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Discussion Forum

Exterior Vapour Barriers

Morgan23 | Posted in Construction Techniques on October 18, 2006 02:19am

I am currently working on a residing job. The building in question is a wood frame structure 2 x 6 @16″ with plywood sheathing. We are using a asphalt base water proofing t( Bakor, “Blueskin” to assure no water penatration through the plywood. The building inspector suggestes that by using this type of mebrane it will trap moisture in the walls, therefore causing rot. He contends that “tyvek” type of barriers are superior. I question that perhaps he should try wearing a tyvek jump suite for a few minutes. I have found that to be the case, perhaps I could promt some discussion. Many Thanks for your time.

 

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Replies

  1. BarryO | Oct 18, 2006 03:04am | #1

    'have never heard of exterior asphalt waterproofing on plywood walls.

    If there is an interior vapor-impermeable barrier as well, I would think the inspector's concerns are justified.

    Air-infiltration barriers such as Tyvek, and even plain ol' tar paper rolls, allow water vapor to escape.

    1. Morgan23 | Oct 18, 2006 03:26pm | #6

      I am curious, have you ever worn a tyvek suit ? I cannot believe that this material let moisture pass through.

      The same building inspector said that he had a contractor who used ice and water shield over the entire roof and had to disallow him to do so. If it helps keep the rain out then what would the problem be? Especially with HRV units that are required to keep the moisture of the interior controlled.

      Thanks for the input.

       

      1. DanH | Oct 18, 2006 07:45pm | #8

        There should be no problem with ice/water shield over the entire roof if it's a cold roof and well-ventillated below. If the roof is insulated ("hot"), though, then (in cold climates) it's probably not a good idea to have the membrane there.

        If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison

        1. MAsprayfoam | Oct 19, 2006 06:07am | #18

          uh... why is the waterproof barrier bad on the outside of the roof deck when the roof is insulated "hot"?

          I do not understand the logic.

          I also question the Tyvek being as permeable as advertised to be.  I would much rather use tar paper on the outside with foam in the wall to stop all air movement then no VB inside either.  That's in my climate tho.  Of course I sell foam too...  Just my $0.02

          Stu

          1. DanH | Oct 19, 2006 12:22pm | #21

            The issue, as always, is moisture entrapment. With no ventillation moisture can condense in the decking or fiberglass insulation, or leak in, and have no way to escape.Tyvek is more moisture-permeable than tarpaper.With closed-cell foam of course no VB is needed to protect the insulation itself from condensation. One still needs to guard against moisture entrapment between foam and any exterior membrane, though.

            If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison

      2. BarryO | Oct 18, 2006 07:53pm | #9

        I've worn Tyvek shirts.  The material is supposed to let vapor through, but block moisture, from what I understand.

        There was an article in FWW a few years ago on the different types of air-infiltration barriers

      3. Piffin | Oct 18, 2006 08:17pm | #11

        What the HI and everyone here has told you is absolutely correct. It depends on your climate. A VB should never be applied to the exterior of the wall without special other comnsiderations in a heating climate. It WILL trap moisture and cause mold and/or rot in the wall system.Your Q about roofs has a variable answer, it depends on the way the roof and ventilation systme is designed. Moisture management in a wall system is very different from moisture management in a roof/cieling systme. One is because the wall is sealed without venting most times while the roof is far more often vented and has additional insulationLocation of the VB is only one of many components that go into designing and building a wall properly.Now then - you have everyone on the edge of thier seats awaiting your reply as to what your climatic location is. No way to accurately advise you without knowing. Heating, cooling, and moderate in-between climates each have their own peculirarites, but the principles are all the same. It would be good to also state what sort of insulation you are considering - that has an impact on the answer. 

         

        Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

    2. GOLDENBOY | Oct 19, 2006 06:23am | #20

      I live in a 48" frost zone.  I have a walk out basement.  Instead of adding deeper footings I insulated with styro SM, 4" thick, 4 ft wide.  This has worked out fine.

  2. cliffy | Oct 18, 2006 03:42am | #2

    If you think a tyvek suit is bad try a blueskin suit or live inside a plastic bag!  The inspector is right on this one.

