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Thread: Vapor barrier / "Moisture retardant"

  1. #1

    Vapor barrier / "Moisture retardant"

    Almost two years ago I moved into a newer house (5 years old).

    I've setup my humble workspace in our attached garage - which has been completely sheet rocked.

    I live in MN, I want my heat for winter!

    So last year I installed faced bats on all the exterior walls. This year I want to do blow-in insulation above the garage. The rest of the house is blown-in on top of a vapor-barrier.

    My problem presents itself in the form of the "vapor barrier" for the ceiling in the garage. I want to know what your thoughts are on my options without having to drop all the drywall.

    Is it acceptable to place the barrier over the ceiling joists and into the spans between, and overlapping electrical - as long as it's between the warm side and the insulation?

  2. #2
    Join Date
    Jan 2004
    Location
    Lewiston, Idaho
    Posts
    23,684
    Justin,

    Do you live in a city or an area where there are buiding codes? If so, contact the inspector or regulatory agency and ask them.

    When I built my shop, the building code inspector told me I didn't have to put up plastic, I could just use a PVA sealer/primer over the wallboard and then paint the wallboard.

    I didn't have faith in that method but I confirmed with inspector, that putting 4 mil plastic and then hanging the sheetrock would still meet code. It did and that's what I did.

    My point....it's possible that using a PVA primer over the sheetrock might do the trick. Check with the local code inspector.
    Ken

  3. #3
    Yea I don't have much faith in the paint-on stuff either.

    putting 4 mil plastic and then hanging the sheetrock would still meet code.
    That's the standard isn't it, or did you mean it the other way around like I'm mentioning?

    Most people have (in a layers kind of list for blown-in):

    insulation
    joists
    plastic
    drywall

    I'm thinking of:

    insulation
    plastic
    joists
    drywall

    Since the drywall is already up =|

    I suppose I'll know tomorrow when I call, kinda hard to find out who to call in this little town.

  4. #4
    Join Date
    Jan 2004
    Location
    Lewiston, Idaho
    Posts
    23,684
    Justin,

    Yeah I put it up in the normal fashion.

    I was building a new shop and wanted to do it the first time.

    Good luck!
    Ken

  5. #5
    Join Date
    Oct 2006
    Location
    Mid Michigan
    Posts
    3,539
    I installed 6 mil plastic sheeting prior to adding OSB then added 14 inches of blown in insulation.
    David B

  6. #6
    Join Date
    Jan 2010
    Location
    Bellingham, Washington
    Posts
    778
    PVA (polyvinyl acetate) primer has been used as interior vapor barrier for decades now. If it is done properly, it works very well. Important thing is to lay it on in a couple of coats. Much less work and will be easier to get right than trying to retro fit plastic sheeting.

  7. #7
    I wouldn't mess with trying to retrofit vapour barrier on top of your joists. I'd be worried about it stretching and ripping.

    The paint-on stuff is probably quickest. Other options would be using spray foam (expensive) or ripping down the ceiling drywall and using plastic (time-consuming).

    Also, check out how much it costs to hire someone. I had R30 of fiberglass blown into my garage attic for less than it would have cost me to buy batts.

  8. #8
    Remember: to be effective, a vapor barrier MUST be on the warm side of the insulation. The object is to prevent water vapor from entering the insulation where, during cold exterior temperatures, it will find the dew point WITHIN the insulation and condense, thus ruining the insulative properties of the insulation.

    Yes, a thick film of high quality latex-acrylic paint or PVA primer (or both!) will perform the function of a vapor barrier installed behind the dry wall. However, there are still penetration issues at windows, joints between wall and floor, wall and ceiling, utility outlets, etc. You can improve the integrety of a vapor barrier using an intelligent application of sealants.

    A few years ago, I worked with the Super Good Cents program, retro-sealing houses. We had good success with clear silicone. The worst offenses were plumbing penetrations, electric outets and window frames. Nonetheless, we were able to improve thermal/infiltration/moisture performance of these houses several hundred percent in some cases.

  9. #9
    Quote Originally Posted by Thom Porterfield View Post
    Remember: to be effective, a vapor barrier MUST be on the warm side of the insulation. The object is to prevent water vapor from entering the insulation where, during cold exterior temperatures, it will find the dew point WITHIN the insulation and condense, thus ruining the insulative properties of the insulation.
    Right, my confusion is that it's still the same 'sandwich', except for the joists.
    The insulation still has the VB between it and the drywall/joists; keeping the moisture at bay.
    My plan is to follow the contour and tack into place so I don't think tear/rip will be an issue, it's just more wasteful of the VB roll.

    Tried to call a few times this AM. No answers so I sent an email.
    Last edited by Justin M Rovang; 09-16-2010 at 12:42 PM.

  10. #10
    Join Date
    Nov 2007
    Location
    Fort Pierce, Florida
    Posts
    2,432
    Best to check with your local officials, but I have seen it done, and is supposed to be legal. You have to tuck tape all joints and seal around penetrations, but that should have been done to prevent fumes from the garage (that's the reason for the drywall in the first place.)
    Retired - when every day is Saturday (unless it's Sunday).

  11. #11
    I would recommend a layer of spray foam up in the attic, over the joists and drywall, and up into the area above the exterior walls (baffle for air venting first). Then top off with cellulose.

    Don't let the foam installer talk you into more than an inch everywhere. Every foam installer I've met thinks the foam is "magic" and "an inch or two is all you need to keep your home warm and cozy". They are delusional salesmen. But the product is fantastic, it just takes the normal amount of it you would think by calculating the actual R-value times the thickness. The expense is the killer, but only using an inch to seal everything, and then the more economical cellulose on top is a great way to go.

    This is a very effective way to seal and insulate an attic in a cold climate, and it's especially good when the drywall is already up and there's no VB.

    I would not recommend blown-in fiberglass, based on my past experiences with it. It's just not a good value. There's a lot of air fluffed into it, and one of the least-known things about fiberglass is that the effective R-value actually drops when it gets cold, and drops even more when cold air moves through it.

    Greg

  12. #12
    I'm not sure I would want the lumber on the warm side of the vapor barrier in a northern climate. The vapor barrier is to prevent moisture from migrating to a cold area where it can condense and cause mold. By wrapping three sides of your lumber with plastic you are likely going to have mold issues on the wood.
    Lee Schierer - McKean, PA

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  13. #13
    Join Date
    Jan 2010
    Location
    Bellingham, Washington
    Posts
    778
    +1 on what Lee said.

  14. #14
    The official answer from my local building + zoning is that it's fine.

    As for the mold worries I had considered that myself. Maybe I'll just fill the gaps instead of draping over.

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