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Musings of an Energy Nerd

Dense-Packed Cellulose and a Wrong-Side Vapor Barrier

An owner/builder in Massachusetts bets that the hygroscopic properties of cellulose will make an otherwise risky wall assembly safe

All of this home's wall insulation was installed on the interior side of the vapor barrier. The Massachusetts house has double stud walls filled with dense-packed cellulose.
Image Credit: Image #1: David Posluszny

Does the exterior sheathing on a double-stud wall accumulate worrisome quantities of moisture in late winter? Several researchers are now looking into this question, and Green Building Advisor has been sharing the researchers’ findings as the information becomes available.

The latest sheathing moisture measurements were made by Bill Hulstrunk, the technical manager at National Fiber in Belchertown, Massachusetts. I’d like to put an important piece of information on the table from the start: Hulstrunk is not a disinterested academic researcher; he is employed by a company that sells cellulose insulation. Clearly, his company is more inclined to share data showing that walls insulated with cellulose are performing well, and might not want to share any data that imply otherwise. That said, some of Hulstrunk’s data are thought-provoking.

Hulstrunk shared his moisture content readings in a presentation (“Hygrothermal Analysis of Superinsulated Assemblies”) at the Passive House conference in Portland, Maine, on September 22, 2014. His co-presenter was builder Chris Corson of EcoCor Construction.

Hulstrunk began his presentation by lauding the hygroscopic properties of cellulose. “Hygroscopic materials want to redistribute and equalize the moisture,” he said. “Hygroscopic materials like cellulose protect themselves and nearby materials. They can pull moisture out of nearby materials like sheathing and studs.”

Hulstrunk also provided a few cautionary statements about cellulose density. “Deep cavities require that cellulose be installed at higher densities,” he said. “I recommend 3.7 pounds per cubic foot for a 12-inch cavity or 4 pounds per cubic feet for an 18-inch cavity. Deeper cavities require more experienced installers, since multiple hose passes are required.” (For more information on this topic, see How to Install Cellulose Insulation.)

Walls without any exterior sheathing stay dry

Bill Hulstrunk set out to visit a number of homes with thick double-stud or I-joist walls insulated with dense-packed cellulose, including several…

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  1. homedesign | | #1

    Conditions at the Apex
    I admire David for being a Pioneer.
    I am not-so-curious about what is happening in the Walls.
    My mind is jumping to the peak of the roof.
    What are the conditions near the apex of the container during winter?
    If it turns out that the tip top of the roof stays "dry enough" ... then why is it dry?
    Is it because the Densely Packed Cellulose prevents significant amounts of water vapor from ever migrating to the peak?
    Or is it the hygric redistribution quality of Cellulose that dries the Ridge?
    Or both?

  2. GBA Editor
    Martin Holladay | | #2

    Response to John Brooks
    The reason what poorly insulated cathedral ceilings have sheathing rot near the ridge (as shown in the photo that you posted on David's blog yesterday) is that the ridge is leaky, and there are air pathways from the interior through the rafter bays to the ridge. The moisture is being carried by exfiltrating air.

    That phenomenon clearly isn't happening at David Posluszny's house.

  3. dankolbert | | #3

    for using "disinterested" correctly.

  4. Airithol | | #4

    Air tightness?
    This house is really air tight, according to his guest blog.

    0.09ACH50 is almost an order of magnitude tighter than PH standard, and close to the Dillingham, Alaska record (0.05ACH50).

    When the rate of wetting exceeds the rate of drying accumulation occurs. When the quantity of accumulated moisture exceeds the storage capacity of the material problems occur.

    Building science basics tell us that exfiltration of warm (moist) air can lead to problems. I wonder if this is a case of the house being "tight enough" to avoid the expected problems. In other words, perhaps a sufficient level of air tightness has been reached to create a drying/wetting equilibrium.


  5. homedesign | | #5

    Moisture Buoyancy
    The photo that I posted yesterday is not exactly a poorly insulated cathedral ceiling.
    It's a SIP roof with a Complex 3-D network of voids that "connect" to the Ridge.
    the photo is from this "Insight"------- alaska
    Quoting Insight-036
    "Note that the damage is all along the upper part of the roof – at the ridge – and at the upper portions of the panel joints near the ridge. The damage at the upper portions of the roof assembly is due to both thermal buoyancy and moisture buoyancy. The thermal buoyancy should be obvious – we sometimes refer to it as the stack effect. However, moisture buoyancy is not always obvious. Yes, moisture-laden air is less dense than dry air and tends to rise. It is one of those counter-intuitive but true things that amaze people in bars late at night."

  6. GBA Editor
    Martin Holladay | | #6

    Response to John Brooks
    Thanks for clearing up my misconception. OK, it's a SIP roof.

    The failure still occurred because of air pathways, so your clarification doesn't undermine my point. David's house is different.

  7. homedesign | | #7

    Exfiltrating Air
    Agreed that David's House is different.
    I thought your point was that IF "Exfiltrating Air" is eliminated then "No Worries".
    David's house may not have a "problem"...for a variety of reasons.
    However, I am still curious about conditions at the Ridge

  8. DavidAlex | | #8

    I'm interested in the roof, too
    The thicker the bay, the more difficult it is too install cellulose -- and apparently the density has to go up, too. How dense does a 25-inch thick thick void at the ridge of the scissor truss need to be to eliminate settling (and thus minimize air exfiltration)? If it works, this is a great way to do a cathedral ceiling. I'm really curious as to what the results will be after several years of moisture cycling.

  9. GBA Editor
    Martin Holladay | | #9

    Response to David Hicks
    Q. "How dense does a 25-inch thick void at the ridge of the scissor truss need to be to eliminate settling (and thus minimize air exfiltration)?"

    A. Even if the cellulose settles -- and I doubt that it will -- what does such settling have to do with exfiltration? David installed Ice & Water Shield over the entire roof, including the ridge. There is no ridge vent, and therefore no chance of exfiltration at this location.

  10. wjrobinson | | #10

    Great topic
    Same home same everything, but use a permeable solution instead of Grace should work well right? And for extra measure, north walls could have and added detail, such as an interior smart vapor barrier.

    Great topic.

  11. GBA Editor
    Martin Holladay | | #11

    A vapor-permeable solution
    Yes, I would vote for a method of establishing an exterior air barrier with materials that are more vapor-permeable.

