Vapor Barrier questions

Discussion in 'Remodel Forum & Blog' started by Nate R, Nov 15, 2008.

  1. Nate R

    Nate R Member

    Jan 11, 2007
    I perform R&D testiing at small engine manuf.
    Milwaukee, WI
    1 1/2 story 900 SF house in Milwaukee built in the early 20s. No basement.
    I have wall cavities that are built w/ studs between 3 7/8" and 4" thick. (The house was built w/ reused wood from another structure.)

    The rafters on this house are also 2X4s (Really nearly 2" wide and nearly 4" in depth) and there is a cathedral ceiling in some of the house on the upper floor.

    In the stud cavities, I've been planning on using R13 Fiberglass batts. But that leaves me some extra space. I do want to insulate this house pretty well, as we plan on being here for a while, and paying the house off in about 7 more years. Anyway, I bought some 1/2" Polyiso rigid foam board w/ foil facing. But this brings up the vapor barrier issue.

    Here's a stackup of the materials from drywall to the outside.
    My first thought, w/o any consideration of vapor barriers/retarders, was :

    R13 Fiberglass Batt Kraft Faced Insulation;
    1/2" Foil Faced Polyisocyanurate Foam Sheet; (R3)
    Original 1X6 sheathing boards;
    3/8 OSB for racking resistance; (The sheathing boards are perpindicular to the studs, not diagonal, and there's no let-in diagonal 1X4 or anything. And there's been some racking over the years because of it. I don't want it to go any further.)
    1/2" or 3/4" foam; (Styrene or Polyiso?)
    Vinyl Siding.

    Obivously, the 1/2" polyiso sheet is a problem w/ water vapor, since putting it tightly against the inside of the sheathing, and then having kraft faced batts makes for a double vapor barrier. The foam I use on the outside could become an issue as well depending on what type I use.

    So, what are your thoughts on how I SHOULD do the stackup here to be proper? And Why?

    I've been reading that Extruded polystyrene is not a vapor barrier, but a retarder, and is OK to use on the outside of the sheathing if the joints are NOT taped.
    But I believe foil faced polyiso is a vapor barrier. Should I use unfaced batts, and put the polyiso just behind the drywall instead of against the sheathing?

    Or should I find some 1/2" extruded polystyrene and use that inside the sheathing instead?

    Or use the polyiso as the current stackup shows, and just leave small gaps around some of the edges between studs and between pieces so moisture can travel in and out? I mean, this works for kraft faced insulation (cutting slits in it) right? So any reason I can't use the iso i already bought as shown above as long as I leave some gaps (1/8" or so) around them"

    Thanks for any input! I need to figure it out soon, as my wife wants to reinsulate a wall cavity we have open. Right now, the exterior side isn't as shown above, but will be eventually. We have the batts and the polyiso to do the cavity, but I didn't want to do it until I was confident in the manner in which we do it.
  2. smokinjoe

    smokinjoe New Member

    Nov 19, 2008
    Nate check at Home Depot or ******* ect for the toll free numbers of like Owens Corning and other insulating manufacturers and talk to the experts...
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  4. jimbo

    jimbo Plumber

    Aug 31, 2004
    San Diego, CA
    You have asked important questions, and there may not be an expert here to really answer it. Us plumbers do not like to admit that there is any area we are not qualified on, but the issue of vapor barriers in well insulated buildings is important. I would try to contact a good general contractor for some advice.

    Stay tuned here because we do have thousands of folks who tune in, so you may still get a professional response here.
  5. Nate R

    Nate R Member

    Jan 11, 2007
    I perform R&D testiing at small engine manuf.
    Milwaukee, WI
    Since I posted this, and didn't get any response in a couple days, I figured I'd better sit down and study what I could find that was legit. Did some reading of papers, etc written by experts, and understand vapor barriers, vapor retdarders, etc a lot better.

    Did calculations for my own case to see what the temperature on the inside of the sheathing would be, and how close it is to the dew point in the house, etc. Basically, what I've concluded is this:

    My home is on the ragged edge of requiring a vapor retarder on the inside based on the calculations.

    BUT, I'm adding polyiso to the inside of the sheathing, with no air gap. This, to me, would make the interior surface of the polyiso what I would consider the inside surface of the sheathing, essentially. So, there shouldn't be any condensation in the wall cavities even with no interior vapor retarder under average outdoor temps in winter and average indoor temperatures and humidities.

    BUT, by having a Class II Vapor retarder on the inside, (Kraft faced batts) I should definitely slow the diffusion of moisture into the wall cavities from inside, and further decrease the outside temperature or inside humidity at which condensation would occur.

    By either leaving small gaps around the polyiso foam seams and ends, or getting a spiked roller and punching many tiny holes through it, I should increase the perm rating dramatically. Enough so that it won't trap too much moisture in the cavities. Some extruded polystyrenes increase the perm rating this way.

    If I use extruded polystyrene foam on the outside of the sheathing and don't tape the seams, there should still be enough "breathability" between the cavities and the outside.

    Basically by having something that is a vapor retarder on both sides, I decrease the amount of moisture that can move into the cavities from inside or out But, they are not vapor BARRIERS, but retarders. So the cavities should still be able to move water vapor out that does get in.

    So, as long as I don't make the polyiso fit TOO tightly between the studs, or put a lot of tiny holes in it if I do, I should be OK putting it against the inside of the sheathing.
    On the outside, I'll use the more permeable polystyrene.
  6. Wet_Boots

    Wet_Boots Sprinkler Guy

    Mar 23, 2007
    Metro NYC
    Don't forget to look at some variable-permeability products like Mem-Brain.
  7. frenchie

    frenchie Jack of all trades

    Dec 30, 2006
    Brooklyn, NY and Fire Island, NY
    As far as the extra space, just use a thicker batt. A compressed batt won't achieve it's true rating, but it'll still be better than the next-one-down. \

    This is hard to explain: just check out the chart, attached.

    ...I see you already bought the foil-faced.

    If you're on the ragged edge of needing a VB inside, why not just mount the polisio on the inside, and use unfaced fiberglass? that gives you a VR on the inside, where it belongs, and no worries...

    Attached Files:

  8. Nate R

    Nate R Member

    Jan 11, 2007
    I perform R&D testiing at small engine manuf.
    Milwaukee, WI
    Ah, I didn't think a compressed batt would be that good! Never saw a chart showing compressed R-Values before. Thanks!

    I would've, but I already had kraft-faced insulation. Given that chart though, I'll just use up the sheet or so of Polyiso I have left, and then just use 5.5" thick R21 batts for the rest. Compressed at 3.5" they're R15. So I'd think at 4" they'd be at least R16, which is what I'm getting now w/ R13 3.5" Batts and the R3 Polyiso.

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