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Thread: Vapor barrier at exterior wall surface ?

  1. #1
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    Vapor barrier at exterior wall surface ?

    As I recall; a vapor barrier is best placed on the interior of the structure's outside walls. I assume this is to avoid the condensing of moisture inside the walls. Paint and the various sidings are somewhat porous and so allow moisture to escape outwards and the interior vapor barrier restricts the movement of moisture into the wall.

    Please correct me if I am wrong about that.

    I routinely experiment with envelope-improvements - most recently centered around the 'first; keep the heat out' concept. This in Florida.

    I have found that a white solar reflective roof is astonishingly effective in that regard. I have also done white selective-coating metal and also latex costing on other substrates.

    Recently I painted an entire house, to below grade, with white latex roof coating. With the idea that if sun reflects away from the roof - the walls will also benefit from the same effect. But . . . what afterwards occurred to me is that I have essentially created a 'vapor barrier' at the exterior wall surface of the entire house.

    I guess my basic question for you all now is: How bad of an idea was this? <g>
    PHM
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    The conventional view serves to protect us from the painful job of thinking.

  2. #2
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    I think you may be fine doing that in Florida but probably isn't advisable in most other areas.
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  3. #3
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    The vapor barrier just needs to be on the warm/humid side of the wall.

    In Florida, it's outside the wall.

    If you were doing this in Joisey I might be a little concerned for "trapping" moisture between your existing and "incidental" vapor barrier.

    Let's see... If I think of air handlers in attics, there is typically foil-faced insulation inside of the cabinet, and then the metal of the cabinet would act as another vapor barrier. On properly functioning systems this doesn't seem to cause problems. Nor does externally-insulated sheet metal ductwork, for that matter.

    So I think you've got nothing to worry about.

    Sent from my Moto Z (2) using Tapatalk

  4. #4
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    My thought was that using a good interior paint almost certainly 'water proofs' the inside of the wall and now I have actually water-proofed the outside of the wall. <g>

    Freezing weather is extremely rare here - although every winter there are some nights in the 30-40º range.

    This house is slab-on-grade' and was pretty 'damp' inside when I bought it. I started running a dehumidifier set to 45% inside the house some months ago and that made a notable difference. But the property was sort of bowl-shaped with the house at the bottom of the bowl. All rainwater ran towards the house. The Gulf is very close and the house is 3' above sea level - so actual drainage was a problem.

    It appeared to me that the walls were actually 'wicking' water up into them. Under the peeling paint they felt 'damp' to me and joint compound repairs could sometimes take days to dry. And the lower on the wall they were - the slower they dried.

    After recently painting the exterior with white latex roof coating to well below grade I re-contoured the entire property with a backhoe/loader - creating a swale at the property line and using the removed soil to raise the grade along all the walls of the house.

    Oh; and installed rain gutters and ran the downspouts out to the swale.

    There is a city maintained drainage ditch along the front street and I tunneled 8" PVC under the front sidewalk to connect my new swale to that ditch. I was too lazy to get down in the mud and pack the dirt back into the 'tunnel' around the pipe so I drilled 1/4" holes in the sidewalk and 'back-filled' around the 8" PVC pipe with cans of spray foam. <g>

    I don't actually have the drainage ditch across my property - mine is a buried culvert - but my east neighbor has the exposed ditch I could connect to. I thought he might object to me installing the PVC over to his ditch but he never said anything and it's done now. <g>

    PHM
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    Quote Originally Posted by pageyjim View Post
    I think you may be fine doing that in Florida but probably isn't advisable in most other areas.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  5. #5
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    I was going back and forth about it. On one hand the sun and exterior ambient heat would tend to drive moisture out the tops of the walls.

    BTW: the walls are concrete block. They are stucco'ed outside. Inside there are 3/4" furring strips, rock lath, and wet-wall plaster finished.

