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Thread: Foam board insulating sheating?

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  1. #1
    DIY Senior Member Nate R's Avatar
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    Default Foam board insulating sheating?

    Hypothetical wall from inside to out:
    1/2" drywall
    vapor barrier
    insulation/stud
    7/16" OSB sheathing
    1.5" Foam board insulating sheathing
    Tyvek

    So, if I install a new construction vinyl window, it gets secured to the foam board!? If I secure it to the OSB, the integral J channel in the window would get buried by the foam board.

    Seems not very secure to me. Also makes for larger jamb extensions.

    Obviously foam sheathing and vinyl windows go together all the time in new construction, so how is it done? Anyone know?

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    Jack of all trades frenchie's Avatar
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    1-1/2" of foam and a vapor barrier? Trouble. How's your wall assembly going to dry out? Might want to spend some time over at Lstiburek's site:

    http://www.buildingscience.com/bsc/

    The typical detail for windows over foam is to block out, with wood, flush to the foam - then attach your window to that. Requires careful detailing of the drainage plane to prevent water intrusion at the seam between them. Either housewrap before, or over, the foam.

    a few relevant links:

    http://www.eere.energy.gov/buildings...ndows_foam.pdf
    http://www.coastalcontractor.net/cgi...icle.pl?id=123
    http://www.umass.edu/bmatwt/publicat...e_outside.html
    http://www.ibiblio.org/london/renewa.../msg00741.html


    These other links, are from a pro-only forum, be careful about posting there if you're not in the trades; the forum regulars can be very mean. The building science sub-forum is open to anyone, as is the electrical. The rest of the forum... only pro's (as you'll see from the 2nd link, from the exterior details sub-forum, where the moderator closed the thread):

    http://forums.jlconline.com/forums/s...ad.php?t=39586
    http://forums.jlconline.com/forums/s...ad.php?t=38479
    http://forums.jlconline.com/forums/s...ad.php?t=33080
    http://forums.jlconline.com/forums/s...ead.php?t=8047
    http://forums.jlconline.com/forums/s...ad.php?t=11328
    Last edited by frenchie; 01-06-2008 at 07:40 AM.
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    DIY Senior Member Nate R's Avatar
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    Quote Originally Posted by frenchie View Post
    1-1/2" of foam and a vapor barrier? Trouble. How's your wall assembly going to dry out?
    Hmm, didn't realize/think about that.

    Does that mean when someone installs vinyl siding w/ the fanfold underneath that they should not have an interior vapor barrier? Since the fanfold is the same material as insulating foam sheathing, no?

    What do you plan on using for insulation ? Dense pack cellulose gives you the most bang for your buck. It also does a decent job of preventing air movement in the structure. Fiberglass is the hardest to install and get the rated R value altough its not impossible.
    I had just defaulted to fiberglass in my mind. Never really thought about anything else. I was thinking sprayed urethane would be too expensive, so glass was next in my mind. The dense pack cellulose: Is that the stuff with the adhesive in it so it can be installed when the walls are still open?

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    DIY Senior Member Nate R's Avatar
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    Oh, and thanks for the links! I'll check out those other forums.

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    DIY Senior Member construct30's Avatar
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    A vapor barrier is required on the inside of the insulation, you can not do it without it.

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    Quote Originally Posted by construct30 View Post
    A vapor barrier is required on the inside of the insulation, you can not do it without it.
    says who ?

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    DIY Junior Member interalian's Avatar
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    Quote Originally Posted by construct30 View Post
    A vapor barrier is required on the inside of the insulation, you can not do it without it.
    It's a code requirement here (6mil), and Alberta is extremely dry ambient. Strangely though, contiguous vapour barrier around exterior wall electrical boxes is required during renovations, but not on new construction. My 3 year old house leaks through all the wall plugs. Exterior is vinyl over tar paper over OSB over 2x6 with R20 fiberglass.

