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  1. #1
    Join Date
    Jan 2011
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    Default Vapor Barrier on both sides of wall?

    I am building a new home in WI and put 1" tongue and grove pink foamboard with taped seams on the outside of the sheathing, I am using LP smartside siding, the walls are 2x6 and I have R19 unfaced insulation in walls, planning on 6mil poly vapor barrier. My questions are
    1. Is this correct for a vapor barrier system?
    2. I am planning on insulating and poly vapor barriering both side of my interior walls - anything wrong with this procedure?

    Realistically if moisture can't get in with the vapor barrier on both sides of the walls, interior and exterior, there shouldn't be any moisture/mold issues, am I right or wrong in this philosophy?

  2. #2
    Join Date
    Mar 2010
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    Canada
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    Default Re: Vapor Barrier on both sides of wall?

    Quote Originally Posted by christinelarry407 View Post
    I am building a new home in WI and put 1" tongue and grove pink foamboard with taped seams on the outside of the sheathing, I am using LP smartside siding, the walls are 2x6 and I have R19 unfaced insulation in walls, planning on 6mil poly vapor barrier. My questions are
    1. Is this correct for a vapor barrier system?
    2. I am planning on insulating and poly vapor barriering both side of my interior walls - anything wrong with this procedure?
    What does you local building inspection department say? Afterall they are the definative authority.

    Hopefully you applied house wrap over the sheathing before the foam board. Judging from your description you are using pink extruded (XPS) rigid foam ( approx. R5 /inch) which has a permeance of about 1.1 for one inch , which is a semi-permeable material. Compared to the common white expanded polystyrene (EPS) rigid foam (approx. 3.6-4.2/inch depending on density) which has a permeance anywhere from 2.0 - 5.8 for one inch , making it a semi-permeable material and most vapor-permeable of the three types of rigid foam.
    Generally, when applying the foam board on exterior sheathing the seams are not taped to allow vapour to pass through. That's one reason ( among others) for the house wrap between the sheathing and the foam board , for the air barrier from the untaped seams on the foam board.
    Personally I have used the higher density EPS on several occasions with very good results but also use the XPS though it's more expensive. The advantage with the EPS since it's more vapour permeable there is more vapour transfer than the XPS.





    Realistically if moisture can't get in with the vapor barrier on both sides of the walls, interior and exterior, there shouldn't be any moisture/mold issues, am I right or wrong in this philosophy?
    Theoretically yes.
    However, in the real world dynamics of buildings water vapour exists and needs an avenue to escape. Also it depends on how well the seal is made to the interior where the greatest vapour transfer originates during the cold season.

    Rigid foam can solve thermal bridging problems because a stud connects the inside of a house to the outside, it can act as a bridge for heat to escape (studs have lower R-values than insulation). Thermal bridging through the studs significantly degrades the thermal performance of the wall. In all climates, exterior foam sheathing improves a wall's performance.
    Installing rigid foam insulation over wall or roof framing reduces this thermal bridging, raises the R-value of the wall or roof assembly, and can eliminate or reduce air leaks.

    Building scientists have reported that adding 1 in. of R-5 insulation to a 2x6 wall insulated with fiberglass batts increases the effective R-value of the wall from 14.4 to 19.4 ,a 35% gain with only a 15% increase in wall thickness. Adding 2 in. of foam raises the R-value from 14.4 to 23.8, an improvement of 65%.

    A layer of insulating foam on the outside of exterior walls also helps the framing stay dry by raising the dew point of the surface where water vapour is likely to condense. Remember though , thicker foam sheathing is safer than thin foam sheathing in that it needs to be thick enough to raise the dew point and is dependant on your regional cold temperatures.
    Thin foam is dangerous, because it reduces the ability of the wall to dry to the exterior without warming the sheathing enough to prevent moisture accumulation (a phenomenon that is usually but incorrectly called *condensation*.