    As I wrote in the article, "Builders who want to establish an exterior air barrier don't have to use a vapor-impermeable product like Ice & Water Shield, of course. Standard recommendations for exterior air barriers include the Zip System (using OSB and a proprietary tape), plywood taped with Siga Wigluv (or some other type of high-quality tape), a vapor-permeable European air barrier membrane like Solitex Mento, or even a vapor-permeable peel-and-stick product like Henry Blueskin VP."

    For someone as meticulous as David Posluszny, my guess is that the vapor-permeable peel-and-stick option (Henry Blueskin VP) would be the most attractive way to go. I imagine that there is an upcharge for switching from Ice & Water Shield to Henry Blueskin VP, however.

  12. kevin_in_denver | | #12

    Blueskin Recommended
    In another one of David's videos, he does recommend Henry Blueskin VP-100 "for the rest of us".

    All the videos of this house are on his Youtube channel:

    The Blueskin line of products is a bit confusing, some of the tapes have conventional rubberized asphalt adhesive, which basically doesn't stick to the rough side of OSB. So just make sure you are getting this stuff:

    Note that a fully adhered and self healing product is theoretically superior to taped joints because the siding installation entails hundreds of sheathing penetrations.

  13. DavidAlex | | #13

    Response to Martin #9
    My mistake. Exfiltration is not the word I was looking for. My understanding of this issue is that the hydroscopic properties of cellulose mitigate the problem of the "wrong side" vapor barrier by evenly distributing the moisture throughout the depth of the insulation, effectively minimizing the amount of moisture against the exterior plywood, in contrast to how hydrophobic material would allow interior heat to drive all the moisture to the exterior and then accumulate on the sheathing. But that requires that there not be any voids in that insulation, right? Moisture would eventually be driven through the insulation, and then get stuck in the void and accumulate on the cold wood.

    In this Q&A posting,, you advised a reader to not insulate a scissor truss with cellulose due in part to the difficulty in getting enough material near the peak.

    My reading of all this is that the moisture-mitigating effects of cellulose in the walls would be much harder to duplicate in the scissor truss.

  14. ohioandy | | #14

    Don't forget about LAWB (Martin, #11)
    I would add liquid-applied weather barriers to Martin's list of permeable options to Grace. The STO system (which I used on my house) and Tyvek Liquid Applied are two examples. Like the Grace, these products serve as water/weather barrier and air barrier, but unlike Grace they are vapor retarders, not barriers. Further, they don't rely on the chemistry of stickiness (although they do rely on the chemistry of drying and curing.) Costs are probably higher,, too.

    Can anyone comment on the long-term durability of these products? The house in question relies totally on the long-term integrity of Grace--seems like a vapor-open assembly would reduce the risk of failure. What if the Grace is stressed across the ridge or eaves? Trusses aren't known for remaining perfectly still...

  15. kevin_in_denver | | #15

    Not that vulnerable

    I really don't see that this air barrier is vulnerable. The roof especially is protected. All the shingles are nailed to sheathing that is 1.5" away from the air barrier. The siding is attached with fasteners that only penetrate a self-healing membrane.

    If someone needs to retrofit a pipe or duct through a wall, it's impossible to make a repair on an air barrier that is buried deeply in a wall such as the Perfect Wall.

    Response to Andy CD:

    David and Bill H. also emphasize that thick dense-packed cellulose retards air flow by itself. So at this point, we don't even know how good the air barrier needs to be. Maybe a dense packed wall prevents infiltration without an air barrier? (Obviously a weather barrier is still required, but it comes free with the air barrier)

  16. kevin_in_denver | | #16

    Major Roadblock: The Building Codes
    The way I read the code, this roof assembly is disallowed.

    Venting per the code would destroy the airtightness of the home.

    I think the wall assembly would also be illegal.

  17. kevin_in_denver | | #17

    Moisture Buoyancy and Conditions at the Apex
    Response to John Brooks:

    I agree with you that we need to suspect that a lot of vapor is driven to the roof plane, and that's one of the most important spots to probe. Bill H. anticipated this, and reports:

    "the hygroscopic moisture redistribution is occurring more rapidly than diffusional moisture flow through the cellulose. .... we also probed his unvented roof at the 24 inch thickness and found similar "boring" results. "

    Note the apex is sure to dry out quickly every spring because it is in direct sunlight: ' "there will be a fair amount of drying to the interior if the sun hits a wall,” Ueno said.'

    Most reports tell us:
    1. Cold and wet sheathing doesn't rot.
    2. If wet sheathing gets to dry out completely once a year, there is no rot.

  18. GBA Editor
    Martin Holladay | | #18

    Response to Kevin Dickson
    You wrote, "Venting per the code would destroy the airtightness of the home."

    I disagree. David Posluszny actually included vent channels above the roof sheathing, so it could be argued that this is a vented roof. The only problem is that David didn't provide any way for the ventilation to help dry out the lower layer of roof sheathing in case it ever gets damp.

    The solution is fairly simple: choose a roofing underlayment (or air barrier material) that is vapor-permeable instead of the Ice & Water Shield.

  19. kevin_in_denver | | #19

    Vented Roof Plane
    Martin, You're right of course. David's roof is vented. I was thinking it's an unvented cathedral ceiling, which it is, but the vented roof plane then provides an overall assembly that should be OK.

    I'm still a little worried about the wall and my local plans reviewer. Bill H. told me that an eight inch wall should fly without foam, but I'd like to go to 12"

  20. John Straube | | #20

    Walls with exterior insulation perform better
    Interesting blog. For what it is worth, we have carefully measured a totally controlled 12” cellulose wall (weighed in the cellulose to confirm 4 pcf, hand packed the last bit to get there), and then used OSB sheathing, Tyvek (just like those German vapor-permeable membranes you mention, except affordable and locally available). See the attached illustration (below).

    It worked, but barely — it skirted the 20% MC level and when challenged with small air leaks and water leaks in subsequent years reached 30-35%+ MC and took a while to dry out. It performed worse by far than all of the exterior insulated walls with equivalent R-value. Will write this up soon...