    Have a good day

    Cliffy

  3. andy_engel | Oct 18, 2006 04:19am | #3

    The answer depends on your climate. If you're in a hot and humid climate - say Atlanta - an exterior vb is appropriate. If you're in Manitoba, you may be building in trouble.

    Andy

    "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein

    "Get off your dead #### and on your dying feet." Mom

    1. cliffy | Oct 18, 2006 04:45pm | #7

      Nice answer! Ever been to Manitoba in winter?  I have a chilhood friend who lives in Winnipeg.  Went out a few years ago for a visit.  Now I now why Ontario calls it Winterpeg.

      Have a good day

      Cliffy

      1. andy_engel | Oct 19, 2006 01:13am | #16

        Never had the pleasure. I'd like to go way north sometime though, just for the experience.Andy

        "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein

        "Get off your dead #### and on your dying feet." Mom

  4. BMan | Oct 18, 2006 04:22am | #4

    There are many vapor permeable barriers available for use on exterior walls. Look to Henry products for options. Blueskin is only one of many like products.

     

    "Good design is good business"

                              Florence Schust Knoll (1917-)

  5. Ewan | Oct 18, 2006 04:46am | #5

    I'll echo what others have said... if you're building in a cold climate, using Blueskin (a complete vapour barrier) on the exterior side of the sheathing could be asking for trouble IF you're not also installing exterior insulation (e.g. XPS or polyiso).

    Do a search using the search term "PERSIST"... this is a building-envelope technique that Ray Moore has described a few times. The PERSIST system uses a exterior membrane like Blueskin, but it requires the use of exterior insulation to control the temperature of the interior side of the sheathing. It also requires there not be an interior vapour barrier installed.

    This link might work: http://forums.taunton.com/tp-breaktime/messages?msg=58800.1

    Regards,
    Ewan



    Edited 10/17/2006 9:50 pm ET by Ewan

  6. tek | Oct 18, 2006 08:06pm | #10

    Here in MA, the code requires anything installed beyond the warm-in-winter side vapor retarder be 10x more permeable.  Check the specs on the stuff you put on the plywood. 

    We once had to show an inspector that .060 EPDM is more vapor permeable than a 6-mil poly VB sheet on a flat roof project. 

    1. Piffin | Oct 18, 2006 08:19pm | #12

      check my memory. It's .003perms. Am I right? 

       

      Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

      1. tek | Oct 18, 2006 08:34pm | #13

        The few cut sheets I've seen range somewhere between .03 and .05.  EFI has a 4-mil listed at .0074, MA code says anything less than 0.1 will do.

        1. Piffin | Oct 18, 2006 09:56pm | #14

          looks like I had the decimal in the wrong place. 

           

          Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          1. tek | Oct 18, 2006 10:31pm | #15

            Better here than on your tax forms.  I cheated anyways and looked it up.

  7. User avater
    Matt | Oct 19, 2006 03:31am | #17

    Where do you live?

    If you look at Tyvek under a microscope it looks like a cheap furnace filter.  The idea is that is supposed to let water vapor through, but not water droplets.

    1. MAsprayfoam | Oct 19, 2006 06:18am | #19

      At what time of year or conditions would water vapor pass thru Tyvek?

      Since Tyvek is on the outside of the house it's going to be pretty much outside air temp.  In my climate I'd have to say that 7 months out of the year plus the nights of the remaining 5 would be cold enough to reach the condensing point of any moist air that might be in that wall.

      In other words, when you get condensation in the wall it ain't coming out by diffusing thru the Tyvek (or other similiar).  At least at half a day for half a year it aint.

      Can anyone straighten me out?  I'm in Massachusetts.

      Thanks,

      Stu

      1. DanH | Oct 19, 2006 12:28pm | #22

        You need to get a better understanding of dew point, relative humidity, etc if you're going to be doing insulation.There is always moisture vapor in the air -- it doesn't ALL condense at the dew point. The amount that would condense, absent airflow, when the wall surface temporarily drops below the dew point, is negligible. (And, practically speaking, the temperature at the Tyvek layer will hardly ever dip to the outside dew point.) What you guard against with the vapor barrier on the inside and permeable barrier on the outside is a buildup of moisture over time, as it seeps through from the inside, condenses, and does not have an opportunity to re-evaporate and move on through.