    Those same building scientists have calculated the minimum foam thickness required for different wall thicknesses and different climates. By following their recommendations, your wall sheathing (or the interior face of the rigid foam) will stay warm enough to prevent moisture accumulation during the winter.
    Because foam sheathing reduces the ability of a wall to dry to the exterior, all foam-sheathed walls must be able to dry to the interior. That means you don’t want any materials with a very low permeance especially polyethylene on the interior of a foam-sheathed wall.

    In other words, make sure the foam is thick enough to prevent moisture acculmulation (“condensation”) in your sheathing or framing; and avoid low-permeance layers like polyethylene or vinyl wallpaper on the interior so the wall can dry inward.

    If you are building a house in one of the warmer climate zones — zone 1, 2, 3, or 4 (except for 4 Marine) — you don't have to worry about the thickness of your foam. Any foam thickness will work, because your sheathing will never get cold enough for “condensation” (moisture accumulation) to be a problem.

    Of course, foam-sheathed walls must comply with existing building codes. Until recently, that was difficult, because some building inspectors insisted on the need for interior polyethylene ,even on foam-sheathed walls, where poly definitely does not belong. Fortunately, the 2007 supplement to the International Residential Code (IRC ) came to the rescue. Since that supplement was adopted, the IRC has allowed certain cold-climate walls to dry to the interior. The code now includes a table, listing which types of wall assemblies have minimal requirements for an interior vapour retarder.
    It gives permission to builders of foam-sheathed walls to use a minimal interior vapour retarder — one with the highest permeance values, known as a Class III vapour retarder. (Ordinary latex paint is all you need.)
    It spells out the minimum R-values for exterior foam to be sure that moisture won’t accumulate in a wall.

  3. #3
    Join Date
    Jan 2011
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    2

    Default Re: Vapor Barrier on both sides of wall?

    Leonard Homes - Thanks for your input, it is greatly appreciated. I still have some questions that hopefully you can shed some light on for me.
    1. Does a person put/use vapor barrier (6 mil poly) on any, one or both sides of an insulated R11 interior wall? - I want to do this for noise reduction. Or will the 6 mil poly cause mold to grow?...especially in the bathrooms from moisture from the shower/water usage. The bathroom does have a humidity control exhaust fan.

    2. We also have a 12x12 hot tub room, as of now I have R19 insulation batts in the exterior 2x6 walls, do I put 6 mil poly over the insulation? The room does have a exhaust fan for humidity control while the hot tub is being used. Should a person use durarock/cement board on the walls in place of normal sheetrock to avoid mold growth on the sheetrock?

    3. Where does a person get a copy of the International Residental Code (IRC) with this table listing the different types of wall assemblies and what exactly is necessary for my wall application? Is this available ****** anywhere? If so, can you send me the link?


    4. My house does NOT have house wrap over the sheating under my 1" pink extruded foam, my tongue and groove seams ARE taped, and was told by my local building inspector that was fine, but the seams DID need to be taped/sealed. He also informed me that I would need 6 mil poly on the inside. Is this correct?

    5. Does a person need to put/use 6 mil poly on the ceiling, before installing the sheetrock?

    The last thing I need is to have spent my lifetime savings on my new house and have it slowly be rotting from the inside out. Due to inadequate information on how to use/where to install vapor barrier. Or using or not using vapor barrier in the right/wrong places. Seems to be a lot of MISINFORMATION out there.

    I still don't understand, that if I put 6 mil poly vapor barrier on the inside of any wall (interior or exterior) in my house, why I should have any moisture problems, unless I just simply don't understand thermal dynamics. I would rather "over build" my house, than find out later, that I should have done this or that and end up tearing things apart cause they are rotting and falling apart.

    I am looking for answers that are black/white. I want to know exactly what I need to do for my application. I have 2x6 walls with 7/16 sheating with 1" pink R5 extruded foam that is tongue and goove and the joints are taped. No house wrap was used. I have R19 batts in the walls. Simply put, yes or no, do I need to put 6 mil poly on the interior of the wall. If I do not, why not. If latex paint is enough of a vapor barrier, then what harm is done with putting the 6 mil poly on the interior of the wall and "over building" the wall?

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