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Is it possible to reach a dew point inside of a wall cavity that is insulated with 3.5 inches of closed cell spray foam?

Tim Geiger | Posted in Energy Efficiency and Durability on

My wall assembly consists of fiber cement siding, Tyvek Drain Wrap, 1″ XPS Foam Board, OSB sheeting, 3.5 inches of closed cell spray foam inside of a 2 X 6 stud wall, drywall, latex paint. I have complete the Advanced Green Building Science course and learned how to calculate dew points, but they don’t take in account the permeability of the cavity insulation . I am not finding any information and/or specifics for closed cell spray foam from this site or other creditable sites. My spray foam contractor is saying that moisture can not move trough the spray foam or through the XPS so moisture should never get to the OSB and create a mold problem.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Tim,
    Three and a half inches of closed-cell foam is indeed a very good vapor barrier, with a permeance below 0.8 perm, so there is no need for you to worry about vapor diffusion from the interior.

    Someone is going to come along and criticize you for sandwiching OSB between two layers of vapor-impermeable foam. That's not a great detail -- not because of vapor diffusion, but because the OSB won't dry out very well if a flashing problem ever causes the OSB to get wet.

  2. Expert Member
    Armando Cobo | | #2

    Ditto... I agree with Martin. If you need more Rv, I would go with 1.5"- 2" XPS & open cell (if you want foam); I would rather use celullose.

  3. Expert Member
    Michael Maines | | #3

    Tim, EPS foam board will allow some drying to the outside and the R-value isn't terrible.

    Your spray foam will block vapor, but it's not foolproof. Sometimes there are framing details that cause the spray foamers to miss a spot. Sometimes the framing lumber shrinks enough to leave a tiny crack where it meets the foam. Sometimes a subcontractor hacks away some foam to route an afterthought wire or pipe and it doesn't get re-foamed properly. Sometimes windows fail or the roof leaks. Of course none of those things ever happen on our jobs but the idea is that you should plan for the possibility of failure somewhere in the system.

    More forgiving approaches include permeable insulation in the stud bays (dense pack, open cell, fiberglass or other mineral fiber, sawdust....), tougher sheathing (cdx, sawn lumber), or rigid foam on the inside of the walls instead of the outside.

  4. Brett Moyer | | #4

    Tim,

    Closed cell spray foam is extremely expensive. In my opinion, it only makes sense where there is a cavity issue (significantly higher R-value for small areas). If you are set on foam and a 2x6 wall, put more to the exterior of the sheathing layer.

    I would use a less expensive cellulose insulation for the wall cavities and spend the money on air sealing. If you build a tight, air-sealed wall, there is no reason to use spray foam.

    FYI: I have heard a couple builders talk about "weird" acoustical issues with thick closed-cell spray foam cavity fills. It makes the home feel like an "echo chamber," they said. Both of them told me if they could do it over, they would have chosen open-cell spray foam for the wall cavities.

  5. bpi na | | #5

    Great posts so far. Your wall is not a good design for many stated reasons. Avoid OSB in general and especially in superinsulated construction.

    I didn't see your location mentioned.

  6. Tim Geiger | | #6

    Cold Climate

  7. GBA Editor
    Martin Holladay | | #7

    Tim,
    Assuming you take everyone's advice and switch from closed-cell spray foam to cellulose or open-cell foam, your exterior XPS foam will only work in Marine Zone 4 or warmer. If you are in Climate Zone 5, 6, 7, or 8, your exterior foam will need to be thicker.

    Read more here:
    Calculating the Minimum Thickness of Rigid Foam Sheathing

  8. Tim Geiger | | #8

    My other option is to have dense pack cellulose(NU-WOOL) blown into the cavity. Now I would have vapor diffusion from the interior when the interior RH is not controlled properly by the Homeowner. Correct? Also I am told by my fiber cement Sales rep that I can not install the siding over anything more than 1" foam. He also is not sure about installing it over fur strips creating a air space. I am being told that the Tyvek Drain Wrap is going to accomplish this for me. He says the siding would be wavy.

