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Musings of an Energy Nerd

When Sunshine Drives Moisture Into Walls

Because of inward solar vapor drive, vapor diffusion from the outside inward is often more worrisome than vapor diffusion from the inside outward — so you need a good vapor barrier strategy

Forgetting the air space behind the bricks sure didn't help. After only 10 weeks of occupancy, some new homes built by Zaring of Cincinnati were so wet that most of the brick veneer, sheathing, insulation, and drywall had to be removed and demolished. A portion of the defective walls were sheathed with Celotex fiberboard, which is so vapor-permeable that moisture held in the brick veneer was easily driven into the wall cavity when sun shone on the bricks.
Image Credit: Steve Bostwick

Builders have worried about wintertime vapor diffusion ever since 1938, when Tyler Stewart Rogers published an influential article on condensation in the Architectural Record. Rogers’ article, “Preventing Condensation in Insulated Structures,” included this advice: “A vapor barrier undoubtedly should be employed on the warm side of any insulation as the first step in minimizing condensation.”

Rogers’ recommendation, which was eventually incorporated into most model building codes, was established dogma for over 40 years. Eventually, though, building scientists discovered that interior vapor barriers were causing more problems than they were solving.

Interior vapor barriers are rarely necessary, since wintertime vapor diffusion rarely leads to problems in walls or ceilings. A different phenomenon — summertime vapor diffusion — turns out to be a far more serious matter.

Something is rotten in Denmark

During the 1990s, summertime vapor diffusion began to wreak havoc with hundreds of North American homes. This epidemic in rotting walls was brought on by two changes in building practice: The first was the widespread adoption of air conditioning, while the second was one unleashed by Rogers himself: the use of interior polyethylene vapor barriers.

Rogers conceived of interior vapor barriers as a defense against the diffusion of water vapor from the interior of a home into cold wall cavities. Rogers failed to foresee that these vapor barriers would eventually be cooled by air conditioning — thereby turning into condensing surfaces that began dripping water into walls during the summer.

Zaring Homes goes bankrupt

As with many scientific discoveries, it took a series of disasters to fully illuminate the phenomenon of summertime vapor diffusion.

One early victim of this type of diffusion was Cincinnati builder Zaring Homes. In the mid-1990s, Zaring Homes was a thriving mid-size builder that completed over 1,500 new homes a year. But the company’s expansion plans came to a screeching halt…

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  1. user-788447 | | #1

    much appreciated
    I’ve heard many times the concept of ‘inward solar vapor drive’ but this is the first time I’ve been given a thorough explanation with examples of failures from the field to boot. This fills many gaps in my knowledge.

    This article extends the argument that winter time vapor transference is much less of an issue than previously thought. If I understand correctly its the power of the sun to ‘drive’ moisture caught in a reservoir that’s a real culprit, not so much the temperature differential between inside and outside of an effective air barrier (because the temperature differential can be as great and greater in winter cold climate conditions.)

    I cringe when I see reasons to use vinyl siding. I like how GBA’s companion website Environmental Building News (EBN) provides reports on the impacts of material manufacture, disposal, toxicity concerns, etc. Content on Green Building Advisor on the other hand tends to consider on site performance only. While this makes sense in terms of energy efficiency and creating a durable structure and assembly I think the two pools of knowledge could integrate to create product recommendations that both work in the field and include more considerations of the full life cycle impacts of manufactured components.

  2. GBA Editor
    Martin Holladay | | #2

    On vinyl siding
    J. Chesnut,
    In a discussion of the phenomenon of inward solar vapor drive, it's important to be scientific and dispassionate. Although plenty of green builders prefer stucco to vinyl siding, it's important to report evidence that walls with stucco are more prone to wet-wall problems than walls with vinyl siding. That's a fact. Understanding that fact helps us make better decisions and helps our homes last longer.

    It's up to each reader to weigh all the pluses and minuses of different materials when selecting products like siding. Of course, the manufacture of PVC is nasty. However, PVC insulation is routinely used for Romex wiring in most homes.

    I'm not an absolutist when it comes to the use of vinyl. Try to buy a car without vinyl! Perhaps you don't ever drive -- perhaps you ride your bicycle everywhere. If so, good for you. The rest of us make compromises every day.

  3. Bill Rose | | #3

    Old and new research

    ASHRAE funded a major study on solar vapor drive a few years ago. The final reports are probably only a few weeks away from being publicly available through ASHRAE. Should be interesting and very helpful. Dominique Derome was the PI. She has moved from Concordia University in Montreal to ETH in Zurich. The work was conducted in labs in Montreal and Leuven, Belgium as well as in the field.

    I think the first reference to this condition was A. G. Wilson "Condensation in insulated masonry walls in summer" in Moisture Problems in Buildings, RILEM/CIB Symposium, Helsinki, August 1965. Grant Wilson was Head of the Building Services Section, Division of Building Research, NRC Ottawa Canada. (h/t Joe Lstiburek) This work was done in heated and humidified test huts in Ottawa that had insulation and interior vapoUr barriers. The abstract notes "No moisture problems were encountered in winter, but some undesirable wetting conditions developed during the summer period." The conclusion of the paper reads:

    Moisture contained in masonry walls exposed to the sun is transferred by diffusion toward the inside of the building in summer. This vapour movement can result in condensation of water in the insulation and on the vapour barrier of walls incorporating permeable insulation on the inside of the masonry. Under adverse conditions, the amount of condensation can be sufficient to encourage deterioration of inner components of the wall. Problems arising from this source can be avoided by using an exterior cladding that is not readily wetted by rain, or where absorptive mansory is used, by ventilating with outside air between the masonry and inner components. Such venting may also result in less wetting of masonry during driving rain conditions, by equalizing air pressures across it, and in more rapid drying of wetted masonry during dry weather.


  4. GBA Editor
    Martin Holladay | | #4

    Response to Bill Rose
    I was unaware of A.G. Wilson's 1965 report and am grateful that you pointed it out.