    The commentary seems to be thinking of air flow. I think we can assume these houses are crazy airtight. Say like a low-slope commercial roof with a good sealed roof membrane. The biggest moisture risk is then not air leakage but diffusion. Cellulose has massive storage capacity for vapor, especially if you use 12” or more of it. So the MC would be expected to rise over winter, then fall as it dried back to the interior. Makes sense to me. Of course, if the interior RH is high, or the inward drying is restricted, we would expect the capacity to be overwhelmed. Hence, we would expect maximum MC due to this vapor diffusion flywheel not in Januay, but later in the season, maybe March or April.

    It seems odd that someone with a vested interest who goes out and takes a few readings (not aware of the need to insulate pins that you push into cellulose or temperature corrections) can be taken credibly when others who have carefully controlled and measured temperature and moisture at dozens of spots every hour for years show different results. That said, the technique of highly open (not OSB) sheathing should work fine based on the science we have (Bill is not doing science) and that with peel-and-stick outside should be marginal, depending highly on interior RH, perfect details, perfect installation. The latter has not been tested or measured properly by reputable people, and so I am intrigued but highly suspicious.

    Of course, Bill still thinks dense pack cellulose roofs never fail….


  21. Expert Member
    MALCOLM TAYLOR | | #21

    Location of the air barrier
    One of the strategies that both Joe L's Perfect wall and a number of double walls share is that the air barrier is buried near the centre of the assembly, making it more resilient to damage and the depredations future inhabitants may cause when they alter the structure. I wonder about the longevity of a house that relies of maintaining a very tight air barrier for its success - especially in such a vulnerable position.

  22. ErgoDesk | | #22

    Rules of Building Construction
    1. Choose the least expensive materials and slap them together.
    2. Watch for immediate problems, slap solutions on to fix. asap, continue watching.
    3. Discover that you are a moron for not using real professionals, keep watch and fixing.
    4. Take course in accounting so you can asses financial damages.

  23. Expert Member
    Dana Dorsett | | #23

    I like the flywheel analogy (response to John Straube)
    The massive moisture buffering of the cellulose may not even hit full stride even in the first season if he's keeping it 68F/45%RH indoors (dew point= 46F) with a humidifier, as-reported. His averages may be higher next season, due to the comparative brevity of the drying season when the sheathing can't dry toward the exterior.

    The average daily temp in Shirley MA doesn't exceed the 46F average interior air dew point until the second week of April in an average year. (!dashboard;q=shirley%20ma ) Taking a reading on the cusp of January/February (even if done correctly) wouldn't be anywhere near the peak reading- the party is just getting rolling by then!

    The drying season for sheathing the north side of the house doesn't even start in earnest until after tax-day, and ends well before Thanksgiving!

    The rainscreen gap built into his stackup is somewhat wasted, given that the sheathing can't dry into the exterior, but would be way better than nothing if the membrane were highly permeable.

    If he's keeping it that humid indoors in winter even a smart vapor retarder isn't going to be much use, but if he held the line at 35% RH max it might help. Best he can do now is to keep it around 30% RH as much as possible to extend the drying season, and start it earlier.

  24. AndyKosick | | #24

    Perfection is not Scalable.
    I'm not actually that surprised that Hulstrunk's wall works. I've seen cellulose defy some very steep odds and have attributed this to it's hygroscopic properties. That said, the biggest problem with this assembly is that it probably depends largely on the perfection (.09ACH50) of the building envelope, the interior air barrier to be exact. Putting this assembly into production (which I assume should be the goal of any wall assembly) would be to subject it to inevitable errors, even with a blower door tests things happen, or down the road a homeowner put an accidental chink in the interior armor. Where as say Chris Corson's walls (vapor open) could put up with some moisture intrusion, Hulstrunk's wall seems like it would have serious problems in short order, especially with elevated indoor humidity (occupant behavior). Please correct me if I'm wrong.

  25. Ken Levenson | | #25

    a couple of material notes
    John Straube says: "...Tyvek (just like those German vapor-permeable membranes you mention, except affordable and locally available)." John, not true. Tyvek is not airtight, is not waterproof and uses convection through micro-pores/tears for moisture permeability - and can become vapor closed if those pores are clogged with water. It can also be degraded by building site contaminants and wood tannins. Those "German...membranes", like our SOLITEX Mento 1000 and Plus are available, are affordable and are made of solid/nonporous membranes that are extremely airtight and waterproof while allowing for moisture transmission at very low pressure differentials (at the molecular level, not through pores), unaffected by wood tannins or other contaminants, making a far more robust control layer. Finally, the SOLITEX Mento Plus is reinforced specifically for installation with dense-pack insulations. Apples and oranges.

    Dana Dorsett, you say "...If he's keeping it that humid indoors in winter even a smart vapor retarder isn't going to be much use". Also, not true. The INTELLO intelligent vapor variable retarder can be of great help in such cases particularly when faced with vapor closed layers outboard, as the INTELLO permeability doesn't "open up" significantly until the RH is at approx 70%. Of course for indoor swimming pools and other relentlessly high humidity uses the INTELLO is not recommended, but for 45% to 50% indoor RH, INTELLO can significantly increase the drying reserves of the assembly.

    And yes, we sell these products:

  26. Bill_Hul | | #26

    Now that the heavy hitters have weighed in...
    Let me first cast aside the notion that these building assemblies are untested, unproven or even new. Tony Walker had been doing Larsen Truss and double wall assemblies since the late 1980's in the Pioneer Valley of MA when I met him a decade ago at the NESEA Conference. Aside from these buildings not being as air tight as more recent super insulated buildings, these buildings continue to be durable and have maintained their energy performance over time. I would like to be able to take credit for this affordable and refreshingly simple system, but unfortunately I cannot.

    In regards to my extended moisture probes, I used electrical shrink wrap to insulate the entire length, leaving only the sharply pointed tips exposed. These extended probes read within 1% of the pins on the moisture meter.

    Here are some pointers when using cellulose in high performance buildings:
    • Use a qualified installer, half the price means skimping on the material and not taking the time to dense pack properly.
    • Cellulose density is important, the deeper the cavity, the higher the density needs to be self supporting.
    • Keep your indoor relative humidity levels within the reasonable levels (30 – 55%) during the winter months; a properly sized HRV/ERV is essential.
    • In mixed heating and cooling climates, vapor open assemblies allow drying in either direction depending upon the season.
    • Focus on your flashing details and back vent your siding, cellulose is robust but like any insulation system, is not designed to cope with water running through it.