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    Nate,

    Why do you feel that you need a vapor barrier ? you live in a cool enough climate where there is nothing wrong with using one. However its just not necessary. Most moisture enters a wall cavity on air currents very little enters through vapor dispersion. A wall that leaks a lot of air leaks a lot of heat and money. Drywall is a good air barrier especially with some vapor barrier paint.

    What do you plan on using for insulation ? Dense pack cellulose gives you the most bang for your buck. It also does a decent job of preventing air movement in the structure. Fiberglass is the hardest to install and get the rated R value altough its not impossible.

    Many products are available that render the sheating un necessary. Simpson makes several shear panels that accomplish the same thing. Nothing bad will probably come of installing the sheathing its just not needed. I would just install the foam directly to the studs and tape all the seams. If you wanted you could block out around the windows with a 2x4 flat. The details can be fairly complex. but doable JLC has some good articles and books on the subject

    TYVEK: just do a Google search on Tyvek and Dick Seibert and see what comes up.
    Jamb extentions are done everyday. most window places will add them for you if you are confident in the final all thickness

    hope some of this helps

    Lou

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    Homeowner geniescience's Avatar
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    i'd listen to the guy from Alberta. His climate is closest to yours.

    Where to put the vapor barrier, (inside or out) or whether or not to put one at all, -- it all depends on your winter temperatures and dew point. Local climate.

    david

  10. #10
    Jack of all trades frenchie's Avatar
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    Quote Originally Posted by Nate R View Post
    Does that mean when someone installs vinyl siding w/ the fanfold underneath that they should not have an interior vapor barrier? Since the fanfold is the same material as insulating foam sheathing, no?
    No, 1/2" is fine; but 1-1/2", you've got a vapor retarder.

    Permeability decreases as thickness increases: 1-1/2" blocks moisture transport 3 times more than 1/2".


    Quote Originally Posted by construct30 View Post
    A vapor barrier is required on the inside of the insulation, you can not do it without it.
    That depends on his local climate, and codes.


    Quote Originally Posted by interalian View Post
    It's a code requirement here (6mil), and Alberta is extremely dry ambient.
    You mean, because Alberta is extremely dry ambient. Vapor pressure is related to the difference in humidity between inside and outside. So for the same indoor humidity, a drier outside means there's more vapor pressure.


    Quote Originally Posted by geniescience View Post
    i'd listen to the guy from Alberta. His climate is closest to yours.
    Not sure I agree. Alberta's drier, and colder, than Wisconsin; I think they're on different sides of the line for plastic. But I'd still check local codes.


    Anyways, the point is that you can't do both, 1-1/2" of foam on the outside, and plastic vb on the inside, in any climate, and not cause problems.


    Nate -

    Another thing to look out for - you'll pick it up in those links - is that if you want foam on the outside, you can only put so much insulation inside the wall. The inside of the foam has to be kept warm enough to not get condensation. If you over-insulate inside, you move the dewpoint* further in, and the framing rots.

    *"first condensing surface", if you want to get technical.


    Is this new construction, or retrofit? There are other ways to get high-R-value walls:

    - spray foam
    - dense-pack cellullose
    -Thicker walls, either by using bigger studs, or (if you want to avoid thermal bridging) strapping cold spots
    - Insulated Structural Panel construction
    - rigid foam on the inside, would work pretty well in your climate; use the foil-faced, and it's a vapor barrier on the correct side...

    ...Lstiburek's climate-zone-specific builder's guides, from the buildingscience.com site, are the bible for this sort of thing, and well worth the price. Written for contactors, rather than academics: clear explations, detailed recommended specifications and drawings... makes it all very easy to understand how everything plays out in your specific climate.


    Last note: bear in mind that nominal R-value is just one piece of the equation. The average house loses a lot more heat through drafts, than through not having enough insulation. I remember Calgary as pretty freakin windy in the winter...
    Master Plumber Mark:

    there is nothing better than the
    manly smell of WD 40 in the air
    while banging away on brass with a chisel and hammer...

    it smells like......victory......

    do not hit your thumb...
    __________________
    Just so everyone's clear: I'm the POODLE in the picture ("french", get it?) The hot woman is my wife.

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