  5. wwLuke | | #5

    reservoir claddings
    I really appreciate articles such as this on GBA. It's an excellent example of why we need building "science". But here's my question, Many of the components of these buildings experiencing catastrophic failures are more or less essential to the process of homebuilding in this day and age. AC, although problematic, is here to stay, Likewise with modified stucco, fibercement, and lick'stick stone. (-no excuse when building with brick IMHO) What is the best way to accommodate these types of materials? They're all more durable, lower maintenance, and more aesthetically pleasing than vinyl, LP, Canexel, etc.... and are thus chosen by many builders and designers who are looking to build 100 year + houses. Even if a builder isn't hitting the mark on energy efficiency they ought, at least to have the option of building something durable with on the shelf materials. I personally am a big fan of rain screens, but here in the dry inter mountain west its rare to find a builder who is familiar enough with the detailing to use them in a cost effective manner. In the vein of, "above all else, do no harm" I would appreciate your thoughts.

    BTW, rant warning ,

    I refuse to accept that vinyl siding is the only cost effective solution to solve this problem. We're stuck with PVC cladding on Romex, but as responsible builders and designers we cannot justify the continual use of PVC in every part of buildings, especially when the Europeans are phasing it out of buildings entirely.



  6. GBA Editor
    Martin Holladay | | #6

    Some materials are here to stay
    I'll address vinyl siding first: I never said that vinyl is the only cost-effective solution to this problem. I don't have vinyl siding on my own house and I'm no fan of vinyl. But if you and I choose to avoid vinyl, it's important to understand why we are avoiding it. It's not because of performance issues. Vinyl siding performs very well. We're avoiding it for other reasons.

    Back in the early 1990s, I used to hear project managers and experts from Vermont's Dept. of Historical Preservation announce in public meetings that the use of vinyl siding was dangerous because wrapping a building in plastic would trap moisture. This was utter nonsense, of course. It's important to clear the table of misconceptions and establish a few facts. Then you can choose any siding you prefer.

    You say that air conditioning, modified stucco, fiber-cement, and adhered manufactured stone are here to stay. (You may be right, but I certainly would never shed a tear if the manufactured stone industry withered and died.) All of these can be used in a home without the catastrophic failures experienced by Zaring Homes. The key is to include details to prevent inward solar vapor drive.

    If you install adhered manufactured stone over OSB without an air gap, your risks of failure are extremely high. If you install adhered manufactured stone over rigid foam sheathing, or in an installation that includes an air gap, you should be able to sleep at night without those fretful nightmares experienced by all the GBA readers who are now muttering, "Gee, I installed adhered manufactured stone over OSB on my last house. I wonder if the OSB has turned to oatmeal yet."

  7. Kevin Dickson, MSME | | #7

    Unintended Consequences
    It's fascinating to me how incremental changes in methods, starting in 1938, can add up to disaster by the 1990's.

    The lesson for today's builders is to pick a wall section from the BSC-approved list, and be very careful when changing anything or substituting anything.

  8. Kopper37 | | #8

    Down South
    Martin - You're correct when you say that this disaster requires four elements.

    Down here in the south, there are plenty of production builders that use brick w/ air gap, a combination of OSB and Celotex sheathing, fiberglass batts, and drywall.

    Some use housewrap, some do not . . . and I'm not sure how they get away with it. Of course, the housewrap doesn't help with solar driven moisture, but it does inhibit bulk water flow.

    Although I would never want to own such a house, they work. At least they aren't the disasters as seen with Zaring . The walls get moist (or wet during wind driven rain events, at least those without the WRB). Then they dry out.

    I question their long term durability. And their energy efficiency is obviously poor. But it goes to show you how a simple change can wreak havoc on a wall system (interior poly).

  9. wjrobinson | | #9

    mildew stink plus...
    Just stopped at a hotel... open door to lobby and the musty mildew smell almost knocked me off my feet.

    So needed a pitstop.. worse in bathroom, looked down to baseboard of vinyl wallpapered wall to find black line of yuk oozing.

    Imagine what's behind ?

  10. Phil M. | | #10

    Where does that leave the ICF (Insulated Concrete Foam) blocks that have Styrofoam on the inside and outside of a 6" concrete slab? Is there going to be a moisture problem with condensation in the deep south of Alabama and Florida with the HIGH moisture and heat/cool changes? All of the ICF sales people say there is no problem. Thank you.

  11. GBA Editor
    Martin Holladay | | #11

    Reponse to Phil
    The two layers of EPS included in most ICF walls are vapor retarders. Even if an ICF home has a reservoir cladding, any inward solar vapor drive is stopped by the EPS foam.

  12. GBA Editor
    Martin Holladay | | #12

    Response to Daniel Ernst
    I think you're right -- these houses are saved by the lack of polyethylene. Inward solar vapor drive is undoubtedly occurring in these homes, but the moisture isn't condensing in the wall cavities. Instead, the damp drywall is drying to the interior.

    Such homes are risky, if only because all it takes is a homeowner's decision to install vinyl wallpaper to push the homes into failure.

  13. mikey | | #13

    vapour barrier
    would you use an internal vapour barrier for a building in the UK? This would be solid brick wall (no cavity), with internal insulation behind. No ventilation gap and no air con. Internal insulation would likely be fiberglass in stud wall.

  14. GBA Editor
    Martin Holladay | | #14

    Response to Mikey
    1. No, I would not use an interior vapor barrier. I would use an interior vapor retarder (for example, vapor-retarding paint, kraft paper, or a "smart" retarder like MemBrain).

    2. I wouldn't use fiberglass in contact with a solid brick wall. Instead I would use closed-cell spray polyurethane foam or rigid foam insulation.

  15. mikey | | #15

    that's interesting, I don't
    that's interesting, I don't think membrain is available in the UK, I think we have an equivalent - intello plus but it's expensive. Why would you avoid fiberglass in this situation? Presumably polyurethane would not allow the brick to dry to the inside, perhaps EPS under a certain thickness would.

