in the spirit in which it's intended---educate me.
I have a home in zone 2B with OC foam, conditioned attic. If I take RH readings along the attic high points and the RH is too high, then you're suggesting I need to add a supply and return duct or a dehumidifier. At what point do I start thinking about ripping insulation off the ridges?

[Editor's note: To read a response to this comment, as well as other comments, click the "2" box below to proceed to page 2.]

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