  9. GBA Editor
    Martin Holladay | | #9

    Tim,
    It's perfectly possible to install fiber-cement siding on furring strips over 3-in. or 4-in. foam. If your siding rep hasn't heard of the technique, tell him to contact Brice Hereford at FastenMaster Screws (BHereford [at] olyfast [dot] com; Tel: 800-518-3569 or 413-537-4219).

    FastenMaster has come out with recommendations for the number of screws needed to hold furring strips over foam for a variety of siding types, including fiber-cement. (This is the topic of one of my upcoming blogs.)

  10. Tim Geiger | | #10

    Martin,
    Would I be better off using cellulose in this wall assembly or changing from XPS to EPS to allow for drying of the OSB?

  11. Riversong | | #11

    That's something of an indictment of the Advanced Green Building course if it led you to this kind of wall system and didn't give you the tools to model hygrothermal performance.

    It's easy to find dewpoints from a psychrometric chart, but that data is meaningless without knowing the local relative humidity at each point through the building envelope, which depends on interior and exterior conditions, the thermal resistance and the vapor permeance of each layer.

    But even that detailed modelling is immaterial, since moisture problems are far more a function of air movement and bulk leakage than of vapor diffusion.

    As others here have noted, sandwiching a highly moisture vulnerable material like OSB between two relatively impermeable layers of plastic is a recipe for premature failure - not mold but decay.

    Remember that both mold and decay organisms require two environmental conditions to flourish: moisture and warmth. The "warm sheathing" approach can provide both if there is ever moisture intrusion because of exterior weather-sealing defects, poor maintenance or aging, shrinkage or settling of materials.

    The single most important factor in durability of wood-based building materials is the ability to quickly dry once wet. Contributing to that is the ability to store and redistribute moisture rather than concentrate it. Non-hygroscopic plastics undermine rather than enhance these qualities.

  12. GBA Editor
    Martin Holladay | | #12

    Tim,
    I think the closed-cell foam is a mistake, so I definitely think you should switch to cellulose and beef up the thickness of your exterior foam layer. You still haven't told us your climate zone or location.

  13. Tim Geiger | | #13

    I am in The Thumb of MI, CZ 6. Had to find the map and blow it up, we are right on the edge of the green(CZ 5) and blue(CZ 6)

  14. GBA Editor
    Martin Holladay | | #14

    Tim,
    That means that your exterior foam sheathing should have a minimum R-value of R-7.5 or R-11.25, depending on which climate zone you want to join.

  15. Riversong | | #15

    we are right on the edge of the green(CZ 5) and blue(CZ 6)

    Ah. That might be the problem. Perhaps you should have taken the Advanced Blue Building Science course. ;-)

    By the way, if anyone is willing to come to Vermont for the weekend of May 28-29, I'll be once again offering my highly-acclaimed HygroThermal Engineering class.

    "Robert's presentation on moisture mechanics was the best presentation I have seen on the subject. I would highly recommend this workshop to all builders, architects and building trades people. I believe the issue of moisture as it relates to residential (and commercial) construction is one of the most important pieces of building science a builder today should have a strong working knowledge of."

    - Jay Walsh, Energy Analyst, Energy Star Homes and LEED-H Rater, Center for Ecological Technology

    http://www.yestermorrow.org/courses/detail/hygro-thermal-engineering-managing-moisture-in-the-home

  16. Tim Geiger | | #16

    If I eliminate the foam sheeting and increase the CC spray foam to 4.5 inches, thermal bridging would be greater, but the condensation would not occur on the OSB, right? I would still have an average R value of 22 as would the 1" XPS with 6" of cellulose.

  17. Tim Geiger | | #17

    The class was well taught, but should rephrase myself. I didn't feel the formula took into account the vapor profile of the assembly.

  18. GBA Editor
    Martin Holladay | | #18

    Tim,
    I don't think you get it. There is no condensation. The closed-cell spray foam is a vapor barrier.

    To learn the basics, I suggest the following articles:
    How To Avoid Condensation in Your Walls

    Vapor Retarders and Vapor Barriers

  19. Riversong | | #19

    If I eliminate the foam sheeting and increase the CC spray foam to 4.5 inches, thermal bridging would be greater, but the condensation would not occur on the OSB, right?