  16. Chris Ladner | | #16

    Retrofit in Arkansas
    Martin, as a long time subscriber to Building Green and a practitioner of commercial energy consulting and commissioning, I have really enjoyed the blog and resulting conversations. I am in the process of practicing what I preach through my own "deep green" renovation of a 1968 brick home. I have been researching cost-effective roof and wall insulation retrofits. The house is fairly typical for our area; brick, air gap, black fiberboard, poorly installed r-11 insulation and gyp board. Interior insulation, using rigid polyiso, has been my focus but your recent blog has scared the smarts out of me. It seems that many of us in the south want airtight homes, good wall insulation, levels, and NO water-related issues. Any ideas for a retrofit strategy?

  17. GBA Editor
    Martin Holladay | | #17

    Response to Mikey
    It would probably be best for you to get advice from a consultant who is familiar with British construction methods -- something I'm not familiar with.

    Fiberglass can dry out after it gets wet, but it performs poorly in basements where it is in contact with damp concrete. I'm not sure how it would perform in your house, but without an air gap, I'd be nervous.

  18. GBA Editor
    Martin Holladay | | #18

    Response to Chris Ladner
    Were you planning to remove the existing gypsum wallboard and fiberglass? You might be better off with open-cell spray polyurethane foam.

  19. Daniel Ernst | | #19

    Arkansas Retrofit
    Chris / Martin - I just saw something in the Rural Arkansas (July issue) that had me shaking my head.

    Doug Rye is a licensed architect in Saline County, host of the the "Home Remedies" radio show, and a columnist for the Rural Arkansas (Arkansas Electric Cooperative publication).

    In his latest column he was touting the benefits of Reftrofoam, a tri-polymer insulation product.

    Here's a quote from Doug Rye's article:

    "I have also learned that this (Retrofoam) can be a great application for brick veneer houses. In this case, small holes are drilled in the mortar joints and foam is injected into th entire airspace behind the brick. I am anxious to see this done soon."

    And that is "expert" advice!

    I don't have any solutions to share with you. You have a difficult task ahead. I guess my point is that you should carefully research any advice you get . . .

    Is the house a ranch, or a two-story?

  20. Chris Ladner | | #20

    Retrofit in Arkansas
    Yes and no. We are trying to be frugal with the amount of waste we generate so i was going to maintain the existing walls that were not affected by the remodel (60%?). That being said energy trumps waste on our priority list. We are also trying to balance the costs of wall sealing/insulation/drywall with the attic sealing/insulation and new windows (currently looking at r-5 minimum). The ductwork and evaporator are in the attic spaces and the benefits/costs of sealing the attic adds another variable to the list.

  21. Chris Ladner | | #21

    Retrofit in Arkansas
    Daniel, That solution was presented to me by a local insulator. I asked why they thought the air gap (drainage plane) was there in the first place????? This "solution" will be a problem for many homeowners in the future.

    No comment on Doug. He is heavily sponsored by the elec Co-Op's.

    It is a 2-story colonial with a 1-story kitchen/mud room (25% of the total).

  22. Cliff McAuliffe | | #22

    Retrofit in northern California
    I am in the process of replacing the cedar shingle siding on my home in northern California. I am replacing it with OSB sheathing, Tyvek house wrap and fiber-cement siding. This meets the local code requirements. My home is not air conditioned. Even with high efficiency windows, the temperature difference on the hottest summer days is 12-16 degrees.

    The existing cedar siding has not shown any moisture problems during its removal and there is no evidence of moisture problems existing inside. With this new exterior construction, would you think that I am in danger of a moisture problem?

    Thank you.

  23. GBA Editor
    Martin Holladay | | #23

    Response to Cliff
    The short answer is no.

    The long answer is, any wall can be subject to moisture problems if there are problems with the details -- for example, flashing details.

    I don't know how cold it gets in winter in your location, but I usually advise anyone who is replacing siding to seriously consider the installation of exterior rigid foam insulation. It will never be easier than it is now.

  24. Carl Mezoff | | #24

    Inward Solar Vapor Drive

    A fascinating article -- particularly for an Architect who has specified 6 mil poly under the gypboard regularly for years here in lower Connecticut! Perhaps I have been lucky, but I have not seen any problems yet. Perhaps because I regularly also call for wide roof overhangs, I have dodged the bullet by keeping the walls relatively dry in the first place.

    Another detail that may have saved me is the use of a 2" layer of foil-faced iso foam under the interior gypsum board. Presumably the foam prevents inwardly driven summer water vapor from reaching the gypboard's cold surface during the cooling season, and keeps the vapor in the wall cavity above the dewpoint.

  25. steve mindel | | #25

    Interesting article. Would typar over plywood help stop vapor drive? We use fiber cement clapboards almost exclusively. We've also used felt paper and would continue to do so, more labor, hard to get it to lay flat on the sunny sides. We've used cedar breather behind the fiber cement but it creates a ripply wall as it pulls in where the nails are. We've done rain screens with wood strapping and plastic products but it creates a lot of detailing problems around windows, corners and other trim. I would be interested in comments on the Delta dry product.

  26. GBA Editor
    Martin Holladay | | #26

    Response to Steve
    Typar does not stop inward solar vapor drive. Like all housewraps, Typar is designed to be vapor-permeable. When it's time for a damp wall to dry to the exterior, the high vapor permeability of housewrap is a useful characteristic. However, when moisture in the siding is being driven inward, its vapor permeability is a liability.

    Plywood is a vapor retarder, so it's more effective than Celotex or Homosote at resisting inward solar vapor drive. When dry, plywood has a permeance of about 0.5. The permeance of plywood rises as its moisture content rises, up to a maximum of about 20 perms -- making plywood a "smart retarder."

  27. steve mindel | | #27

    reply to Martin
    So, would felt paper work better? Is a rain screen still necessary? Better I'm sure, but necessary behind fiber cement? Would taping the joints of the plywood gain anything?


  28. GBA Editor
    Martin Holladay | | #28

    Second reply to Steve
    Q. Would felt paper work better?