        If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison

        1. andy_engel | Oct 19, 2006 04:14pm | #24

          And, practically speaking, the temperature at the Tyvek layer will hardly ever dip to the outside dew point.)

          I'm not sure this is right. Assuming good insulation within the wall, why wouldn't the surface temperature of the housewrap fall below the dewpoint? That's very easy to imagine. I'd like to see some science done on this topic.Andy

          "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein

          "Get off your dead #### and on your dying feet." Mom

          1. DanH | Oct 19, 2006 05:00pm | #25

            The outside temperature essentially never dips below the dew point, just to it. Assuming that we're not in the tropics, so the dew point is below the indoor temp, any warmth coming through from the inside (or simply heat storage in the wall from the day) will keep the temp at the housewrap elevated above the dew point. It doesn't have to be much -- just a degree or two will suffice.

            If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison

          2. MartinHolladay | Oct 19, 2006 09:31pm | #26

            Even if water does accumulate behind Tyvek, through condensation or small flashing errors, the idea is that the water will soon evaporate (turn to vapor) and diffuse through the Tyvek to the exterior.  That's the whole idea behind vapor permeability -- it doesn't really matter if a little water gets behind it, because it allows for diffusion.  Of course, in the real world, the question of whether it works comes down to a simple comparison:  whether water is accumulating behind the Tyvek faster than it can dry by diffusion, or slower.  If the diffusion rate beats the accumulation rate, then the wall system works.

          3. DanH | Oct 19, 2006 10:12pm | #27

            True.
            If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison

          4. andy_engel | Oct 20, 2006 03:53am | #28

            The dew point relative to what level of humidity? That of the outside of the wall, or the inside? I don't care what the dew point is for air outside the wall. I'm thinking of circumstances where the relative humidity inside the wall is greater than outside, and the moisture drive is outward. In that case, which is the situation that housewrap is supposed to address, I'd expect condensation somewhere undesireable.Andy

            "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein

            "Get off your dead #### and on your dying feet." Mom

          5. DanH | Oct 20, 2006 03:56am | #29

            Why is the relative humidity inside the wall higher than outside?  How did it get that way (assuming an effective vapor barrier on the inside)?
            If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison

          6. andy_engel | Oct 20, 2006 02:54pm | #31

            Why is the relative humidity inside the wall higher than outside?  How did it get that way (assuming an effective vapor barrier on the inside)?

            Who knows? That's sidestepping the question. Framing assemblies sometimes get wet, and that can happen by way of a myriad of construction defects. The point is that housewraps are supposed to relieve that very problem. Which brings me back to my last post. How does drying take place, assuming that moisuture is being driven outward, when the sheathing and housewrap is below the dew point of the escaping vapor? Andy

            "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein

            "Get off your dead #### and on your dying feet." Mom

          7. MartinHolladay | Oct 20, 2006 03:06pm | #33

            Andy,

            If the sheathing and the housewrap are below the dew point, condensation will occur.  The usual place for this to happen is on the inside face of the sheathing.  However, you are correct that condensation can make the sheathing wet as well as the housewrap wet.  Now both are wet.  The usual time for such wetting is winter, when the surfaces are cold.  Mold is unlikely to grow when it is cold.  When conditions change, the housewrap and sheathing warm up.  As the temperature rises, the possibility of mold or rot increases.  However, the rising temperature causes the water in the wet sheathing (or behind the housewrap, if there is any there) to evaporate -- vaporize -- turn to vapor.  Depending on the type of sheathing, the sheathing will be more or less permeable.  If the housewrap is #15 felt or Tyvek, it will be fairly permeable.  The water vapor begins passing through the housewrap toward the exterior, and drying begins.  Whether it dries fast enough to avoid all problems depends on the rate of wetting as well as the rate of drying.

          8. andy_engel | Oct 20, 2006 03:12pm | #35

            Yup, understood. But, as a general rule, it's pretty well accepted that moisture moves from warm to cool and from wet to dry. Simplistic, but on the whole, true. It seems to me that the direction of moisture drive is likely to be inward in the scenario you describe. Not always, but when it wasn't, the delta-T, and hence the drive, would be pretty low.