    We continue to use this affordable high performance building system in super insulated and passive house construction across the country without durability issues. There is simply no other insulation system out there with this same proven track record.

  27. user-1135248 | | #27

    okay, stupid question
    I was *just* going to ask about the long probes. They looked
    uninsulated in the video, but it was kind of hard to tell. If
    they're shrink-tubed that's reassuring, but there's a remaining
    question: it's a resistivity measure, right? The probes are
    generally designed to jam pretty hard into wood at a known
    separation; how can you be sure you're getting the same quality
    of electrical contact inside a relatively squishy mass of
    cellulose?? Did you run through enough to actually sink into
    wood at the outside part of the wall, and what were readings
    like at those points?

    It does seem reasonable that the dense-pack would allow very
    little intra-wall convection, even with minor voids at the top
    from settling later on, so it makes sense that the usual mechanisms
    that wet inferior walls wouldn't really be at work here.


  28. jinmtvt | | #28

    Very interesting follow up of the prior blog ...
    I think i'll go through David's entry one more time now ...
    Might have missed a few technical points here and there.

    But, this wall assembly seems "fragile" to me.
    It might perform superbly now, but what about future ?
    Future owners?
    Then, the Grace membrane is not the easiest to get to fully stick without some heavy roller/heating sessions, What if it does peel up with time in some spot? would it cause any problem since it is installed on a sheathing neway ...

    How can one be sure that there are no voids in the cellulose filled sections ?
    I understand the packing and density technique, but i have hard time believing that it completely fills every corner and details in the roof sections ... might be just ignorant intuitions though ..

    As some pointed out, this particular installation with controlled and methodical work might work perfectly, but what about using a similar wall system with labor crews ?
    Would any small imperfection cause major problems ?

    I still prefer ( again personal intuition ) interior vapor membrane with all of insulation at the exterior...
    much easier to "vapor seal " in empty framed walls from the interior than risking loosing control

    I still can't believe he's used so much ( in weight ) of cellulose in such a small building !
    incredible! :)

  29. kevin_in_denver | | #29

    Let's Think About Cellulose for a Minute
    Bill H. has a hypothesis that "the hygroscopic moisture redistribution is occurring more rapidly than diffusional moisture flow through the cellulose." I haven't been able to find any reports investigating this, but it's plausible, given the properties of cellulose.

    The term "hygroscopic" hardly conveys the true power of cellulose to move liquid water.

    Cellulose molecules were originally designed to conduct water straight up against the force of gravity. The observed maximum height is 379 vertical ft. That's the equivalent of a pump putting out 164 psi.

    So it doesn't surprise me that ground cellulose can conduct liquid water just 12 inches horizontally back to the heated space as fast as vapor diffusion can force vapor through dense packed cellulose out to the adsorption zone.

    Additionally, liquid water is 1700 times denser than water vapor, so it doesn't need much capillary cross sectional area to get back inside the house.

    Last November, Martin did a review of the literature on this topic:

    In that article John Straube advised that if you build a double stud wall filled with cellulose, “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.” I'd say that's great advice to reduce risk without adding much cost.

  30. GBA Editor
    Martin Holladay | | #30

    Response to John Straube (Comment #21)
    Thanks for sharing the results of your careful measurements.

    GBA readers who are tempted to imitate David Posluszny should carefully read John Straube's comments.

    To recap:

    1. A wall with OSB sheathing and a vapor-permeable housewrap (not Ice & Water Shield) "worked, but barely — it skirted the 20% MC level and when challenged with small air leaks and water leaks in subsequent years reached 30-35%+ MC and took a while to dry out." That's why GBA has long recommended that double-stud walls insulated with cellulose need something other than OSB as exterior sheathing -- plywood, board sheathing, or DensGlass Gold is better -- and a ventilated rainscreen.

    2. "The MC would be expected to rise over winter, then fall as it dried back to the interior. ... Of course, if the interior RH is high, or the inward drying is restricted, we would expect the capacity to be overwhelmed." In other words, the rate of drying must exceed the rate of wetting, and these walls are close to that balance point.

    3. "The technique of highly open (not OSB) sheathing should work fine based on the science we have ... and that with peel-and-stick outside should be marginal, depending highly on interior RH, perfect details, perfect installation."

    Again: thanks, John.

  31. GBA Editor
    Martin Holladay | | #31

    Response to Bill Hulstrunk (Comment #26)
    Thanks for the reminder that energy-conscious builders have been insulating double-stud and Larsen truss walls with cellulose since the 1980s. I agree with you that we have decades of experience with these walls.

    In your comments, you were curiously silent on the main focus of my article: David Posluszny’s decision to cover his walls and roof with an exterior vapor barrier. Since you have long promoted the installation of dense-packed cellulose between the rafters of unvented roof assemblies -- in spite of the fact that building codes forbid the practice -- and since most types of roofing do not allow any outward drying, perhaps you aren’t worried about David’s approach. Yet at the Portland conference, I heard you express reservations about David’s decision to install Ice & Water Shield on the exterior of his walls.

  32. GBA Editor
    Martin Holladay | | #32

    Response to Dana Dorsett (Comment #23)
    You wrote, "The rainscreen gap built into his stackup is somewhat wasted, given that the sheathing can't dry into the exterior."

    That's the element of this situation that is so heart-breaking: David included these perfect rainscreen details for the wall, as well as ventilation pathways above the roof sheathing... but then he chose a material for his WRB / roofing underlayment that isn't vapor-permeable. The ventilated gaps provide tremendous drying potential -- the ability to pull moisture out of damp sheathing -- and yet that pesky Ice & Water Shield shuts down all drying to the exterior.

  33. Airithol | | #33

    Re: Bill Hulstrunk (#26), Martin Holladay (#32) & Dana (#23)

    I do not think anyone here is really questioning the virtues of dense pack cellulose, or it's successful use for decades. It's the location of the vapour barrier in conjunction with it (DPC).

    If Martin had titled his article Fiberglass Batts and Wrong-side Vapour Barrier, I doubt there would be much discussion about the success of this assembly. However, because of the cellulose, or rather, it's properties, this assembly has a fighting chance.

    Personally, my confidence in this assembly goes up, or down, with it's air tightness.

    Martin & Dana,

    I would not characterize his (or any) rain-screen gap as a waste. His cladding is still back-vented, and the real purpose of the gap is to ensure the durability of his Grace Ice/Water shield when the house is re-roofed/sided. Yes, it's self-healing, but given the assembly's reliance on air tightness, I think it was worth putting in.