    A. As I wrote in my article, "Homes with asphalt felt experience fewer problems with inward solar vapor drive than homes with plastic housewrap." Asphalt felt is less permeable than most plastic housewraps.

    Q. Is a rain screen still necessary?

    A. It depends on the details of your wall assembly -- your siding type, your choice of WRB, your sheathing type, your insulation material, and your choice of an interior vapor retarder -- as well as your climate and how much roof overhang you have. The less risky choices are provided in the bulleted list at the end of my article.

    A. Would taping the joints of the plywood gain anything?

    Q. Taping the joints of plywood sheathing improves air tightness but doesn't affect vapor permeance.

  29. wjrobinson | | #29

    Carl Mezoff's post is worth discussing further
    Martin could you discuss Carl's theory to do with foam sheeting under drywall?

  30. GBA Editor
    Martin Holladay | | #30

    Response to Adkac
    Moisture condenses on cold surfaces. In an air-conditioned home, interior poly is a cold surface. If there is no interior foam under the drywall, fiberglass batts allow the exterior moisture to contact the cold poly.

    If a builder installs a layer of rigid foam between the studs and the interior drywall (or between the studs and a layer of interior poly), and if the seams of the rigid foam are taped, then the first surface encountered by the inwardly driven moisture is the back of the rigid foam. But it's not a cold surface like the poly; it's warm. (The foam insulation separates the warm air between the studs from the cold air-conditioned air on the other side of the foam). With no cold surface, there is no condensation.

    Interior rigid foam addresses the problem of summertime condensation, just as exterior foam addresses the problem of wintertime condensation.

  31. Dan | | #31

    Rigid foam in attics
    Could rigid foam be safely installed between the rafters, spray foamed in place and then recovered with loose celulose?

  32. GBA Editor
    Martin Holladay | | #32

    Response to Dan
    Are you talking about rafters or joists? If you are talking about rafters, I'm not sure how you intend to install the cellulose.

    If you are insulating between your rafters, you need to decide whether to go with an unvented or a vented roof. It's possible to install rigid foam between the rafters, foamed in place as you describe, to define the ventilation channel. Ordinarily, 1.5 inch x 1.5 inch spacers are installed first, to keep the foam away from the roof sheathing.

    If you fill the rafters with cellulose, you'll need to install netting to hold the cellulose in place.

    If you're talking about joists, not rafters, the answer is yes.

  33. David M | | #33

    Would felt paper work better?
    I would like to add a couple of comments with respect to the paper vs housewrap issue.....

    1) The MVTR of asphalt saturated felts and papers increases in hot/humid environments. So in the exterior moisture drive situation, they are "dumb" membranes.

    2) Not all housewrap manufacturers believe that higher is better when it comes to MVTR. For theresevoir cladding/air conditioned interior combination, it is best to use a housewrap with an MVTR near the building code minimum.

    Thank you opening up the discussion.

  34. Dan | | #34

    Rigid foam in attics 2
    yes, Joists, not rafters. confusing my terms. I experience alot of heat in the attic (vented) that is transmitted to the second floor rooms. I also have a valted ceiling facing south that is packed full of cellulose to the point that the foam stand offs on the bottom of the roof deck has collapsed. My thinking to correct this problem is to add several inches of sheet foam to the roof deck, apply a second layer of sheathing (basically making a SIP panel) and then add shingles or standing seam panels. Would I need to correct the failed air channel in the rafters if I created an air channel between the new sheet foam and new sheathing? what would be a good method to pursue?

  35. GBA Editor
    Martin Holladay | | #35

    Response to Dan
    It's certainly possible to insulate a roof deck from above with one or two layers of rigid foam.

    To avoid moisture problems on the lowest level of roof sheathing, you need to be sure that your rigid foam has a high enough R-value to keep the sheathing above the dew point during the winter. It's also a good idea, as you propose, to include a vent channel between to top surface of the new rigid foam and the top layer of roof sheathing.

  36. bruce | | #36

    Use of polyethylene vapor barrier

    I am building a house in Iowa with the following wall system: Vertical 12" wide steel siding (board & batten), Tyvek, OSB, 2x6 studs, R-19 fiberglass insulation (asphatic inpregnanted kraft paper on one side of the insulation), sheetrock.. The house will be air conditioned. Would you use a polyethylene vapor barrier under the the sheetrock if was your house? What are the pros (if any) and cons of using a polyethylene vapor barrier in this situation?

    If I don't use a poly barrier, what are your comments regarding the use of the kraft paper faced R-19 insulation verse using an unfaced insulation?

    I am constructing the house now so I would appreciate your comments soon. Thanks.

  37. bruce | | #37

    Effectiveness of polyethylene vapor barriers

    Many years ago, I read some research regarding vapor barriers which I think was conducted at the University of Minnesota. If my memory is correct, I believe the researchers found that a polyethylene vapor barrier was ineffective if there was small tears or punctures in it. I think they said two pencil punctures in the 16" space between the studs was a problem.

    If that is true, then my concern with using a poly barrier in the midwest has been this: If the
    the vapor barrier allows passage of vapor into the wall cavity due to relatively small damage and condensation occurs, then the poly sheeting would hinder the drying of the wetted materials. Therefore, I have thought that it is better to not use it. What are your thoughts regarding this observation?

    In a stud wall, the poly sheet laps over the vertical edge of the bottom and top 2x6 sill plates and is held tightly in place by the sheetrock. Can vapor get into the wall cavity behind the poly barrier through the horizontal faces of the 2x6 material?

  38. GBA Editor
    Martin Holladay | | #38

    Response to Bruce's first post
    Interior polyethylene should never be used in an air-conditioned house.

    You have designed a wall with very poor performance. Depending on where you are building in Iowa, it may even be illegal.

    I assume you have visited the GreenBuildingAdvisor Web site because you want to build a green home. If I'm guessing correctly, then I strongly advise you to choose a wall system with a much higher R-value. Fiberglass batts perform poorly, and you will have a lot of thermal bridging through your studs.