    Unless the interior has uncontrolled humidity (such as a swimming pool enclosure), diffusion is really not the issue and creates an over-simplified model if considered in isolation. That wall assembly has not only thermal bridges in the framing but also moisture bridges through the wood, and there may be any number of imperfections or voids or separations that can allow air movement. Except in laboratory conditions, condensation occurs everywhere all the time - the only question is how much and can the materials and assemblies tolerate it?

    I would still have an average R value of 22 as would the 1" XPS with 6" of cellulose.

    Assuming R-6.25 for the foam and 16" oc framing, the clear wall R-value would be 19.6, but with the typical 25% framing proportion would offer a whole-wall R-value of 15.9.

    With dense-pack cellulose and 1" XPS exterior, the clear wall R-value would be 22.2 and the whole wall would be R-20.

    While all spray foams are excellent for air sealing, they vary, often widely, in their density, R-value, blowing agent, water resistance, vapor permeability.

  20. Tim Geiger | | #20

    Martin,
    Assuming I don't have any flashing problems I will not have moisture in the walls. So the answer to the original question is simply NO! I could not get my classmates to agree with me when I said there can't be condensation in the wall because moisture (vapor) could not get there, without a failure somewhere else.

  21. Bill . | | #21

    Tim, number one issue with your wall is OSB. Not a good material in a superinsulated assembly.

    If you move the foam sheet to behind the drywall, you are stopping thermal bridging and not putting foam on two sides of OSB which is the worst thing you can do.

    Robert, what is your opinion on this assembly?

    fibercement siding
    Typar
    1/2 plywood
    2x6 24" O.C. advanced framing filled with dense pack cellulose
    2 inches of XPS taped w/ taped seams
    electric boxes in foam but backed up with another piece of 2" XPS.
    2 layers 5/8 drywall for thermal mass all plates acoustic sealant applied.
    Latex paints inside and out.

    Whole assembly R value is _______? close to R 28-30?
    Moisture handling rating __________? very good?

  22. Riversong | | #22

    I could not get my classmates to agree with me when I said there can't be condensation in the wall because moisture (vapor) could not get there, without a failure somewhere else.

    Good for them for not accepting such an over-simplified answer. There are far too many variables with sprayed foam, including the environmental variables and mix ratio and application rate and technique that all conspire to determine the actual percentage of closed cells, the quality of adhesion to substrates and the amount of shrinkage or voids.

    The note for spray urethane foams in the BSC permeance table warns: "While all spray foams are excellent for air sealing, they vary, often widely, in their density, R-value, blowing agent, water resistance, vapor permeability. These last two can have the greatest affect on just how you use spray foam in various building assemblies."

    And the most perfect foam installation only seals the stud cavities, not the rest of the framing assemblies and intersections.

  23. Riversong | | #23

    Bill,

    I'm not a fan of fiber cement siding, and the BSC table gives it a perm of only 1.5 and advises the use of furring strips behind it. So I don't think it makes the most breatheable exterior cladding. I still prefer real wood with latex stain (not paint).

    Plywood is much better than OSB in both durability and vapor permeance, particularly since its perm increases with increased moisture content (kind of like MemBrane). But it's dry cup perm is only about 1, which is low for an exterior cladding. I've always aimed for the old rule of thumb for a cold climate: keep the exterior skin 5x as permeable as the interior skin. Since codes require an interior perm of 1, I aim for an exterior perm of 5 (which, by the way, is the perm of #15 felt, the IRC standard for WRB).

    I'd much rather see interior foam board than exterior, and placing it inside allows the use of foil-faced polyiso, which is more environmentally benign and creates a radiant barrier if strapping is used (which is my preference).

    If the backs of the electrical boxes are foamed shut, there is no need for an additional piece of rigid behind them, since there will be nearly 5" of cellulose there.

    I'm not convinced that the extra thickness of DW will add much usable thermal mass. I'd rather see direct-gain passive solar mass in floors or wall wainscoting.

    Your wall assembly will have a whole wall R-value, at 16% OVE framing, of 27.3.