            And we've not even talked about the effects of latent heat! Where is ORNL?Andy

            "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein

            "Get off your dead #### and on your dying feet." Mom

          9. MartinHolladay | Oct 20, 2006 04:50pm | #37

            Andy,

            Yes, housewrap can be wet from condensation.  This should not come as a huge surprise;  one reaction is, so what?  Condensation occurs on asphalt shingles, too, and many other places.  The real question is, does it damage the wall?  Does the housewrap stay wet, for weeks, during warm weather?  Or does it dry quickly?  Condensation during cold weather doesn't matter much, if it stays cold.  That's why the Antarctic huts of the Scott expedition are in such terrific shape, even though they are made of wood (with cork and seaweed insulation) and about 100 years old.  The temperatures of McMurdo in Antarctica are generally so cold, that even if snow is banked against the wood walls, mold and rot are never going to happen.  You are also correct that when sun hits wet siding, the vapor drive is often inward.  This is sometimes a problem, and sometimes not.  If the sheathing is vapor-impermeable, like polyiso, there is no problem.  If the sheathing is vapor-permeable, like fiberboard, the vapor can be driven into the wall assembly very easily;  if the house is air-conditioned, and if it has a poly vapor barrier, you can have problems very quickly.  Moisture will condense on the back of the poly, run to the bottom plate, and begin rotting the wall at the bottom plate.  If the sheathing is plywood or OSB -- that is, a little bit permeable but not very permeable -- the inward solar vapor drive may or may not be a problem, depending on whether the wall can dry to the interior, and the rate of wetting.

            So, housewrap gets wet, and housewrap can dry.  It all comes down to the same question:  is the rate of wetting greater than the rate of drying, or less than the rate of drying?

          10. andy_engel | Oct 20, 2006 07:52pm | #43

            I guess my interest here is more academic than practical. You're absolutely right - as long as walls dry faster than they get wet, it doesn't matter. And I think in most instances, the vapor permeance of housewrap v. tarpaper is also of academic interest relative to its importance in keeping out bulk water. I'm admittedly nit picking.

            What piqued my interest here was the idea of condensation at the housewrap. For some reason, that just never occurred to me. My mental model has condensation happening at the inside of the sheathing. Andy

            "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein

            "Get off your dead #### and on your dying feet." Mom

          11. Piffin | Oct 22, 2006 01:38am | #56

            "t's pretty well accepted that moisture moves from warm to cool and from wet to dry. "Andy, the scientific basis for this is that energy in the form of heat energy is the dynamic that carries the moisture. When the heat energy is lost via radiation to the cool spot/dew point, the moisture stops moving. For it to pick up and move again later, letting material dry, take extra energy moving in the same direction.if I recall properly - this thread has wandered around a bit in these several days - the original Q referred to a spray foam. If this foam is a polyurethene, then no moisture wil be moving thru, nor will a significant amt of heat energy. So sheathing that gets wetted would only be likely to do so from leaks on the exterior package and not from moisture migration through the wall. In that case, I wonder where the heat energy is going to come from to dry it again. This is where flashing detailing is so much more important in modern homes thanin older heat losing monsters. 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          12. DanH | Oct 20, 2006 04:58pm | #38

            > How does drying take place, assuming that moisuture is being driven outward, when the sheathing and housewrap is below the dew point of the escaping vapor?Conditions at the surface of a body of water are always at the dew point, yet water evaporates. The only thing that stops this evaporation is if the air some distance (meters) away from the water surface is at the dewpoint as well (which is to say that it's foggy or raining).The only real question is can this evaporation occur faster than new moisture is being added, either from the inside or from rain intrusion. This is why you have vapor barriers (which housewrap is not) and flashing (which again housewrap is not, though it may serve as a secondary rain barrier).
            If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison

          13. DanH | Oct 20, 2006 05:03pm | #39

            > Framing assemblies sometimes get wet, and that can happen by way of a myriad of construction defects. The point is that housewraps are supposed to relieve that very problem.No, that's not the purpose of housewrap.
            If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison

          14. andy_engel | Oct 20, 2006 07:46pm | #42

            It may not be their purpose, but is one of ther attributes. Andy

            "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein

            "Get off your dead #### and on your dying feet." Mom

          15. MartinHolladay | Oct 20, 2006 01:30pm | #30

            Andy,

            Simplistic dew-point calculations grossly oversimplify the moisture dynamics in a wall system.  Wall systems are complex;  among the factors affecting their performance is the rate of water entry from the exterior, due to rain penetration past the siding and flashing errors;  the amount of vapor diffusion from the interior toward the exterior;  the amount of air movement through the wall -- air carries moisture with it;  the location of the insulation and the type of insulation;  the relative humidity of the indoor air;  and the permeance of the housewrap.  Calculations made in the 1970s often predicted that the dew point occurred in the middle of the fiberglass insulation.  But for several reasons, not all of which I understand, condensation does not actually occur on the fiberglass fibers.  Instead it tends to occur on plywood or OSB sheathing, if the circumstances are correct.  For all intents and purposes, the experts (e.g. Bill Rose) are now teaching us, "forget dew point calculations."  There are two ways to tell if a wall system works:  (1) model it using computer software like WUFI, or (2) build it in your climate and monitor it for several years, under several different scenarios, and hope your house performs the same as the monitored house.  Since most of us can't do either, we use belt-and-suspender systems with large margins of forgiveness and we cross our fingers.

          16. andy_engel | Oct 20, 2006 03:07pm | #34

            I've read Rose. I understand that houses are complex and dynamic, and that more goes on than can be explained by examining one part of the system without context. Which is why I'd said that I'd like to see some science done on this question.

            We, meaning the building community as a whole, accept that housewraps prevent bulk penetration of water in the liquid phase, but allow vapor phase water to pass. And while dew point calculations are simplistic, the dew point is not without effect. Given that in our New England climate at least, a read of the marketing materials would suggest that housewraps allow vapor to pass when the moisture drive is outward, I'd like to know if anyone has examined what happens at the interface of the housewrap and the oustide when the housewrap's temperature is below the nominal dewpoint of the moisture in the wall.

            And I would never, ever allow an author a sentence as long as the latter.Andy

            "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein

            "Get off your dead #### and on your dying feet." Mom

          17. User avater
            CapnMac | Oct 20, 2006 10:42pm | #46

            And I would never, ever allow an author a sentence as long as the latter

            Well, editing for a magazine that makes sense--but, dang if I can find a way to parse that down with out adding additional length to it <g>.Occupational hazard of my occupation not being around (sorry Bubba)

          18. andy_engel | Oct 21, 2006 01:38am | #48

            That's high praise.Andy

            "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein

            "Get off your dead #### and on your dying feet." Mom

          19. User avater
            CapnMac | Oct 21, 2006 08:54am | #49

            That's high praise

            Well, it wasn't like I wrote it out and took a red pen to it or anything like that . . .

            More forum BTDT, and try and get it all on the one t-shirt . . . Occupational hazard of my occupation not being around (sorry Bubba)