  39. GBA Editor
    Martin Holladay | | #39

    Response to Bruce's second post
    Your question shows a confusion between air barriers and vapor retarders. If you are designing an air barrier, small holes matter.

    When it comes to a vapor retarder or vapor barrier, small holes are irrelevant. Interior polyethylene with many holes still works as an effective vapor retarder -- in fact, in most homes it is too effective.

    In your climate, the problem with interior polyethylene is not that it might not stop the diffusion of vapor. The problem is that it WILL stop the diffusion of vapor -- making it hard for a damp wall to dry to the interior during the summer.

    To learn about the difference between air barriers and vapor retarders, I suggest you read:

    Vapor Retarders and Vapor Barriers

    Forget Vapor Diffusion — Stop the Air Leaks!

  40. bruce | | #40

    Response to Bruce's first post by Martin Holladay
    Martin, Two questions: 1) Other than the R value and thermal bridging at the studs, what problems does wall have? 2) I believe there is foam insulation that "fits" behind the steel board and batten steel siding, would that be something to consider adding?

  41. GBA Editor
    Martin Holladay | | #41

    Another response to Bruce
    You're right that the main problem with the wall is its low R-value.

    In addition, you will need a good plan to create an air barrier for your wall, since fiberglass batts are particularly susceptible to air leakage.

    If you can install continuous rigid foam sheathing on the outside of your studs, that would definitely help.

  42. bruce | | #42

    Another response to Bruce by Martin Holladay
    Martin, Tyvek will be used over the OSB. Is that sufficient for the air barrier? You stated ".....since the fiberglass batts are particularly susceptible to air leakage." What is meant by "susceptible to air leakage"? Thanks.

  43. GBA Editor
    Martin Holladay | | #43

    Response to Bruce
    I'm beginning to suspect that you aren't reading any of the articles that I have been suggesting. You really need to start getting a foundation in the basics.

    No, Tyvek is not sufficient as an air barrier, although it can be one component in an air barrier system.

    To answer your second question: fiberglass batts do almost nothing to slow air movement (which is why fiberglass is often used as a furnace filter). Other types of insulation, like dense-packed cellulose, are much more effective at slowing air movement.

  44. bruce | | #44

    Response to Bruce by Martin Holladay
    Martin, I have read the articles that you suggested and others articles. Please bear with me as I try to understand the basics of the problem.

    From the various artices and discussions between respondents, it appears to me that a polyethylene vapor barrier should not be used in a wall in a house in the midwest (say Iowa, Illinois, South Dakota) if it is air conditioned, even though in the winter some vapor will (might) enter the wall and probably condense because the wall without the poly can dry. Is this a correct conclusion on my part?

  45. GBA Editor
    Martin Holladay | | #45

    Response to Bruce's latest post
    Yes, that's correct.

    You can do two things to reduce the chance of condensation:
    1. Install your drywall using the Airtight Drywall Approach.
    2. Install rigid foam sheathing (sized according to the needs of your climate) on the exterior of your wall.

  46. bruce | | #46

    Response to Bruce's latest post by Martin Hooaday
    Martin, Thank you. I appreciate your help. Where can I read about the Air Drywall Approach?

  47. bruce | | #47

    Kraft Paper Backed Fiberglass Insulation (asphat coated)
    Martin, Another question in trying to understand the problem of solar driven vapor. Would the use of kraft paper backed fiberglass insulation (asphat coated) in the walls of a air conditioned house present the same problem as the polyethylene sheet.

  48. bruce | | #48

    Dew Point Temperatures
    Martin, Where can i get information regarding the dew point temperature for various levels of humidity?

  49. GBA Editor
  50. bruce | | #50

    E-mail address
    Martin, Thank you again. Could you send me your e-mail address?

  51. GBA Editor
    Martin Holladay | | #51

    Response to Bruce's 9th question
    I can be reached at
    martin [at] greenbuildingadvisor [dot] com

  52. Gary | | #52

    Inward Solar Vapor Drive
    Ok, here's the deal. Our building is a 23 year old brick apartment complex. There is a vapor barrier on the outside and inside, also has regular insulation. During the summer it's almost raining inside the walls...measured at 91%RM and 72% H. In the past we have cut a hole in the wall to dry it out, but I think this is only a temp patch to the real problem. How do I solve this so we have never deal with it again? Please Help!!!

  53. GBA Editor
    Martin Holladay | | #53

    Response to Gary
    It's impossible for anyone to diagnose the problems at your building without a site visit. I don't know whether you are the owner or a tenant, but the first step toward finding a solution is to hire a home performance contractor or energy rater -- ideally someone certified by RESNET or BPI -- to visit the building and conduct some tests.

    A few comments:
    1. It is highly unusual for a brick building to have an exterior vapor barrier. You don't mention what material is used as an exterior vapor barrier.

    2. The basic solution to high indoor humidity during the summer is to install air conditioning.

  54. Fred | | #54

    New Home concerns
    I live in northern Kentucky and just completed a new ranch home. Construction features are 6" walls with blown in cellulouse on living level. Basement is walkout with approximattley 50% of the walls above ground with 6" studded construction. House is totally sheeted in OSB with Tyvek wrap. There is no interior vapor barrier on living level other than what might be reffered to as the air tight drywall approach. Are studded walls are brick wrapped with 1" air gap. Basement is currentlly unfinished and insulation is 6" J&M poly wrapped batting. What has prompted my concern is that I have stared to finish my basement and have discovered moisture behind the batting due to the cold surface temperature of the OSB board. I have started to run a de-humidifier and am installing 1/2" polystyrene board over the OSB and sealing the edges with foam. I then am putting back the batting and drywalling. Do you think I am OK with this or am I missing something else? Am I OK on the living level?

  55. GBA Editor
    Martin Holladay | | #55

    Response to Fred
    The best wall sheathing behind brick veneer is rigid foam (XPS or foil-faced polyiso), since these sheathings prevent inward solar vapor drive and solve the "cold OSB" problem.