  34. homedesign | | #34

    Where are the Dead Bodies?
    David Posluszny and John Straube bring up a good Question.
    "Where are the Bodies?"
    How many Superinsulated (w/ Dense Pack Cellulose) Houses have "Failed"?
    How many have been torn down or abandoned?
    How many have had to be completely Re-clad or Reconstructed because of a reverse flashing error?

    edit to say...I realize this article is about a "wrong side vapor barrier" ... but I am still curious about the dense-pack casualties

  35. wjrobinson | | #35

    David's reasoning's not addressed directly by Straube, Martin +
    David, says exterior water leaks are the main problem not vapor. So he decided to use the most bullet proof method to asure exterior water proofing during the build out, and for the foreseeable future, roof changes siding changes not damaging his waterproof layer.

    Straube mentions water leaks as the problem in his example. Ding Dong... David directly addressed what Straube found to be a problem. So logically it is an apples to oranges useless debate.

    Those concerned about an interior barrier, David specifically addressed that too in his design and analysis. He does not have a barrier at the drywall on purpose, he wants vapor to get back into the interior with ease. And he feels the dense pack cells is the barrier to entry along with the fact that cells has some great moisture properties to take advantage of.

    I am not advocating his build but I am defending his logic train verses those of you skipping around and missing directly the points David makes in his highly detailed review of the hows and whys and whats of his home.

    David, you have done a great service just in how well you detailed your build. As always many of us would love access to your plans, your spreadsheets of costs and a list of the exact materials used. That info is always missing here, and Fine Homebuilding and at Building Science Corp.

  36. user-741168 | | #36

    A good discussion
    I consider the study of cellulose-only cavities to be critically important. It will be the default assembly for anyone going off the foam grid. This is a good discussion.

    Run the numbers at steady-state. I did, with assumptions of 45% RH inside, 10 perms inside and 10F outside (assumed constant for the entire month). The rate of moisture accumulation was 0.27 lb/sf/mo. If the wall is a foot thick, at 4 lb/cf and the sheathing weighs 1 lb/sf, then that’s 5 lb/sf. And the increase in one month amounts to about 6%. The 7% moisture content values read from the Delmhorst I’d consider a tad suspect. But if the allowable range is from around 10% up to the low 20% range, such an increase under these conditions should make it.

    Twenty years ago in my lab, we had a cellulose-only (low-density, no tests for airtightness) wall with no poly and kept the indoor RH at 50% for a winter. Both the cellulose wall and its neighbor with fiberglass had MC go up above 25% moisture content in the OSB sheathing. There was no mold in the cellulose cavity, but mold in the fiberglass, at the same moisture content. I attributed that to fire-retardant salts in the cellulose. Those same salts made all the moisture content results questionable even after our attempts at correction.

    It’s not quite correct to imagine uniform redistribution of moisture through the thickness of the cellulose. Warmer material will be drier than colder. I leave it to John Straube or our friends at Forest Products Laboratory to say just what the equilibrium distribution will be.

    Our results at the Guggenheim may pertain. It was 5” of shotcrete with an impermeable “cocoon” coating on the outside and cellular glass insulation on the inside. At mid-height of the building (neutral pressure plane), protected from rain washing, the RH in the concrete stayed around 50% year round. At the higher level, with exhausting air and high water exposure, the RH climbed to above 90% RH in winter. So water-protected diffusion-only cavities with high storage lend themselves to slight variation about an annual mean.

    In short, these assemblies should be able to work, provided 1) the drying season can correct for the wetting season, 2) interior vapor permeance comes into play, 3) winter interior RH doesn’t go too high, and 4) we allow some salt inhibition of mold. That’s for walls.

    Roofs may be a somewhat different story.

    We studied three types of cathedral ceiling cavities: vented, slotted (air space without the vents) and stuffed. We were surprised to find the slotted cavities perform quite like the vented, while we expected them to perform like stuffed. It was as if something in the slot was sucking moisture out of the slot. Only recently, with Joe L’s discussion of conditions at the ridge, have I begun to imagine what that might be. I challenge his, and anybody’s, notions of “hygric buoyancy”, which actually may show a water vapor concentration difference as a function of elevation at about the fifth decimal place. Instead, I think ridges get colder than the rest of the roof due to their geometry and radiant exchange with the sky. (Can somebody please do an IR flyover?). So the ridge sucks water from the rest of the slot, or so I presume. I’ve certainly observed ridges in worse condition than the rest of the roof.

    Also, shingle color may play a big part in overall moisture performance.

    I think that cellulose-only assemblies, for walls and for roofs, can and should be made to work. However, it is not a gimmee, the way an exterior foam assembly is a gimmee. I recommend a consortium approach with producers, users and researchers to get a better handle on cellulose-only cavities. We need to do a right-sell of this assembly, not an oversell.

  37. Expert Member
    MALCOLM TAYLOR | | #37

    Comprehensive Analysis
    Among the incisive comments by regular posters and interested parties, Martin Holladay, Dan Kolbert, John Straub and Bill Rose also chime in. No one can claim that the issues didn't get a good airing by people who know what they are talking about!

  38. GBA Editor
    Martin Holladay | | #38

    Response to Bill Rose
    Thanks very much for your comments. I agree that further research on the topics raised here would be very valuable.

    I think that many builders and designers would agree that most assemblies with thick cellulose insulation work very well. What we would all like to know is, What happens when you change one of the parameters of a typical assembly? In other words, how close are we to the cliff?

    What happens when a builder decides to install an exterior vapor barrier on a wall?

    What happens when a builder chooses an unvented instead of a vented roof assembly?

    What happens in a house with very high indoor relative humidity?

    We'd all like to know the answers to these questions.

    I'm also intrigued by our comment that "shingle color may play a big part in overall moisture performance." I assume that this is a reminder to cold-climate builders that white roofing products (or so called "cool roofs") lower the temperature of the roofing (and therefore lower the temperature of the roof sheathing) due to nighttime radiational cooling. Cold sheathing accumulates moisture, and is therefore riskier than warm sheathing. If I am reading your hunch correctly, it's a reminder to cold-climate builders that we should be wary of jumping on the "cool roof" bandwagon.

