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Too many vapor barriers! Help!

sanleera | Posted in Green Building Techniques on

This is the exterior wall assembly on this house: Vinyl siding, 1″ polyiso, then flash and batt (1″ closed cell in cavity. Then clear plastic was applied on the interior face of the wall. I’m thinking that has to go. What are our options? Is it possible to vent the plastic with strategic slits in the plastic? The drywaller is scheduled for next week, and I just pointed this out to the owner today. Thanks in advance for any help I can pass on to add weight to my advice to him!

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  1. GBA Editor
    Martin Holladay | | #1

    1. Where are you located? What's your climate?
    2. Take a deep breath.
    3. If your house is not yet drywalled, you have plenty of time. Just remove the interior polyethylene and put it in the Dumpster.

  2. sanleera | | #2

    Whoops, sorry, that info would be helpful, huh? We're in Western NY, near Buffalo. What would be the easiest/best way to keep the unfaced batts in place until the drywall is on?

  3. GBA Editor
    Martin Holladay | | #3

    Unfaced batts are friction fit. The batts on the walls should stay in place.

    If you have insulated ceilings with unfaced batts, some of them may want to fall down. Solutions include:
    1. MemBrain (which is pricey).
    2. Metal rods available at lumber yards -- they are a little wider than the framing bays, and they bite into the wood. They are designed to hold insulation, and they spring-fit between joists or rafters.
    3. Scraps of lath, cut a little longer than 14 1/2 in. (or 22 1/2 in), and sprung in place.

  4. sanleera | | #4

    Thank you so much, I will pass this along and circumvent a potential disaster.

  5. Anonymous | | #5

    Martin, you have said that with enough foam there is no condensation concern. That means the poly can stay right?

  6. Dan Kolbert | | #6

    No, wrong. There's always moisture somewhere, and it has to escape either inward or outward or it will rot things out (sometimes with remarkable speed).

    And there are other ways to keep the ceiling insulation in place - staple up mason's line or the plastic "twine", etc - you could even just leave strips of the existing poly in place to keep things in place. But the rods (also called Lightning Rods) are pretty easy to use if you need to.

  7. Riversong | | #7

    Ditto "there's always moisture".

    A typical 2200 SF house, with 17,600 board feet of lumber will release 300 gallons of water just from the lumber (never mind the concrete and other water-based materials like paint) in the first heating season as the wood acclimates from 19% moisture content (KD standard) to the typical 10% in a heated house.

    Not only is it foolish to trap that moisture in the walls, but during the winter in a cold climate - when interstitial condensation is an issue - almost all the moisture drive is from the inside out. That will reverse in the summer with high radiant drive, but few modern houses (with OSB and other such junk material) can withstand 6-9 months of wetting before drying inward.

    A good rule of thumb for a heating-dominated climate is to keep the outer skin of the house at least 5 times as vapor permeable as the inside skin. This means no exterior foam or ZIP wall or self-adhering flashings.

    By the way, slitting the poly vapor barrier will only allow more air movement into the walls and ceilings but do little to prevent condensation. An air barrier is fairly open with just a few percent of compromise, but a vapor barrier is 97% closed with 3% compromise. No house in the lower 48 should require an interior vapor barrier - the codes require only a 1 perm vapor retarder, such as VR primer.

  8. GBA Editor
    Martin Holladay | | #8

    I'm going to disagree with Robert about the dangers of winter condensation against sheathing in homes designed to dry to the interior.

    The old saying goes, "Condensation happens when it's cold, but rot happens when it's warm." Condensation that occurs as frost does not damage a house when it is frozen, because rot organisms need warm temperatures.

    Routine minor condensation happens in a lot of walls. But as long as the moisture can evaporate and dry to the interior when the weather warms up, the house will usually be fine.

    That said, it's better to design walls that don't allow condensation to occur. The thicker the exterior foam, the better.

  9. Riversong | | #9

    "Condensation happens when it's cold, but rot happens when it's warm."

    And therein lies the irony. We add lots of exterior foam in an attempt to prevent condensation on the sheathing (typically moisture-intolerant OSB), but the foam raises the temperature of the sheathing so that - if it does get wet - it stays in the danger zone for mold and decay organism growth. Canadian studies have proven this.

    And the solar radiant drying that is necessary for inward vapor drive is significantly reduced by high levels of exterior insulation, even more so if a rainscreen is used.

    Condensation cannot be stopped - it occurs all the time (except at absolute zero or 0% relative humidity). Condensation is not the problem - moisture accumulation is, particularly when it occurs in moisture-intolerant materials like OSB which absorbs moisture from framing, does not redistribute it well, and tends not to release it quickly enough.

  10. GBA Editor
    Martin Holladay | | #10

    We're going in circles here, and I'm not sure whether it's worth repeating myself.