    However, you have already chosen OSB. Your proposed solution -- installing rigid foam on the interior side of the damp OSB, between the studs -- is an acceptable solution, as long as it is carefully detailed and the foam is thick enough. Ideally, you will let the OSB dry out before installing the rigid foam, although the OSB will probably eventually dry out to the exterior.

    The foam you have chosen ( 1/2-in. polystyrene ) has an R-value of only about R-1.8 or perhaps R-2.5, depending on what type of polystyrene you have chosen. It would be better to choose thicker foam, in order to keep the interior surface of the foam above the dew point. Your thin foam may still be cold enough to allow moisture to accumulate on the interior side of the foam. To learn more on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  56. Fred | | #56

    More info
    Foam is GreenGuard from PACTIV Building Products. it's called XPS Insulation Board and is rated R-3. I'm using "Great Stuff" foam to seal all the edges. I did run fans and heaters to dry OSB before installing foam. Think I'm OK. On issue of living space with blown in cellulouse, do you think I'm go to go there? Thanks for your fast responce, lots of good info on your blog.


  57. GBA Editor
    Martin Holladay | | #57

    Second response to Fred
    Your cellulose-insulated walls are probably fine.

  58. Perry525 | | #58

    Water vapor.
    When you look at the weather, you see that the sun heats the area in the tropics causing water vapor to rise and flow towards the Poles. On its way it meets areas of cold, where the water vapor cools and turns into clouds, rain, hail, snow.
    Water vapor always moves towards areas of low pressure and cold.
    In the cold North the water vapor created in our centrally heated homes, by washing, cooking, breathing etc; looks for an exit to the cold outside or the nearest cold surface, usually a window to condense.
    In the South our air conditioned homes attract the water vapor inwards towards the cold interiors.
    Once the action of water vapor is understood, then the solution becomes simple.
    When warm wet air meets a cold area or surface, the air temperature drops, the air cannot hold the same amount of moisture and the water vapor falls out. It has reached its "Dew point."
    The solution is insulation. A thick enough layer of closed cell insulation that will inhibit the temperature drop that will cause the water vapor to turn into condensation.
    Its not the sun, its the differential between the warm wet air and the nearby cold surface, get rid of the differential and the problem goes.

  59. GBA Editor
    Martin Holladay | | #59

    Response to Roger Anthony
    You are half-right.

    You wrote, "The solution is insulation." Not really: there are many solutions to vapor drive and condensation problems, but it is overly simplistic to say, "The solution is insulation." I would say, "The solution is a well-designed wall assembly."

    Many different wall assemblies can work, as long as they address vapor drive and heat flow, and as long as the drying potential of the wall assembly exceeds its wetting potential.

    If someone reads your mantra -- "the solution is insulation" -- and just adds thick fiberglass batts to a poorly performing wall, their "solution" would be unlikely to address a condensation problem.

    Finally, you wrote, "It's not the sun, it's the differential between the warm wet air and the nearby cold surface." You are wrong when you write that "it's not the sun." In the type of wall failures that I am addressing, it is indeed the sun that is the engine of the vapor drive. The sun creates a microclimate on the south side of the house, raising the temperature of the damp siding and causing evaporation.

    Here's the proof that the sun is the cause of these problems: they never occur on the north side of a house.

  60. Perry525 | | #60

    Condensation Water Vapor
    You have to wonder how a designer or builder could

    forget all that they learnt as a child?

    How could they think that a thin sheet of plastic could

    stop condensation when one side has warm and wet air

    and the other side has colder air, air that is below the

    "Dew Point" of the warm air.

    Once the air temperature reaches its "Dew Point" the

    water vapor will fall out as condensation, rain, hail, frost,

    snow depending if you are indoors or out.

    Every day the majority of things that are round us are

    warmer than the air, condensation never forms on a

    warmer, than the surrounding air, surface.

    That is why insulation stops condensation.

    The problem is that water vapor is a very fine gas the water vapor molecules are small enough to pass through many building materials. They easily pass through drywall. To understand this, a useful comparison is that air molecules are the size of footballs and water vapour the size of ball bearings, that can exist in the spaces between the air molecules.

    The typical builder needs to show the utmost diligence when trying to install a water vapor proof barrier and because of the way buildings are designed they usually leave gaps, cracks that water vapor promptly enters.

    To understand the process you need to understand that like heat, water vapor is always attracted to cold and it will always head for the nearest cold area or surface. (Regardless of what the sun is doing ... there is no sun indoors on a cold winters night, yet water vapor still heads for the cold coil in a dehumidifier to condense.)
    This is where insulation comes in. It is critical to ensure that no cold surface is presented to warm wet air. Insulation is the simple barrier that stops the movement of water vapor to a cold area or surface.

    The manufacturers of SIPS have it right.

  61. GBA Editor
    Martin Holladay | | #61

    Roger, you are still only half right.
    Of course water vapor passes through drywall. That's what we mean when we say that drywall is vapor-permeable.

    However, you are still wrong when you write, "insulation stops condensation." The most common type of insulation -- fiberglass -- does not slow down the passage of moisture-laden air; nor does it slow down diffusion.

    If you are worried about solar-driven vapor drive, you need a vapor retarder, not insulation, on the exterior of your wall, and you need an interior surface that is vapor-permeable. Just piling on the fiberglass batts won't help.

  62. user-227802 | | #62

    Appling this principle to concrete basement walls.
    Now, think about this in regard to solar heating up the above grade portion of an 8" poured concrete basement wall facing south and west. Also solar heating up wet soil next to the first foot or so of the below gradeportion of that same wall. And you have an inch or two of thermax foil faced isocyanurate board attached to the whole interior surface of that wall. And the basement is at a early summertime temperature of around 75 degrees, or even lower. It's in the northern states, so the ground is still pretty cold down a few feet. What is to prevent condensation occuring at the first foil surface this solar driven moisture sees? ( I've seen this precise kind of moisture-drive where the basement walls are insulated with fiberglass and a 6 mil polyethylene; and of course that's asking for it.)