  39. user-741168 | | #39

    roof color
    Yes, my short comment was intended to highlight the growing concern for moisture conditions under light-colored roofs. Cooling to the sky, especially on clear nights, is quite strong, and with white roofs the daytime heating may not compensate sufficiently. Hard to find a good reference on the subject. Here is one paper

    My comment above also addressed roof geometry, and peculiar radiant exchange conditions at the ridge. This is speculative on my part. I've not yet measured cooler conditions at the ridge. By Lambert's cosine law, one would expect greater sky exchange at the ridge, leading to cooler conditions.

  40. wjrobinson | | #40

    David's unusual vented roof
    David's roof has an air space above the sheathing that is impermeable due to ice and water layer.

    Benefits may be many:
    1 less nails in his Grace material, which is to be a 100% waterproof air barrier.
    2 air space must help dry the roof
    3 air space may lesson night time cooling reaching the cellulose containing sheathing that we don't want to get too wet and rot.

    Either way, one thing noted many times is that cold sheathing is less vulnerable to rot than warm wet sheathing. Night time radiation cooling could be killing rot organisms every 24 hours which is a good thing.

    If David's build works well, we should work to understand why and spread the lesson.

    If David had used foil backed sheathing for roofing layer, he could stop the radiational cooling from getting to his inner sheathing layer that is of concern.

  41. dankolbert | | #41

    As I often warn at our discussion groups, developing an interest in building science is an excellent way to never sleep well again.

  42. GBA Editor
    Martin Holladay | | #42

    Response to AJ Builder (Comment #40)
    You wrote, "Benefits [of David's roof ventilation channels] may be many: ... air space must help dry the roof."

    I'm not sure what you mean by "the roof." The ventilation channels will help dry out the upper layer of roof sheathing, but the lower layer of roof sheathing -- the layer touching the cellulose, which is the layer that is most in danger from winter moisture accumulation -- can't dry out to the exterior because of the Ice & Water Shield.

    You also wrote, "one thing noted many times is that cold sheathing is less vulnerable to rot than warm wet sheathing." In fact, the reality is a little more complicated. Cold sheathing is actually more vulnerable, because it will accumulate more moisture during the winter than sheathing that is kept warm by exterior rigid foam. If the damp sheathing could be kept cold year 'round -- a situation that occurs in Antarctica, for example -- then rot would be impossible. But in Massachusetts, temperatures during the spring rise to the point where mold and rot are real worries. If we want to avoid these worries, warm sheathing is generally safer than cold sheathing.

  43. GBA Editor
    Martin Holladay | | #43

    Dan Kolbert's tsuris
    OK, let's say it's been a long day, and your head is finally hitting the pillow. You're tossing and turning, trying to fall asleep.

    I will agree with you that, of all the things you might want to think about at that point, it's probably not a good idea to ask yourself, "I wonder how my wall sheathing is doing right now..."

  44. wjrobinson | | #44

    Yes the roof to me is the roof shingles and it's sheathing layer. That airspace sets up the very bullet proof under roof to be an order of magnitude more safe if that is ever possible as far as EXTERIOR moisture causing any moisture harm by LEAKING INTO the inner layer that is the vulnerable layer of sheathing. David believes that exterior water is the main problem in a build like his.

    His in house moisture has to get past dense pack cellulose. The cellulose near the inner wall is warm enough to be not wet from in home humidity. One side of this cellulose sponge is trying to stay dry. There is reason to believe David has built a working system. Bill Rose is the expert, I am all ears.

    As to cold and rot... the worst rot I see is when an inch or so of rigid is attached to exterior sheathing. There may have been flashing leaks too. Basic carpentry done by most carpenters, AT JLC Providence RI there is not what not even 1% of the nations carpenters standing at close attention to the 30 minute window flashing event. 10-15 of us... out of millions.

    I like this home and the explanations David posted.

    David and others, we should take this home and try to even half the cost and double the value of the look and double down on making sure it can handle moisture.

    Someone should post a spreadsheet of the material costs and use and then suggest improved alternatives. Better yet build better improved lower cost higher curb appeal higher moisture resistance homes.

    David's super insulated home blog being so detailed to me is just plain exciting and energizing. Makes my work effort day to day even more funner than the fun twas prior to it's posting.

    Job overly well done David!!

  45. wjrobinson | | #45

    Straube posted comments as to moisture too high in a build like David's.

    Please comment on the fact that Straube posted a wall detail with inner 6 mil plastic.

    That plastic layer is a moisture trap if outside or inside moisture gets behind it. Yes? Of course it is.

    That is not David's wall. David slows inner moisture down by dense packing. David allows moisture back into the home because he has NO 6 mil plastic.

    Comments Straube thoughts and detail shown Martin.

  46. GBA Editor
    Martin Holladay | | #46

    Response to AJ Builder
    John Straube was sharing data. He posted a detail that illustrated the wall he monitored. He never claimed that the wall he monitored was identical to David's wall.

    I am grateful to John Straube and Bill Rose for their research and their willingness to share data. The shared data allows all of us to have more information that can help us design better wall and roof assemblies.

    As most building scientists have understood for decades, interior polyethylene is a double-edged sword. It limits outward vapor diffusion in winter, but it also inhibits summertime drying. The net result of these two mechanisms is climate-dependent. WUFI or field monitoring studies can be used to try to determine whether interior poly is a net help or a net hindrance.

    The farther north you go, the more likely that the wall shown by Straube makes sense. (John Straube is Canadian.) The wall he illustrates is designed to dry to the exterior, not to the interior.

  47. Expert Member
    Dana Dorsett | | #47

    Response to Ken Levenson( #25) Jason Hyde (#33) Bill Hulstrunk
    Ken: "...INTELLO permeability doesn't "open up" significantly until the RH is at approx 70%"

    That's great for slowing the rate accumulation, not so great for speeding up the drying speed, potentially problematic for a fat assembly with hygroscopic insulation. An RH of 70% for sustained periods is at high risk of mold/fungus growth.

    Jason: " the real purpose of the gap is to ensure the durability of his Grace Ice/Water shield when the house is re-roofed/sided. Yes, it's self-healing, but given the assembly's reliance on air tightness, I think it was worth putting in."