    Nevertheless, here goes: the exterior foam reduces the chance of condensation by keeping the sheathing warm. You hypothesize a situation in which condensation occurs and the sheathing keeps cavities warm, encouraging rot. But I'm saying that studies show that foam of the proper thickness prevents the condensation in the first place.

  11. Anonymous | | #11

    Sandra's wall will not have summer condensation when rot happens. None. RH in wall will match RH in home.

    The poly could stay in place in most rooms.

    Personally, I would make sure bathrooms have vapor barrier paint, no poly, and use schluter systems under tile.

    The main concern for Sandra now and in the future is bulk water leaks from the exterior that gets past exterior foam board to OSB. The OSB will then rot easily because any water getting in her foam sandwich will not evaporate fast enough to avoid decay starting. Once decay starts it takes very little moisture to keep the process going.

    Window and door flashing must be done very well. Sandra, this flashing area is your main concern. Check into it here and at the home.

  12. GBA Editor
    Martin Holladay | | #12

    I agree with most of your advice -- except your comment that "The poly could stay in place in most rooms."

    Since the poly prevents drying to the interior, leaving it in place is risky. The poly is clearly unnecessary.

  13. Dan Kolbert | | #13

    "RH in wall will match RH in home."

    Not sure how you can say that. The whole point of a "barrier" is to prevent the flow of something across is.

  14. GBA Editor
    Martin Holladay | | #14

    I believe the answer to your question has more to do with air barriers than vapor retarders. If the main air barrier is established at the exterior sheathing, then the RH of the air inside the stud cavities is likely to be close to the RH of the interior air.

    If, on the other hand, the exterior sheathing leaks air and the interior drywall is installed according to the Airtight Drywall Approach, the RH of the air in the stud cavities is more likely to be close to the RH of the exterior air.

  15. Riversong | | #15

    studies show that foam of the proper thickness prevents the condensation in the first place.

    Martin, please link to any such studies. I've seen nothing but conjecture, based on theoretical (not real world) assumptions.

    And I'm not "hypothesizing condensation". Building assemblies and materials get wet from not just condensation, but also from exterior leakage, ice dam formation, mechanical system leakage, and capillarity. They also start out very wet.

    Joe Lstiburek used to say:

    "Things get wet from the inside, the outside and they start out wet."
    "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."
    "Ideally, building assemblies should be designed to dry to both the interior and exterior. In heating climates, the primary drying potential is to the exterior."
    "The drying potential of an assembly decreases with the level of insulation and increases with the rate of air flow."
    "As such, energy conservation has the potential to destroy more buildings than architects."

    The more we slow down heat flux through a building assembly, the more we inhibit its ability to dry. Most of what passes for "smart" building today has significant unintended consequences. Using relatively impermeable foam to insulate our homes will ultimately be understood to have caused more problems than it was intended to solve.

  16. GBA Editor
    Martin Holladay | | #16

    I'm not aware of the study you're asking about, although I'd be all in favor of any agency willing to fund such a study.

    In the meantime, there is accumulating evidence that exterior foam helps protect wall sheathing from moisture problems. Needless to say, walls need to be able to dry out, which is precisely why I advised Sandra to get rid of the interior poly.

    My own experience and observation, including inspection of walls in my own foam-sheathed home built in 1980, supports my understanding of the value of exterior foam sheathing.

  17. Anonymous | | #17

    The layers of foam will set the average temp in the fiberglass close to the inside temp unless as Martin said there is an exterior leak, which there will be a few leaks.

    The closed cell foam is isolating the OSB and exterior very effectively for almost all wall area. Just the frame has to dry past the barriers. Certainly Membrain or no poly is best but the poly is not as great a concern as bulk water getting past flashing and board foam to wet and rot the OSB.

  18. Riversong | | #18

    Sandra's wall will not have summer condensation when rot happens. None. RH in wall will match RH in home.

    It's easy to post unsupportable statements when done so anonymously.

    You and Martin are both wrong about this. The RH at any point in the building envelope is determined by the relative vapor permeance and relative thermal resistance (R-value) of each layer, as well as the potential for moisture intrusion in any form and through any mechanism, including the substantial initial stored moisture in wood framing and concrete.

    Moisture can intrude into what might appear to be a sealed cavity by air leakage through either exterior or interior surfaces as well as gaps and joints in the framing or unsealed mechanical penetrations from any direction. It can intrude through capillary conduction, from bulk leakage from unsealed openings or failed caulking, from vapor pressure differential, concentration gradient, wind pressure differential, stack-effect pressures, gravity differential, osmotic and diffusion pressures and surface diffusion due to hydrogen (electrostatic) bonding.