  63. GBA Editor
    Martin Holladay | | #63

    Response to Mark Faultersack
    Whenever you insulate a basement wall from the interior, you want to install foam as tightly as possible to the concrete wall. You want to either use closed-cell spray polyurethane foam (which adheres tightly to the concrete) or rigid foam that is attached with adhesive and/or mechanical fasteners.

    The side of the foam touching the concrete will be at exactly the same temperature as the concrete, so it won't be any colder than the concrete. Moreover, there won't be much of an air space there (if there is any air space at all). So really, there is no opportunity for condensation.

    Let's face it: the concrete will always be damp. After all, it's underground. You aren't going to prevent the concrete from being damp. You just want to be sure that you have a good layer of insulation between you and the concrete.

    With fiberglass batts in this location, the origin of the moisture is the interior air. Warm humid air from the basement condenses against the cold concrete. That's what causes problems.

    None of this has anything to do with inward solar vapor drive.

  64. user-227802 | | #64

    Applying this principle to concrete basement walls
    I think you've convinced me: water driven through the concrete would be in vapor form, the solar driven vapor that reaches the foil is no colder, so it won't condense, the foil stops any further progress of the moisture, yet does not absorb any, and the concrete remains wet as usual. Well, I'll try a test section on a wall, and check it in a year from now. In reference to the fiberglass wall I was describing, it was in the very hot period of the late summer and this house had a U-shaped floor plan facing south, so it trapped a lot of heat. The moisture was on the cavity side of the basement wall polyethylene sheeting, so I still believe it was driven in from the outside.

  65. BHons | | #65

    Mobile Home Mold Problem
    Hello all. I'm new to the site. I realize this discussion is rather old, but I found it while researching my problem and hope that someone can help. I have a 2002 double wide mobile home with Hardie Panel on 2x6 exeterior walls. The Hardie Panel is nailed straight to the studs. The interior walls are all prefinished drywall with vinyl wall paper. Very poorly installed faced fiberglass insulation is all that's in the walls. I noticed the drywall getting soft and a musty smell. I removed the drywall to find a small amount of mold and damp insulation. What I need is some suggestions on how to fix this problem (how to insulate) that won't brake the bank. I really don't want to reside the house and I do not want to put foam board on the interior of the walls. I should mention that we were keeping it way too cold in the house given the situation. Probably in the upper 60's to low 70's. Thanks in advance for any help.

  66. GBA Editor
    Martin Holladay | | #66

    Response to Brian Hons
    At a minimum, you have to remove the vinyl wallpaper and put it in a dumpster. Then repair the damp drywall, tape it, and paint it.

    If you have enough money to do that, and some left over, we can talk about insulation improvements.

  67. BHons | | #67

    Mobile Home Mold Problem
    Thank you for the quick response, Martin. Affording the drywall and finishing it is not going to be a problem. I am mostly concerned about the insulation. Closed cell foam is really not an option given the price. Would filling the cavity with open cell foam work? That may be a bit expensive, too. What about cut and cobble with 1 inch R-Matte Plus-3 Rmax and sealing the edges and seams with either foil tape or closed cell foam in a can, and then fill the cavity with fiberglass insulation? If so, should I go faced or unfaced with the fiberglass? Those are the things I have thought. Thoughts? Any other suggestions? Oh, I guess I should have mentioned I have already removed all the old insulation and drywall. That's how I discovered that the Hardie Panel is attached directly to the studs.

    By the way, I really like this site. Lots of great info. I'm a kind of a jack of all trades, but master of none, so I'm always looking to learn new things. Thanks for the help.

  68. Expert Member
    Dana Dorsett | | #68

    Since the Hardie is nailed to the studs...
    You need some amount of air space (doesn't take much) as capillary break between the siding and the interior. It doesn't take much- even 1/4" will do. The next layer should in should be as air tight as possible, and a weather resistant barrier WRB.

    What you might do is buy 1/4" fan-fold XPS siding underlayment- the unperforated kind (sold at some box stores, eg: ) pink, blue, green or whatever- they're all pretty similar. Use it both to establish a 1/4" gap, and as the WRB. Cut some 1" wide strips and tack them to the Hardie with dabs of foam board construction adhesive on each side of the stud, and another mid-way between the studs. Then cut pieces that fit the full length of the stud bay but about 1/2" narrower than the stud bay, tacking them to your space strips at the studs and, then seal the edges to the studs with a bead of can-foam. You'll then have a 5" deep cavity to work with, and a reasonably rigid air barrier on the exterior.

    If the budget allows, compressing R23 rock wool batts in there would work pretty well, as would high-density R21 "cathedral ceiling" batts. If the budget is tight a "contractor roll" of low density R19s would also work (unfaced or kraft faced, but not foil faced.)

    What is your climate zone/location? (It makes a bit of difference as to what to do with the interior side stackup.)

    Some fan-fold XPS has thin facers that bring it's permeance under 2 perms, but as long as it's at least 1 perm it's fine in this application. Some "dampproofing" have lower permeance facers, which might work, or maybe not depending on your climate and the rest of the stackup. Unless you're in US climate zone 2 or lower you DON'T want the fan fold with the reflective facers, but if that's where you're located, having the shiny-side facing the Hardie would give you a slightly better performance.

  69. BHons | | #69

    Response to Dana
    Thank you so much much for the info, Dana. We live 50 miles southeast of San Antonio, Texas.

  70. BHons | | #70

    One More Question
    Out of curiosity, would it be ok to use closed cell foam with no capillary break, or would this cause problems?

  71. GBA Editor
    Martin Holladay | | #71

    Response to Brian Hons
    Your house has no wall sheathing. If you install closed-cell spray foam against the back side of your siding, you will be permanently gluing the siding in place. That will make it impossible to remove any of the siding in the future for repairs or remodeling -- for example, if you ever want to remove the existing siding and install real sheathing on your house.

    Dana's suggestion that you include a capillary break behind the siding is a good one.