  24. Expert Member
    Armando Cobo | | #24

    Tim,
    The Advanced Building Science class you just took gives you all you need to figure this out (I teach the same class); and if you go back in the book, you’ll see that the formula given to do a dew point analysis is a very simplistic way to calculate the dew point of a wall. Also, it assumes you have a very good seal and moisture detailing. With the psychrometric chart you can figure out the Dew Point for any temperature @ any humidity and with the Climate Consultant 5 you should have found that Saginaw (Closest I know) the 3 month average low monthly outdoor temperature is 21°F.
    So for example: 70°F, RH30% = DP @ 36°F, so you need 2” XPS outside the plywood. or 70°F, RH40% = DP @ 36°F, so you need 4” XPS outside the plywood. That means you must make sure you maintain the RH below the designed DP.
    If you really want to figure this out with certainty, I recommend you hire a ME to do a WUFI analysis, which is a much more detailed program. That was also studied in you class, I’m sure.

  25. Expert Member
    Armando Cobo | | #25

    Sorry, the DP=45°F @ 70°F, RH40% not 36°F.

  26. Riversong | | #26

    Armando's calculations only indicate how useless such simplistic models are. Nowhere in Michigan would 4" of exterior insulation be required.

    The Canadian Building Code, which has been way ahead of US practice, requires only 20% of the R-value to be exterior up to 9000 HDD at indoor RH of 36%. And this ratio was developed from extensive research in cold climate construction.

    The IECC allows a reduction to class III vapor retarder (vapor semi-permeable) when, in climate zone 5, a 2x4 wall has R-5 exterior or a 2x6 wall has R-7.5 exterior.

    Armando, perhaps you'd like to explain how you derived that 4" XPS (R-20) conclusion.

  27. Expert Member
    Armando Cobo | | #27

    Here is the way Dew Point is taught in the class. In Climate Consultant 5 I found the three coldest month average for Saginaw, MI is 21°F. In the psychrometric chart the dew point for 70°F, 40%RH is 45°F. The simplified formula is Ts=Ti-DT(Rc/Rt), where Ts is sheathing temp, Ti is interior temp, DT is delta T between inside temp and outside temp, Rc is the cavity Rvalue and Rt is total wall assembly. Using R20 cavity, 4”XPS R20, Int. temp at 70°F and RH40% > Ts=70-(70-21)(20/40) > Ts=70-49(.50) > Ts=70-25 > Ts=45°F which is the same as the dew point.
    As a comparison, in the psychrometric chart the dew point for 70°F, 30%RH is 36°F. The simplified formula is Ts=Ti-DT(Rc/Rt), where Ts is sheathing temp, Ti is interior temp, DT is delta T between inside temp and outside temp, Rc is the cavity Rvalue and Rt is total wall assembly. Using R20 cavity, 2”XPS R10, Int. temp at 70°F and RH30% > Ts=70-(70-21)(20/30) > Ts=70-49(.67) > Ts=70-33 > Ts=37°F which is 1°F warmer than the dew point.
    Obviously, this means that if the occupants live at a higher interior temp. or higher humidity, the wall assembly may fail. Again, this is a very simplistic way to figure the dew point on a wall. Perhaps Peter Yost would chime in since I think he wrote the teaching manual. For what is worth, I learned this same technique from BSC over a decade a go, and again, if you need more assertion, hire a ME.
    As full disclosure, the class book and manual have plenty of disclosures about this quick method.

  28. Riversong | | #28

    That "simplified" formula assumes that the dew point remains the same throughout the envelope, and ignores the permeance of each layer and the outdoor relative humidity (the vapor pressure differential). There is a humidity gradient, depending on delta-RH and permeance the same as there is a temperature gradient depending on delta-T and R-value (or U).

    In fact, with a 70° indoor and a 21°/80% outdoor (winters in most areas have high RH), an R-20 cavity insulation, R-10 exterior foam, and an interior 1-perm vapor retarder, it would only be necessary to keep the interior RH below 38% to prevent condensation at the sheathing.