    At least Martin is smart enough to warn against leaving the poly in place. The only safe assumption about any building envelope is that it starts out very wet and will get wet during its lifetime. Therefore maintaining the maximum drying potential is critical to durability and health.

  19. Dan Kolbert | | #19

    Ah Robert, you charmer you. Martin - if there's a poly vapor barrier btw. framing and drywall, isn't there going to be a differential from one side to the other? If not, why would anyone bother to use one?

  20. GBA Editor
    Martin Holladay | | #20

    Good question, and a complicated one best answered by WUFI. Walls are three-dimensional and complicated, and every one is different -- so there is probably no single answer to this question.

    My answer was based on what I know, which is that air leaks account for a much greater percentage of moisture transport through walls than vapor diffusion.

  21. Riversong | | #21


    I missed out on charm school. Martin is quite right that walls are three-dimensional and complicated, but even WUFI can't capture the entire picture.

    And, while it's true that most transport of indoor ambient moisture into the building envelope is by air movement rather than vapor diffusion, that considers only one moisture source and two modes of moisture transport into the structural and thermal envelope.

    In existing houses, according to HUD, 80% of moisture problems are from damp basements and crawlspaces. Both building science and experience indicate that the primary moisture problems in wood-frame buildings are due to environmental bulk moisture leaking through or being driven through the exterior or rising from the ground.

    Water is both the most common substance on Earth and the most powerful and persistent. Without its ability to break apart solid rock, turn it into soil and dissolve and precipitate minerals, life on Earth would not be possible. It is the only substance that exists in all three phases (solid, liquid and gas) at normal atmospheric conditions, is essential for metabolism and the thermo-regulation of both our bodies and the Earth, has abnormally high melting and boiling points, has one of the highest heats of vaporization of any liquid, gets more dense just before freezing than as a solid, has very high surface tension allowing it to rise by capillary action to as much as six miles, and is the universal solvent. In other words, water has a magic that no WUFI can predict nor any building technology mitigate.

    When it becomes a flood or a tsunami or a hurricane, humanity is helpless before its power. There is nothing we can to do make our homes immune to its effects and we cannot maintain a healthy indoor environment without just the right amount of moisture. If we, in the building community, were intelligent we would work with it rather than try to fight it (just ask the Army Corps of Engineers how successful they've been in controlling Mother Nature). This requires materials and methods which can tolerate, absorb, buffer and release moisture on a diurnal and annual cycle. This means natural hygroscopic materials like wood, cellulose, gypsum, lime, straw and clay - not unnatural petrochemical products like plastics and foam insulation or phenol-formaldehyde saturated sheets of Spam imitation wood.

    You can't fool Mother Nature. If we don't cooperate with Her, we will be defeated by our own foolishness.

  22. Anonymous | | #22

    Scientific discovery and it's capitalistic deployment is winning the race against population growth.

    I believe all is fine on planet Earth and will continue to be so.

    Mysticism is just that. Balony.

  23. homedesign | | #23


    ...water has a magic ...

    I agree that WATER has "Supernatural Powers"
    Water is Fascinating and Very Complex.

    I thought Robert's Description was Very Good.

    Here is a Bill Rose Quote:

    "The polarity of the water molecule is what renders it endlessly interesting, interacting as it does with practically every other molecule on Earth"

  24. user-757117 | | #24

    Funny, I don't really read anything mystical about Robert's post...

    In other words, water has a magic that no WUFI can predict nor any building technology mitigate.

    It is naive to think that any complex process can be modelled 100% accurately regardless of computing horsepower... The unknown quantities may as well be called magic.

  25. Riversong | | #25

    Anony Mouse: Have the honesty and courage to put your name to your posts.

    As for the Bill Rose quote: water proves that bipolarity is not a disorder, but rather essential for the ordering of our universe.

    Anyone who thinks that's "mystical" can't differentiate between authentic science (that is, untainted by capitalist capitulation) and SPAM.

    Here's some more water magic:

    water has the highest specific heat of all liquids except ammonia
    water has the highest heat of vaporization of any molecular liquid
    except for liquid metals, water has the highest thermal conductivity of any liquid
    hot water freezes faster than cold water
    solid water (ice) is less dense than liquid water and hence floats
    boiling water will turn instantly to snow at temperatures below -25°
    water has at least 5 liquid phases and 14 solid phases (remember Ice 9?)
    when supercooled will turn instantly to ice when disturbed
    when superheated will turn instantly to vapor when disturbed
    at -184° F water turns super-viscous like molasses
    at -211° F water turns amorphous like glass
    homeopathy is correct that water has a "memory" of what was once dissolved
    there is evidence that it also retains thoughts, moods & conscious intentions (viz Dr. Emoto's research)

  26. GBA Editor
    Martin Holladay | | #26

    In an attempt to settle, once and for all, the long-standing debate over whether hot water freezes faster than cold water, I ran my own test. According to my observations, the cold water froze faster -- but not by much.

