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Unvented roof deck with dense-packed cellulose — vapor barrier?

mscarried | Posted in General Questions on

Hi I just dense packed my attic ceiling (it is a livable space, not crawl) with dense pack cellulose (8″), held in with insulweb. we put the cellulose right up against the roof deck. I am concerned whether I need a vapor barrier and where it should go. My plan was to just put rough cut wood (not tongue and groove ontop of it for the ceiling, but a few people who looked at it thought I might need a vapor or air barrier. The roof is not vented. Though I also need a new roof come spring, so possibly just putting a metal roof ontop of the old one will solve this – or they are two separate issues. I guess it is too ate to ask whether it is ok that it is against the roof deck. But before putting the wood up (I don’t have it yet and money is short), should I put plastic or tyvek up..? Any suggestions?

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  1. GBA Editor
    Martin Holladay | | #1

    1. Your method of insulating violates the building code and puts your roof sheathing at risk for moisture accumulation and rot. You can't install cellulose in a sloped roof assembly unless either (a) you have a ventilation channel between the top of the insulation and your roof sheathing, or (b) you have installed a substantial amount of rigid foam insulation above your roof sheathing. For more information, see How to Build an Insulated Cathedral Ceiling.

    2. If you can find a way to fix your current mess, you still need an air barrier (not just boards) on the underside of your insulation layer.

  2. wjrobinson | | #2

    Why? Why do we do and then... ask how to .... do?

    People... ask... confirm... plan.... then... do.

  3. iLikeDirt | | #3

    Do not put a vapor barrier under the studs. That would be a costly mistake. You have already created a condensation trap. Putting a vapor barrier under it will simply collect the condensation and direct it into your exterior walls.

    The solution is to warm your roof sheathing so water won't condense on it. To do this, you need to insulate it. If you need a new roof soon, then you have an opportunity to salvage the situation. After the shingles are off and the roof deck is exposed, cover it with 4+ inches of foam insulation board with staggered seams; more is better. Then attach a plywood nailing surface over the foam, apply roofing felt, and install the new metal roofing over over 1x purlins. Installed this way, you can get away with exposed-fastener metal roofing and save some of the cost of upgrading to standing seam. Martin is right about the interior air barrier, too. If you really want wood T&G on the interior, just put up like 1/4" drywall first and seal the seams, and then apply the board sheathing under that.

  4. charlie_sullivan | | #4

    I see three options:
    1) Dump the cellulose out and re-do it right.
    2) Put in an air barrier and hope for the best even though you know it's not done right and will eventually fail.
    3) Put foam above the cellulose.

    If you don't do option 1, you probably want to put something up sooner rather than later to slow down the moisture getting through the cellulose.

  5. mscarried | | #5

    well, I thought the research I did prior, plus the person doing it thought this was correct, only when it was looked at by another contractor, did this question come up. what I have read about the cellulose is that with 8" act as a vapor barrier, when Iook it up, it seems it is not necessary.. (here's a quicky from national fibre: ("In summary, we do not recommend the use of vapor barriers with cellulose insulation, except
    in circumstances of exceptionally high moisture levels, such as an indoor pool facility")

    OK I will look more into what to do when I put the new roof on, Why not just put metal Over the old roofing there, won't that act as an air barrier?
    For the inside, instead of 1/4" drywall, how about sealing the ceiling with plastic (to go under the wood) (will it work same as drywall, just less work. - hey its not like putting insulweb and dense packing was easy, cheap or not thought out :).. I've heard of conflicting opinions on this vapor barrier.

  6. GBA Editor
    Martin Holladay | | #6

    You don't want a polyethylene vapor barrier. For more information, see:

    Do I Need a Vapor Retarder?

    Vapor Retarders and Vapor Barriers

    And you want a good air barrier on the interior -- not something flimsy like polyethylene. Gypsum drywall is cheap, and once you have taped the seams you'll have a good air barrier.

    But you really need to put several inches of rigid foam on top of your roof sheathing to make this assembly work.

  7. iLikeDirt | | #7

    You don't need or want a vapor barrier. What is it with vapor barriers? Why do so many people focus on and obsess over them? What pointless, detrimental things they are in 99% of all cases. You don't want a vapor barrier! Not here, not there, not anywhere! You do not need them, Sam-I-Am!

    The fact that you are talking about your roof situation as well leads me to believe that you think the danger comes from rain leaking in or water vapor migrating from outside to inside. Nope. The danger comes from moist air that's already inside your house migrating through the air-permeable cellulose and condensing on the underside of your roof deck, which you just made very cold by insulating it on the inside.

    Since you've mentioned that need to replace your roof anyway, the best solution here is to do that right now in conjunction with adding exterior insulation. If you add insulation over your exposed roof deck, it will be warmer, and moist interior air will be far less likely to condense on it. That will give you some breathing room before you put up your interior-side air barrier which really should be drywall. The combination of an interior air barrier and exterior insulation will solve the problem everything will be good again.

    Do not try to save a buck by using sheet plastic as your interior-side air barrier. Moisture loves to condense on it and can't get through it. If your air-sealing around the sheet plastic is less than perfect, moist air will get up there anyway, and if it condenses on the roof deck and falls, it will fall on top of the plastic and be carried by gravity into the exterior walls. Don't do it! It should be obvious why a metal roof is not an air barrier. It is outside of your house. It is not sealed to anything. Etc.

    It sounds like you need to fire your contractor and get a new one. His ignorance has endangered your house. If you let this go and a major moisture problem develops, he would be legally liable.

  8. mscarried | | #8

    ok thanks -
    Solution (I mean it isn't BAD yet) sounds like, A) what I did is not so bad - so an air barrier inside with thin drywall, totally do-able and B) when I redo the roof, put some outside insulation on roof before putting on the new roof. WHy is it taking off the old roof is necessary (I see for putting insulation on, but I don't see (sorry) why the original roof wont act as an extra barrier.

    For future knowledge, what would a better solution have been to insulate this large attic space as I could not afford spray foam, was it to put foam boards in first then cellulose (or fiberglass.). ?

  9. wjrobinson | | #9

    Carrie, you should have installed vent chutes. then the cellulose, then on the inside I would add continuous rigid foam taped and then the T&G. That is a good ceiling and economical.

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