    To claim that

    Obviously, this means that if the occupants live at a higher interior temp. or higher humidity, the wall assembly may fail.

    is absurd, since failure depends on the time/humidity/temperature relationship, the moisture vulnerability of the materials, the moisture redistribution and storage capacity of all adjoining materials, and the mechanical properties of the materials and their connections.

    A formula that results in an exterior/interior R-value ratio that far exceeds what both code and field studies have suggested is necessary or prudent is clearly a useless tool. Sometimes simple is convenient, but simplistic is often worthless when it comes to complex energy and mass flux through multi-media building assemblies.

    Only when designers and builders understand the principles involved, can intelligent decisions be made.

  29. Michael Martinez | | #29

    I jsut happen to have taken the same class and came up with the same answers that Armando Cobo did. I think its a good way for us "normal" builders to get some answers. It's amaizing how some people feel their point of view is the only right point of view. Im pretty sure NAHB did not asked several highly educated folks to develop a course with wrong information, and I do believe more Dr. Joe Lstiburek than some know it all. Enough is enough.

  30. Riversong | | #30

    I think its a good way for us "normal" builders to get some answers

    If by "normal builders" you mean those who are satisfied with wrong answers and slightly enlightened ignorance, you've just described what's wrong with the building industry.

  31. Riversong | | #31

    Reformatted for you reading pleasure:

    I think its a good way for us "normal" builders to get some answers

    If by "normal builders" you mean those who are satisfied with wrong answers and slightly enlightened ignorance, you've just described what's wrong with the building industry.

  32. John Brooks | | #32

    I do believe more Dr. Joe Lstiburek than some know it all. Enough is enough.

    Michael M.,
    I think if you knew more about Dr. Joe... you would also know that he is famous for his mistakes.
    He is also generous about admitting and discussing his mistakes.

    Enough is NOT Enough

    I think the odds of Joe being "correct" are not much better than Mr. Know it All's
    I urge you to consider both possibilities.

    I will wager this image over any image I have ever seen at the NAHB
    https://www.greenbuildingadvisor.com/sites/default/files/images/Riversong%20window%20rough%20opening.jpg
    If you can post a better image of an elegant solution... I dare you

    And we all know that Mr. "Know it all" has never made a mistake
    at least according to Him ;--)

  33. Joe | | #34

    Well, maybe NHAB need to stop teaching their "wrong" class and they need to stop promoting the GBA website. Having looked at this website for a short time I can tell some people enjoy calling out everyone they disagree in mean spirited way.
    It was said the course teaches a simple way to do a wall analysis, and it was sais if you need aa sure answer to hire a mechanical engineer or do a wufi analysis... what else do you want.
    Maybe I would like to see Mr Ribersong and Mr Brooks blog about"their way" of doing dew point analysis and have the folks at ASHRAE, Bill Rose, Joe Lstiburek and Peter Yost amongst others critisize their post. But must important for this website is having difference of opinion in a respectful manner and not in an arrogant, insulting and glorified way,

  34. John Brooks | | #35

    Joe,
    you miss my point
    I encourage you to consider both possibilites.

  35. John Brooks | | #36

    I would love to see Bill Rose and Joe Lstiburek comment on this very Thread.
    I am not a Know it All
    Here's a virtual Beer for Armando & all
    I can not debate Joe or Bill on Building Science ... but I think Robert can

  36. Riversong | | #37

    And we all know that Mr. "Know it all" has never made a mistake
    at least according to Him ;--)

    If it's this Mr Right you're all referring to, I would agree with the above (at least in the realm of building).

    I have made a couple of errors in calculation that I've posted on this forum, and then quickly discovered and revealed those errors. To err is to stray or deviate from one's target - to miss the mark - and that we all do occasionally, those who are careful less often than others who are not.

    And, as I've taught others how to be capable carpenters, I've stressed that the difference between an amateur and an expert is that the expert knows how to turn mistakes into advantages, or at least how to quickly correct them with the least effort and the lowest cost so that they are invisible to others.

    But a mistake is normally understood as a misjudgement worthy of condemnation because it's based on a flaw in character - and this notion is based on the subconscious dominance in Western culture of the absurd notion of "original sin".