          20. user-217211 | Oct 21, 2006 11:52pm | #51

            Ya'll are having all the fun without me again. I really have a hard time jumping into a thread that has grown to 50 posts and is close to dying a natural death. Just a few thoughts.Vapor diffusion is a sllooowwwww process but insidious. It rarely is the primary wetting mechanism but it does happen. Usually there is air movement. Oh hell, I'm going to regret this but here goes. Water molecules are very small. One billion end to end equal about one foot. They get excited when warm The more excited they get the more they can fit in a given volume of air (lets say a bucket). They also stick to surfaces (adsorbed moisture) but not for long. They land and jump off. The warmer the surface the shorter their stay on the surface. The dryer the air, the more they tend to stay in the air. So we have this bucket of air with all these excited little water molecules. They bounce off the surface and each other. Let's say it is 70 degrees in the bucket and the air is holding about half the total water molecules that it is capable of holding.(50% RH) In that condition the number of molecules adsorbed on the surface of the inside of the bucket may be about 5 layers deep on average.The molecules right on the surface don't move around much and act more or less like the molecules of the bucket. The molecules 5 layers out are constantly in motion and act more like water molecules. If the air in the bucket is cooled the molecules get less excited and start to get tired and slower. They stay attached for slightly longer periods. They start to cling to each other more and the RH goes up. If the surface of the bucket is cooled the molecules that contact it stay attached longer and more layers build up. The more layers that build up, the more they start to act like water. These layers of water molecules exist on everything you normally come in contact with. At some point they act so much like water, that they become available to things like mold to carry on it's life cycle. That is why mold likes to start growing at around 80%RH and higher but before a surface is even visibly wet. As the air and or surface become colder, eventually the water molecules get so bored that they decide to just hang out. They build up and eventually enough energy is lost to allow them to reorganize into liquid water. That extra energy that it takes to reorganize the molecules into a different phase is significant and is called the heat of fusion or the heat of vaporization depending on which way the shift is occuring. In reality, the bucket surface and the air can act independently of each other but both will affect the water molecules in our bucket. The bucket can be warmed by radiation or cooled by radiation. At night, when facing the clear night sky, the bucket can radiate it's heat to deep space but deep space doesn't radiate much heat back. The bucket can then be cooled to around ten degrees cooler than the outside air temperature. Condensation may form on the interior and or the exterior of the bucket depending on the RH of the air inside and outside the bucket. The cooled bucket may also cool the air and be warmed by it. Everything is in a constant flux but always remember the tiny little excited water molecule. It's constantly flying around sticking and jumping off. If it finds a cold surface it's going to take a nap there and all it's buddies are going to pile on until it becomes available for mold growth and resultant rot.Now let's consider the hot to cold and moist to dry movement of water vapor. In reality it is a sum of lots of movement. The molecules move alot when they are warm and less when they are cold so they will move readily from warm to cold but less so from cold to warm. They just get too tired when they are cold to move as quickly back in the other direction so the net result is a migration towards cooler surfaces and air volumes. Same thing with moist to dry. There are more molecules available to move from moist to dry and less molecules available to move from dry to moist. They seek equilibrium.Now to a wall. If a wall is cold on one side and warm on the other, there will be a net movement towards the cold side because there is less energy there to keep the molecule excited. Over time, there will be a build up. On the other hand the air on the other side of that cold wall is likely to be very dry from the stand point of absolute humidity(not RH). Molecules will jump from the exterior surface into this dry air and few will return so drying will occur towards the exterior. Let's say osb exterior sheathing in winter. The osb is cold so moisture from the interior is attracted to it. The inside of it will become damp. There is more moisture in the inside air than there is in the outside air. Ignore RH here because it is irrelevant. Absolute moisture content is what matters. There is more moisture on the interior surface of the osb so more molecules will move through the osb towards the outside than will move back the other way. When they reach the exterior side of the osb they will jump off into the dry air outside and few will return. If you cover the osb with a permeable membrane this can occur. If you cover it with an impermeable membrane then drying can only occur to the interior.Why isn't RH important? It can really confuse the issue. Absolute humidity is more important to understanding moisture movement. This is better expressed by using dewpoint. Let's say the inside RH is 25% and teh inside temp is 70 degrees f. If that air is cooled to around 30 degrees it's RH will now be 100% even though it's absolute humidity remains the same. So if you are in that 25% 70 degree air it may feel very dry to you. If the air outside is at 20 degrees f, the RH can be 100% and it will still be dryer than that inside air. Movement of moisture will be towards the dry air that is at 100% rh. Curious ain't it?

          21. user-217211 | Oct 22, 2006 12:06am | #52

            This little fun fact about water is why it is more important to think of controlling the temperature of a potential condensing surface than it is to think in terms of RH or where the vapor barrier should go in what climate.Now imagine a world where all the potential surfaces for condensation that can be negatively impacted by moisture buildup were always at room temperature and never approached the dewpoint of the air that they were in contact with. Here's how it works.Put a vapor barrier around the entire structural shell of your building. Building in a condom. Now put all the insulation outside of that. All of your wood components will now be treated like they are furniture. They will be warm and dry. The interior humidity levels will be important only to the degree that you resist condensation on the windows. The vapor barrier is exterior to the shell but interior the the insulation. Don't put insulation in the walls. What a wonderful world it would be......