    With a self-stick membrane penetrated by a nail going into 4'x8' plywood there is effectively ZERO air leakage from those nail penetrations- a weak argument at best>

    Bill: " Here are some pointers when using cellulose in high performance buildings:" (snip)

    "• Keep your indoor relative humidity levels within the reasonable levels (30 – 55%) during the winter months; a properly sized HRV/ERV is essential."

    At 55% RH/68F the dew point of the interior air rises to 51F. In a Shirley MA location with an impermeable membrane over the sheathing that level of humidity delays the beginning of the drying season for the sheathing until the LAST week of April, and the moisture accumulation season begins the last week of September. That means you have only ~5 months of drying season to 7 months of accumulation season.

    "• Cellulose density is important, the deeper the cavity, the higher the density needs to be self supporting."

    Settling of cellulose is not about the weight, thickness or depth- it's all about the moisture cycling and the mechanical creepage that creates. Higher density DOES allow higher moisture cycling- the minimum density to avoid settling relative to the moisture cycling has been well modeled & validated experimentally by a Torben Valdbjørn Rasmussen at the Aalborg University in Denmark. (He has published numerous scientific papers on the properties of cellulose insulation, as well as other relevant building science topics. Start here: )

    By putting an vapor barrier on the exterior the peak moisture levels of the cellulose will clearly be higher than if it had been allowed to dry into the rainscreen cavity year round, and may need to be even higher than 4lbs density do not suffer at least some creepage/settling over time.

  48. DEnd2000 | | #48

    Response to aj builder
    aj you wrote: "Night time radiation cooling could be killing rot organisms every 24 hours which is a good thing."

    I agree that it would be a good thing, but that likely is not going to happen. Below freezing temperatures does not generally kill mold spores. If you have the temperature and moisture levels to support mold growth, then occasional freezing will only slightly slow down the decomposition. After a season of mold growth there will likely be enough spores on the material that growth will quickly reach previous levels. So are you slowing down decomposition by an occasional freezing cycle? Absolutely. Is it enough to make a real difference? Nope.

  49. wjrobinson | | #49

    yes Donald agree
    The borate may be helping.... So say some.... With critters.... Is rot a critter? Nature's external stomach biotic creator of rich fertile soils... Leading to life forms with internet symbiosis....

    Aj -(an internet connected biom community)

  50. [email protected] | | #50

    Borate treated osb or plywood will make the sheathing more resistant to rot and mold caused by excessive moisture in the panel. Borate in the insulation maybe not so much.

  51. lutro | | #51

    Question for Kevin Dickson #17 (and for everyone)
    Kevin says, "2. If wet sheathing gets to dry out completely once a year, there is no rot."

    This doesn't match what I have read on GBA and elsewhere, including a few of the comments on this thread. Sheathing that is wet and warm for a while each year has grown mold, even if it eventually dries each year, according to some reports. Kevin, can you say more about the parameters that you were addressing?

  52. kevin_in_denver | | #52

    Response to Derek Roff
    I've seen several reports with similar results to the quote above:

    "As I reported in a November 2013 article called “Monitoring Moisture Levels in Double-Stud Walls,” Kohta Ueno, a researcher at Building Science Corporation, has measured wall sheathing with moisture contents that range from 20% to 25% on the north wall of a double-stud cellulose-insulated wall in Massachusetts — and even higher readings when the interior relative humidity was high.
    I called Ueno up recently to get an update of his ongoing monitoring study. “We opened up both of the walls on the Carter Scott house — the north wall and the south wall — and we found shockingly little damage in spite of two winters of high moisture content readings at the sheathing,” Ueno told me. “When we opened up the north and south walls, everything looked fine. When we pushed on the cellulose, there were no signs of major voids. It looked like a classic dense-pack installation. There was just some surface rust on the staples and fasteners, and a little grain raised on the sheathing, but no mold or problems like that. I don’t think these walls are showing a problem.”

    But I agree with you, "sheathing that is wet and warm for a while CAN rot". Someone has to quantify HOW wet, HOW warm, for HOW long.

    There have been "dead bodies", but only in cases of high air leakage or water intrusion.

    David P. gets the credit for seeing that, then trusting his gut, and possibly steering the industry toward a simpler wall/roof assembly.

  53. user-1102578 | | #53

    Response to: a couple of material notes by Ken Levenson
    Earlier in the thread John Straube mentioned Tyvek "just like those German vapor-permeable membranes you mention, except affordable and locally available" and Ken Levenson disagreed: "John, not true. Tyvek is not airtight, is not waterproof".

    FYI we had success in our deep energy retrofit at using only Tyvek (CommercialWrap) as the sole air barrier (.9ach50): (note the blower door was done when the studs were bare and before drywall was in). I should also note that Tyvek is rated by the manufacturer as an air barrier.

    Many other "off the shelf" WRBs I looked at could not work as an air barrier but Tyvek can. Note that we used it in our project for the exact reasons that John pointed out -- affordable and easy to source.

    Lastly Superstorn Sandy hit before we had the roofing, exterior insulation nor siding on -- just the Tyvek (entire house wrapped in Tyvek button-capped and taped like a x-mas present). I stayed in the house that night because I was worried; we had zero tears and zero leaks.

    I'm sure there are other superior membranes out there but I was very concerned about performance for the dollar in our project. Tyvek performed.

  54. loracekim | | #54

    BSI recent newsletter
    In his latest newsletter Joseph Lstiburek does recommend a fully wrapped house -
    here is the link:
    And he states:
    "Wrapped in a back fully-adhered membrane. This membrane is the water control layer, the air control layer and the vapor control layer. It is clearly continuous. It is mastic sealed to the exterior face of the perimeter concrete foundation wall. We end up with an airtight concrete box at the bottom of the house (the basement floor slab is sealed to the perimeter concrete foundation wall that is sealed to a membrane wrapped sheathed structural frame. It does not get tighter than this. In terms of airtightness, it is so simple, so straightforward, so effective even a caveman can do it. Don’t like black? Use a blue membrane. Don’t like vapor closed? Use vapor open it does not matter. The plywood or the OSB is the vapor “throttle”."

    So my questions is does his suggestion work because we then wrap the house in 4-6 inches of pink or blue insulation vs the double wall under discussion here? and if so why doesn't a airtight house like Joe describes still have moisture issues in the cellulose on the interior walls since technically they would not breath out either. On that note wouldn't a house with a breathable membrane and 4- 6 inches of insulation still technically be air and vapor tight if the exterior insulation is installed tight?