  72. Expert Member
    Dana Dorsett | | #72

    Response to Brian Hons
    In your US climate zone 2 location using low permeance materials like foil facers or vinyl faced XPS on the exterior side of the assembly is not a problem, so long as you don't use low permeance materials on the interior side. (No vinyl or foil wallpapers. Standard latex paints are fine.)

    A vented capillary break (= air gap cavity) is far preferable to gluing it all together with closed cell foam. If you want to use closed cell foam, installing some #15 felt or taut housewrap with 1/4" of clearance to the fiber cement siding and spraying the foam onto the WRB can work. But from price/performance point of view you'd be better off with 1/2" foil-faced polyiso or 1/4" XPS as the exterior side air-barrier. With a foil facer on the air-gap side you'll get a modest thermal performance boost out of it, primarily during the cooling season on siding that gets direct sun, but it's not enough to warrant paying extra for.

    Also, closed cell foam isn't exactly the greenest stuff (for that matter, neither is XPS) between the polymer content and high global warming potential HFC blowing agents on the order of 1000x CO2. Polyiso and EPS are blown with pentane at less than 10x CO2, and are typically 1-1.5lb per cubic foot vs. 2lbs. for closed cell polyurethane. The greenest foam-R is open cell polyurethane, which is 0.5-0.7lbs density and blown with water. If you're insisting on a foam solution, detailing an aluminized fabric type radiant barrier as the exterior side facing the 1/4" gap, and installing 5.25" of open cell foam works, but still has a higher environmental impact than the 1/4" fan-fold or 1/2" polyiso + fiber batt solution.

    Your original moisture problems were created by two MAJOR factors: A: The vinyl wallpaper reduced the drying path toward the interior to near-zero, and B: The fiber insulation was in direct contact with a porous siding type that stores rain & dew moisture, and the fiber wicks that moisture out of the siding, toward the wallboard. Either one on it's own would raise the mold risk considerably in your climate, but the combination is all but guaranteed to fail. With a capillary break to block the wicking, a vented channel to lower the moisture content of the siding, a lower permeance air-batter & vapor retarder on the exterior side of the fiber to limit the rate of moisture getting in via vapor diffusion (even during the intense vapor drives of sun on rain/dew wetted siding), and a more vapor open interior, you will have done about as much as can be done without re-siding.

    And it will be enough. Some amount of moisture is still going to get in via capillary wicking from the siding into the stud edges, but the enhanced drying of the siding to the exterior via the 1/4" gap helps with that, and the much faster drying rate from the cavity to the air-conditioned interior is also a huge benefit. If you filled up the cavity with closed cell foam, the comparatively lower vapor permeance of the closed cell foam would keep the studs from letting that moisture out via the sides of the studs. A fiber cavity fill would help redistribute the moisture, improving the overall drying rate for the studs, making mold/rot on the studs less likely.

  73. BHons | | #73

    To Dana
    Would 1/2" vinyl faced XPS be ok? None of the area stores have 1/4" in stock.

  74. GBA Editor
    Martin Holladay | | #74

    Response to Brian Hons
    Q. "Would 1/2-inch vinyl-faced XPS be OK?"

    A. Yes.

  75. BHons | | #75

    RE: Since the Hardie is nailed to the studs
    Dana mentioned that the XPS should have a perm rating of at least 1. The insulation I am looking to buy is Owens Corning Foamular XPS faced insulation sheathing. It has a perm rating of 0.2. Is this too low for my climate region? Again, we live about 50 miles southeast of San Antonio, TX.

  76. GBA Editor
    Martin Holladay | | #76

    Response to Brian Hons
    You seem to have forgotten that Dana modified his advice when you identified your climate zone.

    Dana wrote, "In your U.S. climate zone 2 location, using low-permeance materials like foil facers or vinyl-faced XPS on the exterior side of the assembly is not a problem, so long as you don't use low permeance materials on the interior side."

  77. RTL90 | | #77

    Hi Martin,

    Hope you don't mind another comment on this article. I was hoping to get your suggestions on the best solutions to improve the wall r value and airtightness in my home. I own an 1950s brick ranch located in Michigan in climate zone 5 but bordering a zone 6. The exterior of the home for the most part is a full course of brick, 3/4" to 1" air space (no real intake or exhaust openings for the airspace, the exhaust space is above the soffit and likely moving into the vented attic), fiberboard sheathing, 2 x 4 framing, interior foil faced paper batt then gypsum board with a plaster coat. I had the chance to do a full gut on a couple of our exterior walls as they weren't behind brick. We did new unfaced batt insulation, sheathing, wrb, and exterior xps foam insulation with a lot of attention to air sealing. We currently have the plaster off on an exterior wall that has brick cladding and now we are trying to determine the best solution for insulation. We use hydronic baseboard heat in the wintertime and a mini split for cooling in the summertime. It does get rather hot and humid here in the summertime. Interior vapor barrier, and likely retarder are out of the question with all of the strikes to this existing wall assembly. I wrote into FHB podcast and they were kind enough to discuss my issue and provide a solution. They suggested that I install felt paper against the sheathing on the interior side the use the batt insulation of my choice (I'm a big fan of mineral wool). How do you feel about this solution? I did initially say that I was hoping to avoid spray foam so that may have affected FHB's response. I am open to spray foam if it would provide a far superior assembly.

    I appreciate your time and any replies. Heck one day I might tear down all of the brick and sheathing and fully start over with new sheathing as well as a thick layer of exterior foam, new windows, and siding.

    Thank you,


    1. GBA Editor
      Martin Holladay | | #78

      As long as there is currently no obvious sign of water entry or moisture damage, either approach will be OK -- either rectangles of asphalt felt installed in each stud bay against the interior side of the sheathing, or a flash-and-batt approach. (For more on flash-and-batt, see "Flash-and-Batt Insulation.")

      1. RTL90 | | #79

        Thanks Martin. I think I'll go with the asphalt felt approach as I already have enough of those materials to complete the job. I did run a bead of caulk around the sheathing and framing members to help air seal. I'll then install felt against the sheathing followed by mineral wool batts.

        Appreciate your help.


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