          22. Fbart | Oct 22, 2006 12:44am | #53

            Reading this thread made me question if I was about to make a mistake.  I just started a residing project on a wood framed house, that has this gypsum type boxing on the exterior of the studs.  I was planning on adding a layer of 1/4" fanfold foam insulation prior to Hardi plank siding. ( have done this before, with no knowledge of adverse results).  Am I asking for trouble with possible condensation on the wall side of the foam in the winter.  We are in central Kansas, and have both hot, and cold conditions.  Unheated areas, such as tha garage are getting just Tyvac.  Any thaughts would be appreciated.  Thanks.   

            Fred

          23. DanH | Oct 22, 2006 04:44am | #60

            The fanfold stuff is generally perforated to let moisture through. It's not great from a moisture escape standpoint but it works. Of course, the perforations tend to negate what little insulating value the stuff has, because they let air through too.Housewrap is a better choice -- lets more moisture through and less air. In most nominally insulated homes (dating from roughly 1950 to 1980), air infiltration is a more serious problem (causes more heat loss) than lack of insulation.The fanfold should only be used when you need it to provide a level surface over existing siding. (That's why the pros use it.)
            If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison

          24. User avater
            CapnMac | Oct 22, 2006 03:20am | #57

            Absolute humidity is more important to understanding moisture movement ... Curious ain't it?

            Which was another thing I found humorous in the direction the thread was going.

            There was starting to be a lot of absolutism (NPI) on where things had to be.

            I have to disagree, a small bit, with Bill, though--Dallas is not but 200 miles north of me--but their Absolute humidity is easily half mine.  This does not prohibit the DFW are from being muggy and awful outside in the summer (or the winter, too, soemtimes).

            BUt, here in my area, in a river valley, with gravity effects and sea/land breeze effects--that AH almost means I need a different "answer" than DFW does--and perversely, different than metro Houston, too (not maritime enough).

            So, yes, it's all curious, especially, sometimes even knowing what curiosity did to the cat . . . Occupational hazard of my occupation not being around (sorry Bubba)

          25. Piffin | Oct 22, 2006 01:24am | #54

            Here are mmy thoughts and specualtions on why this happens at the exterior sheathing rather than in the center of the FG batts - we now know that batts allow a convection loop inside the stud space. The loop carries air warmed near the interior wall up the inside and down the outside.The place I observe ( as a remodeler who does a lot of demo in prep for the new work) most moisture staining from condensation inside stud spaces is in the lower fourth, the very place where this theory would predict that most condensation would occour, because that is where the convection loop is at it coldest. 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          26. User avater
            CapnMac | Oct 22, 2006 03:28am | #58

            FG batts - we now know that batts allow a convection loop inside the stud space

            AH ha, bingo, you just said what I could not remember on Friday in the replacement window thread.  Somebody gave a linnk to the hysterical building preservation site, which had all sorts of nifty advice on insulation (like between heated floors, but not in walls at all).  There was a filip inthere about ventilating newly-insulated spaces to allow the moisture a route 'out' that had bothered me, but I could not set a finger upon the why.  Here, let's ventilate the FG batts, the better for them to fill up with damp air--that's a success story waiting to happen, ain't it?Occupational hazard of my occupation not being around (sorry Bubba)

          27. MAsprayfoam | Oct 20, 2006 03:16pm | #36

            My experience has been different.  I have seen dew on the housewrap.  That is proof enough to me that is was at or below the dew point of the air.

            Secondly, Dan, I do understand condensation, moisture drive, heat transfer, and the latent heat of transformation for that matter.  Spent a lot of time at Rensselaer Polytechnic Institute and the university of Hartford getting that stuff drilled in.  I'm always willing to learn tho.

            When condensation occurs, heat is absorbed by the change of state reaction from vapor to liquid.  This is 540 calories per gram of water.  That is actually pretty high.  Doesn't is seem to take forever for that water to boil even after it reaches 212F?  To change one degree only takes 1 cal per gram but the change of state takes 540. 

            So, condensing absorbes 540 calories of heat from the air and the material during condensation of each gram of water.  The air temp is very often at or slightly colder than the dew point.  This is when you find frost or dew on the lawn.  Of course as condensation happens the air gives up moisture and the DP goes down.