    If, as is the case, we were born into "original harmony", and the only sin was believing otherwise, then a mistake is nothing more than an error, a straying from the path, which is a necessary part of the process of learning and maturing and evolving (evolution occurs only because of genetic mutations, or mistakes). The path is typically not revealed to us except insofar as we wander from it, so we cannot be blamed for erring unless we repeat it.

    I have, for instance, long been a guide for those wandering the wilderness, both without and within. But I identify with the Hassidic notion of "guide" as one who does not necessarily know how to get out of the forest, but has been wandering in it long enough to know which paths NOT to take.

    The lesson, in fact, that I would strive to teach those I guided was that a "mistake" is nothing more than an opportunity for growth. The only REAL mistake is regretting one's mistakes and not learning from them. Once the opportunity for learning has been grasped, the "mistake" dissolves its negativity and becomes merely a signpost to steer us back to the path.

    "Mistakes are the portals of discovery."
    - James Joyce (1882 - 1941)

    "Mistakes are a part of being human. Appreciate your mistakes for what they are: precious life lessons that can only be learned the hard way. Unless it's a fatal mistake, which, at least, others can learn from."
    Al Franken, "Oh, the Things I Know", 2002

    As I've freely shared, here and elsewhere, I spent years making the mistake of building double-wall fiberglass-insulated homes, which had a slew of inherent problems, and then the mistake of applying the original Larsen Truss to new construction and ending up with two isolated insulation cavities, before I discovered the marriage of the two in the form of the Riversong Truss wall. Isn't it the case in life that the marriage of two incomplete entities can make a whole greater than the sum of its parts? Thus I used my "mistakes" to learn to come closer to the mark – that elusive bullseye that the most committed among us (the Zen archers) are aiming for.

    I offer my history so that others may learn from my mistakes, or errant wanderings, along the path to a house system that expresses harmony, and – more importantly – learn from the wisdom I've gleaned along the way.

    "Wise men profit more from fools than fools from wise men; for the wise men shun the mistakes of fools, but fools do not imitate the successes of the wise."
    Cato the Elder (234 BC - 149 BC), from Plutarch, Lives

  37. Riversong | | #38

    Regular Joe: But must important for this website is having difference of opinion in a respectful manner and not in an arrogant, insulting and glorified way

    There are far too many "opinons", most of them worthless, since they are not based on reliable experience or demonstrable scientific principles. Different perspectives, if well-founded, are what we should be inviting here. And we should also welcome the kind of lively debate which separates the wheat from the chaff.

    The real problem is when people cannot differentiate between honest, straightforward (and accurate) criticism and "insult". Legitimate critique is insulting only to those who prefer ignorance to knowledge or insist on a "right" to be wrong, or are satisfied with simplistic answers which are not solutions but only a deepening or perpetuation of the problem.

    "Many people today don't want honest answers insofar as honest means unpleasant or disturbing, They want a soft answer that turneth away anxiety."
    - Louis Kronenberger, journalist, publisher & professor (1904 -1980)

    "Freedom is the right to tell people what they do not want to hear."
    "We have now sunk to a depth at which the restatement of the obvious is the first duty of intelligent men."
    - George Orwell : English novelist, essayist, and critic, 1903-1950

    "Every man who says frankly and fully what he thinks is so far doing a public service. We should be grateful to him for attacking most unsparingly our most cherished opinions."
    - Sir Leslie Stephen (1832-1904), literary essayist, author,The Suppression of Poisonous Opinions, 1883

  38. Tim Geiger | | #39

    In my area many builders have been using wall assemblies like the one I have described. I went to our inspectors when I completed the class and showed them the formula in the book. I asked if they had any reported moisture/mold issues with this. They replied NO, not yet? They gave me some examples of moisture problems using cellulose with XPS over OSB, but haven't heard anything about CC Spray foam yet. They also said they are not qualified to answer any of these questions about dew point, etc. I have had issues with OC spray foam letting moisture into cavities when homeowners do not watch RH inside the house. I agree OSB can be eliminated. As builders we need to educate our customers. Also our inspectors need to get on the band wagon. I asked a few builder colleagues if I could test/check for moisture in the homes I worked on with them and they were against it!

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