    The bottom of the concrete blocks are on the edge of the floor slab, unsealed I am sure, and the top of the block walls just have a 2 by 6 plate that the ceiling joists and rafter attach to.

    But on the other hand; sealing both sides of the walls vapor tight seemed contrary to everything my northern-trained-mind was used to doing. <g>

    PHM
    --------------


    Quote Originally Posted by shellkamp View Post
    The vapor barrier just needs to be on the warm/humid side of the wall.

    In Florida, it's outside the wall.

    If you were doing this in Joisey I might be a little concerned for "trapping" moisture between your existing and "incidental" vapor barrier.

    Let's see... If I think of air handlers in attics, there is typically foil-faced insulation inside of the cabinet, and then the metal of the cabinet would act as another vapor barrier. On properly functioning systems this doesn't seem to cause problems. Nor does externally-insulated sheet metal ductwork, for that matter.

    So I think you've got nothing to worry about.

    Sent from my Moto Z (2) using Tapatalk
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  6. #6
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    To be safe I would invest in a moisture meter to see if it becomes an issue. Might want to log the readings along with the interior space and outdoor weather conditions. I say this based on limited knowledge. I hope this works out well for you.
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  7. #7
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    Poodle there has been a long standing argument on where to place the vapor barrier. I have read several articles (none recently) on this subject and both sides have good points. I don't remember all the arguments but one is if you place the barrier on the inside wall you trap the moisture between the barrier and the wood structure. I put mine on the inside walls and haven't noticed any problems. Most articles I have read on below grade vapor barriers strongly promote the barrier be on the outside. One thing I do know is unless you have a lot of re-bar in a concrete wall the barrier will be stretched beyond it's elastic limit and become useless. In short I don't know the answer to your question and I don't know if you will find a definitive answer.
    No man can be both ignorant and free.
    Thomas Jefferson

  8. #8
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    I agree with pageyjim getting a moisture meter. Putting roof coating on shouldn't be any different than flat roof that has insulation and drywall but if there's a vapor barrier already in the wall that could be a problem trapping moister between the two. Why not call the coating company and ask them what they think.

  9. #9
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    I know this is an old post but I don't think the question was ever answered and somebody might benefit from this. From my perspective, you need to consider where the wall will dry to. Depending on where you live, you may want the wall to dry to the interior or exterior or both. Historically, in a heating climate, the wall may dry to the exterior so the vapor barrier placed on the interior of the wall is better. In a cooling climate, my wall may want to dry to the interior so the vapor barrier would make sense to be on the exterior. In a heating/cooling climate, it does not make sense at all if the wall has to dry in both directions. Also, I don't think something like polyethylene vapor barriers are a good idea in the first place. This article (and an excellent website, by the way) speaks to this sort of thing: https://www.buildingscience.com/docu...ll-design/view

    One more quick thought, depending on the type of wall construction, air movement is a big factor in this conversation. In a heating climate, you can't have air from the home migrating through the walls and the water vapor finding a condensing surface on the back side of the exterior sheathing. Try to keep the pressure boundary on the inside of the wall. I looked at some homes in a heating/cooling climate where poly was used as a vapor barrier on the inside of the wall behind the drywall. The homes were under a negative pressure from the operation of the imbalanced air handling system. Hot, humid air being sucked into the walls from outside and condensing on the back side of the colder drywall. Mold city in every wall cavity.

  10. #10
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    I look at it this way

    Warm moist air migrates to a cool dry environment.

    Fl or NY in the winter the homes RH is low even with a supplemental humidifier. In the summer all that outside moisture will eventually reach the dew point inside the wall. My vapor barrier went on the outside of the exterior walls here.

  11. #11
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    Maybe I should have specified heating or cooling only locations. I don't think a vapor barrier on either side of the wall is appropriate for both a heating and cooling climate as the wall may need to dry in both directions. A vapor retarder makes more sense.