            Stu

          28. DanH | Oct 20, 2006 05:06pm | #40

            > So, condensing absorbes 540 calories of heat from the air and the material during condensation of each gram of water.???
            If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison

          29. MAsprayfoam | Oct 20, 2006 06:59pm | #41

            Sorry- my mistake: change of state from vapor to liquid gives off 540 calories of heat for each gram of water produced. 

            When the air temp is dropping this is what makes it stop at the dew point then only creap down from there as the moisture is condensed out thus lowering the dew point temp while maintaining 100% humidity.

            If the weather at night says it's 40F out and dew point of 35F then the overnight low will only be 32-33F usually for the above reason.

            Stu

          30. User avater
            CapnMac | Oct 20, 2006 10:35pm | #45

            Assuming that we're not in the tropics, so the dew point is below the indoor temp

            Sorry, just have to interject non sequitur laughter here.

            I live at the edge of the tropics as it happens (the Tropic of Cancer is almost directly overhead here at 29ºN).  And, the condition of DP < Indoor Temp describes just 3 weeks ago (and about every week since May).  DP in September were running from a low around 55º to up around 78º just ahead of rain (peak RH typically occurs about 0830, in the upper 90s).

            Hard to not appreciate water vapor, humidity (relative & absolute) and the nifty effect  that "pounds water vapor per cubic foot" has in such climates.

            Alrightly then, that's out of my system--back to VB in colder climes . . . Occupational hazard of my occupation not being around (sorry Bubba)

          31. User avater
            BillHartmann | Oct 21, 2006 11:25pm | #50

            "I live at the edge of the tropics as it happens (the Tropic of Cancer is almost directly overhead here at 29ºN). And, the condition of DP < Indoor Temp describes just 3 weeks ago (and about every week since May). DP in September were running from a low around 55º to up around 78º just ahead of rain (peak RH typically occurs about 0830, in the upper 90s)."And for a signficant part of the year it also discribes areas well to the north of you.I am in Kansas City and my guess is that our DP average 60-70* about 8-10 weeks a year.Thus an interior, impervious VB, such as poly, can dump enough water that it will have a hard time drying out.

          32. MAsprayfoam | Oct 20, 2006 02:57pm | #32

            Thanks for coming to my defence Andy.

            I just got back from Icynene in Mississauga Ontario and have been behind on email.

            Check out the Walls that Work on the Corbond.com web site and it steps thru it rather nicely.

            Think about it one step at a time and you come to realize we have a problem here.

            Stu

          33. Piffin | Oct 22, 2006 01:26am | #55

            Of course, with Corbond or other urethene sprays, the moisture migration issue is removed in comparison to other types of wall assemblies. 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          34. MAsprayfoam | Oct 22, 2006 03:45am | #59

            Exactly why I offer Corbond to my customers!

            Thank you for the endorsement of that fine product.  It makes the wall system as bullet proof as it can get IMO.

            Stu

          35. Piffin | Oct 23, 2006 03:11am | #61

            It's great stuff! And they have the research to back it up, when installed by reputable installers. 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

      2. andy_engel | Oct 19, 2006 04:10pm | #23

        Holy moly! I never thought of that! Most of the time when the moisture drive is outward, housewrap is below the dewpoint! Slap my face. And of course, in the summer, the moisture drive is inward. It's easy to imagine a water trap, isn't it?Andy

        "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein

        "Get off your dead #### and on your dying feet." Mom

    2. User avater
      CapnMac | Oct 20, 2006 10:23pm | #44

      The idea is that is supposed to let water vapor through

      And that mechanism means vapor comes through very slowly (the surface tension on a droplet of liquid water being too high to pass through the "spaces" in tyvek).

      That's why Tyvek suits are hideously uncomfortable in use.  The usits are Tyvek (IIRC) though, as a rip/tear prevention, not for vapor permeability.  (That, and tyvek is tons cheaper than Gore-Tex <g>).Occupational hazard of my occupation not being around (sorry Bubba)

      1. DanH | Oct 20, 2006 10:43pm | #47

        Tyvek jackets are used by cyclists. They have the three important properties for that use: Block wind, block rain, let water vapor through.

        If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison

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