  27. Anonymous | | #27

    Rather be anonymous than behave like I am the second coming.

    So much vitritude for one man who will soon enough be gone and then what oh man of much water knowledge?

    Sandra how do you like us now? We sure are fun group of greenies ehh?

  28. Riversong | | #28


    "The fact that water has previously been warmed contributes to its freezing quickly; for so it cools sooner. Hence many people, when they want to cool hot water quickly, begin by putting it in the sun. . ."
    - Aristotle

    In 1461, the physicist Giovanni Marliani confirmed that hot water froze faster than cold. He said that he had taken four ounces of boiling water, and four ounces of non-heated water, placed them outside in similar containers on a cold winter day, and observed that the boiled water froze first.

    "Water slightly warm is more easily frozen than quite cold."
    - Francis Bacon (1620)

    "Experience shows that water that has been kept for a long time on the fire freezes sooner than other water".
    - Rene Descartes

    In the 1600's, it was apparently common knowledge that hot water would freeze faster than cold. However, it was not reintroduced to the scientific community until 1969, 500 years after Marliani's experiment, and more than two millennia after Aristotle's "Meteorologica I". The story of its rediscovery by a Tanzanian high school student named Mpemba is written up in the New Scientist.

    In 1963, Mpemba was making ice cream at school, which he did by mixing boiling milk with sugar. He was supposed to wait for the milk to cool before placing it the refrigerator, but in a rush to get scarce refrigerator space, put his milk in without cooling it. To his surprise, he found that his hot milk froze into ice cream before that of other students. He asked his physics teacher for an explanation, but was told that he must have been confused, since his observation was impossible.

    But later that year he met a friend of his who made and sold ice cream in Tanga town. His friend told Mpemba that when making ice cream, he put the hot liquids in the refrigerator to make them freeze faster. Mpemba found that other ice cream sellers in Tanga had the same practice.

    Dr Osborne, a professor of physics, visited Mpemba's high school. Mpemba had asked him to explain why hot water would freeze before cold water. Dr Osborne said that he could not think of any explanation, but would try the experiment later. When back in his laboratory, he asked a young technician to test Mpemba's claim. The technician later reported that the hot water froze first, and said "But we'll keep on repeating the experiment until we get the right result." However, repeated tests gave the same result, and in 1969 Mpemba and Osborne wrote up their results. To this day, it is known as the Mpemba Effect.

    If anyone who is not anonymous asks, I'll be glad to explain the physics of this phenomenon.

  29. Riversong | | #29


    Please stop allowing anonymous posting. It encourages irresponsible argumentation with no accountability.

  30. GBA Editor
    Martin Holladay | | #30

    Tolerance is a virtue, espoused by many wise teachers, including one you have cited in the past -- Gandhi.

  31. Riversong | | #31


    You abuse Gandhi's legacy. Tolerance of honestly held beliefs is one thing. But no one was more intolerant than Ghandiji of injustice, arrogance, the abuse of power or the degradation of humanity.

    We pretend to be a civilized people. But when asked what he thought of Western civilization, Gandhi responded "I think it would be a good idea".

    By allowing anonymous (that is, irresponsible and unaccountable) posts, you are encouraging intellectual dishonesty and a general diminution of the quality of discourse on this site.

  32. HDendy | | #32

    Good to have you back Robert! These are the kinds of discussions I've been missing!

  33. Riversong | | #33


    I don't think we've met, yet. But I'm glad that a few people here appreciate deep, broad, honest and critical dialogue.

    I'm back simply because I got bored.

  34. Dan Kolbert | | #34

    Have you tried needlepoint?

  35. Sandra Heiser | | #35

    To Anonymous #27:
    I like the greenies just fine, and declare myself as one! I haven't posted because I've been on vacation and was 'unconnected'. (It felt a bit liberating actually!) I'm glad my question started this discourse, it's one of the core issues of building science.
    And ending up discussing Gandhi's viewpoints is a bonus.

  36. Riversong | | #36

    And ending up discussing Gandhi's viewpoints is a bonus.

    Of course Gandhi was unalterably opposed to barriers of all sorts: barriers to freedom and self-determination, barriers to equality and human dignity, barriers to the gathering of nature's salt - and would no doubt be appalled at the notion of a double vapor barrier.

  37. sanleera | | #37

    Exactly Robert, he would have advised us to live with the elements and develop greater fortitude, or move to Florida.

  38. Riversong | | #38

    I don't think Gandhi would much care for Florida (and he would probably have been arrested for walking around in a loincloth).

    But he would likely have suggested to either spin your own yarn or spin a good yarn.

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