  12. #12
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    A Vapor Retarder it is for Sure , also Now there are 15-20 Different Breathing - Wicking - Exterior Wall Mats that may be applied to Block Before all Finish Walling Materials Go On ..

    Wow in Mikey's Place , I'd be Incline to - Over Do the Roof - with a Complete Roof Over - Trusses & All 6" - 8" 100% Complete with IR Reflective Barriers and Light Color Metal to Boot ..
    But that's Me ..

    Real Work come in On the Old Soffits - I'm afraid I'd be doing some Major DIY work or Rent a Kid of Two .
    Drilling Out Old Sill Plate Right above Walls Cut back Sheathing 18" , Have to Add Double Screen Material , leave No Access for Bugs ..( Steel Wool ) for Plugs during Work
    Let the Top of Block Walls Breathe Up with Heat of Day passing through Old Roof - Into New Improved Upper Cooler Metal Roof - to Go New Big Ridge Venting .. They Use Double Wide & Two Rows Brutal .

    New Upper Roof - Takes Care 2-3 Birds with 1 Stone .. 1. It Allows may more Insulating Properties from Heat Sun ..
    2. Raising the Height Up Off the Old Roof say 1.5" with 2x4's on Side for New Designed Metal Roof .. with @ Least 1" Strapping Strips Crossing back every Ft - length of Roof
    Allows for Big Air Movement .. 1. Keeping Roof Cool . 2. Pulling 4 Times Moisture , so Dry Out Time - before Soffits are Closed back in ( Foam Block Walls )
    3. This also Acts Sound Deadening from Big Storms there - that will have U Jumping out of Bed in Middle of Night - Thinking a Plane Crashed Near_By ..
    4. Give Time to Re_Design the Old Stuffy Soffits with New Longer Shady Soffits ( Beef them Up ) - Keep Sun Off the Building ..
    5. Drying out the Older Building should be done Slowly ( Bust this Move in Nov. in Florida )
    6. There are Migrating Moisture Breathable Sealers - that will move 2 inches or more into Block to stop Moisture .. ( Stripping Walls for Prep ) is even Larger Task . .
    It maybe Possible at Point 18" of Plywood sheathing Removal and Holes - Great Time to FOG the Interior Cavities of Blocks from Above !
    x 180 Holes - There is Actual Spray Head for doing this Type of Work ..
    Stop There or Go for 110%
    If You have Coin - When everything is Open ~ Foam Spray Cement Blocks Interiors 100% that will Suck Moisture from Walls
    as Foam Uses Water to Cure . . then It will NOT come back .. above may run 14K , but the Heat Removal from Building alone will Pay Back in Ten Years or Less
    It may sound like a lot but a 1-2 Punch to Home - will bring Home Trophy in Returns .. way more Comfort . .
    Plus there are - Big DIY Foam Kits now for $660~$1500. so If You did that Part save 5K . .

    Sure Master Dry - will say something that could be done to Footer all way around Home - and Drain it all away . . 5K this Pay back may Mean You don't get as much Humidity in Building ..
    so U see a 5% saving in Electric Bill . .

    First Step on Old Roof is to do a small Cut Back 18 Inches on Plywood Only - Leaving Good Size Air Pocket to Pull from Lower Roof and Holes in Sill Plate ..just some Food for Thought
    Everyone Let Me know If there is anything to Improve upon or I missed . .

  13. #13
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    I Actually had to Remove of of these Heater from few Homes in Florida ~ Maybe they were good Idea 25 years ago ..
    Not in Florida

    Great Idea in NC , SC , GA
    TN, VA , TX , etc CA . .

    https://www.ebay.com/itm/333712579813

  14. #14
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    I remember the fiasco with artificial stucco during the 70-80's. Houses had interior vapor barriers installed and the insulating board was an exterior vapor barrier. Major law suits over wood rot due to moisture being trapped between the two barriers. Companies made a business of "stucco" removal and siding replacements.

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