    A different question what about a hybrid design - airtight walls and lower roof but above the insulation in the attic have a open ridge above the cellulose?

    Thanks for this great discussion!

  55. kevin_in_denver | | #55

    Response to Mike Strecker
    Joe's house doesn't have any moisture issues because the foam keeps the cellulose warm and above the dewpoint. For the same reason his membrane can be vapor closed. And covered tightly with impermeable foam.

    "airtight walls and lower roof but above the insulation in the attic have a open ridge above the cellulose?"

    In the Shirley Wall, the cellulose should be dense packed against any cold surface in order to pull the moisture out of it. Including the lower roof sheathing.

  56. user-1061844 | | #56

    70% RH or 80% RH mold growth (or 18M% or 20M%)
    Dana, would like to put the facts next to each other. If 70% RH in insulated cavity causes mold per your comment, then 80% is must be really bad. It happens that 80% is about equal to 20MC% in OSB or Plywood - which as indicated by Ueno and the like is the level where the real concern for rot and mold starts. We advocate for keeping the MC% below 15% in OSB/ply/cellulose to steer clear of any issues, just like you do advocate for drier insulation/assemblies. do like to note that the chance for mold to occur above 80%RH is substantially higher then below it. (ASHREA 160P uses it as a save limit too).

    Coming back to INTELLO's property, which rapidly becomes more vapor permeable right after reaching 70% (check the WUFI material library for that if you would like to know exactly). This property prevents issues w. construction moisture or even short term elevated humidity in bathrooms rushing into the insulated cavities in cold weather (see also <a

  57. GBA Editor
    Martin Holladay | | #57

    Response to Mike Strecker
    Your questions were addressed in the article. I wrote, "Posluszny covered the exterior side of his wall and roof sheathing with Grace Ice & Water Shield — a peel-and-stick rubberized asphalt membrane that is impermeable to water vapor. This approach is typically used for a PERSIST house, where 100% of the insulation (usually rigid foam) is installed on the exterior side of the Ice & Water Shield. Unlike PERSIST builders, however, Posluszny put all of his insulation on the interior side of the peel-and-stick."

    The wall you describe -- one with 4 to 6 inches of rigid foam on the exterior side of the peel-and-stick -- is a type of PERSIST wall. This type of wall works for the reason given by Kevin Dickson -- the vapor barrier is on the warm side of the rigid foam insulation, not on the cold side as it is at the Posluszny house.

  58. loracekim | | #58

    Thanks Kevin and Martin!
    I did miss the comment in your article. Not to open another can of worms but to me it seems logical that the persist type house Joe describes is the way to go. Like Joe said - "it is so simple, so straightforward, so effective even a caveman can do it" I know it misses on some of the Green points but if we are going to have the average builder join in on the bandwagon we have to develop a solid design. Not sure the average builder or homeowner doing there own work would stick to the level of detail that Posluszny does to make his system work. Just a thought and thanks again!!

  59. Expert Member
    MALCOLM TAYLOR | | #59

    Reply to Mike Strecker
    Joe makes a point of calling it a "perfect wall" because it came out of an exercise to design purely for performance and without any considerations of cost. It may come out alright when compared to other complex envelope systems, but Joe acknowledges it is a good step up in price from how most of us build now.

  60. kevin_in_denver | | #60

    Joe vs. David: Buildability
    The perfect wall is robust and can tolerate some field errors. The Shirley wall is simpler, though.

    The perfect wall needs 6"+ screws to hold the furring strips. It's hard to fish those screws through 4"+ of foam and hit the stud.

    The perfect wall also needs these plywood "prebuilt" window boxes that must be installed plumb, level, square, flush and carefully air sealed. I can't even imagine describing the process to a production builder, much less expecting them to implement it correctly and inexpensively.

  61. user-1006180 | | #61

    pretty good wall
    Can I diverge somewhat, while trying to take the results of this "experiment" into the real world?

    How about this for a fairly low cost, 'pretty good wall' for Zone 6- cover plywood sheathing with Solitex Mento Plus and metal panel siding screwed directly over it, with no furring, as the channels in the metal siding would form a decent drainage layer. A 2 x 6 stud wall (necessary to meet IBC for a three-story, 1200 s.f. apartment building) would be furred with on-edge horizontal 2x4s filled with cellulose, with drywall over.

    The idea is to produce a well built, very energy-efficient apartment building while keeping the budget as low as possible.

    I know this is somewhat less satisfactory than a true double wall, but it does provide a chase for wiring. I would frame using the "New USA Wall" model advocated by Gregory La Vardera, which would cut the thermal bridging quite a bit. His model calls for Roxul ComfortBatt insulation and a vapor retarder/barrier attached the the interior side of the 2x6 framing. The problem with that membrane location, of course, is that the insulation would have to be done in two operations. Using the Solitex on the exterior should allow the wall to dry to both sides.

  62. GBA Editor
    Martin Holladay | | #62

    Response to Nick Burnett
    There are lots of ways to build a wall. The method you are describing is sometimes called a "Mooney Wall." The method has been discussed on GBA several times, including here: Mooney wall for greater insulation value.

    The R-value of a Mooney wall is about R-25. If that's enough R-value to satisfy you, the Mooney wall is an acceptable approach.

  63. schreib77 | | #63

    It's been about 5 years since this was published. It would be interesting to know if the results touted by the author are still real. Any chance we could find that out and publish details?

    1. Expert Member
      Dana Dorsett | | #64

      Me too! I'm definitely curious about the peak early spring moisture content readings of the sheathing, now that it's been long enough for it to stabilize to regular seasonal cycling.

    2. creativedestruction | | #65

      For anyone interested and still following this old article, David did post a video showing him opening up a wall and checking the sheathing from the inside in March of 2018:

      Note he actually removes a recessed speaker from the drywall--I'm sure that falls short of an interior air barrier. No actual moisture measurements, but the area he looks at shows no signs of mold or rot. He stops short of saying "it's dry" though, and it could be my imagination but in the video the cellulose looks just a bit darker the closer it is to the sheathing.

      It's a curious experiment. My inkling is that in the near-absolute absence of bulk water exposure and air leakage, the interior moisture sources can in fact be managed by the hygric buffering even with an exterior vapor-closed membrane. Would love to see the inside of the roof ridge. And more actual moisture readings.

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