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  1. #1
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    Feb 2010
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    Default No vapor barrier in ceiling

    I know now that I should have put a vapor barrier in the ceiling when I redid it a couple years ago, but hindsight is always 20/20. That is what happens when you ask the question to the wrong people. Now I have the dilemma of frost buildup on the roof decking when it gets below zero. What are some of my options? Will a couple coats of oil based paint help retard this, or that vapor primer I have read about help. Should I blow in some more fiberglass insulation or cellulose? How about laying some unfaced R19 on top? What is the best route to take. I spent a lot of time making a race track ceiling and do not want to redo it.

    TIA

    Kirk

  2. #2
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    Feb 2010
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    Default Re: No vapor barrier in ceiling

    A vapor barrier probably won't help. The moisture in the attic is probably coming from all the leaks through the ceiling. Fiberglass does not stop air movement. I install spray foam and see dozens of attics with fiberglass that don't work right. I spray the underside of the roof deck and make the attic semi-conditioned space. This works great and cuts down on heating bills dramarically.

  3. #3
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    Feb 2010
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    Default Re: No vapor barrier in ceiling

    What extra vapor in the attic?

    The attic becomes the same temperature as the house, or nearly so. The temperature in the houses I have gone back and looked at, the attic temperature is close to the temperature in the house. There is no more moisture in the attic than in the house. You absolutel;y do not want ridge and soffit vents. The whole point is to make it an unventilated assembly and semi-conditioned space.

    You will not have frost as the attic is now warm.

  4. #4
    Join Date
    Aug 2007
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    Default Re: No vapor barrier in ceiling

    Quote Originally Posted by RJordan View Post
    What extra vapor in the attic?

    The attic becomes the same temperature as the house, or nearly so. The temperature in the houses I have gone back and looked at, the attic temperature is close to the temperature in the house. There is no more moisture in the attic than in the house. You absolutel;y do not want ridge and soffit vents. The whole point is to make it an unventilated assembly and semi-conditioned space.

    You will not have frost as the attic is now warm.
    I'm of the same thinking and have been for awhile after a seminar about attic ventilation/insualtion 18 years ago. This had been put on by government researchers and basically the trend was that conventional thinking with ventilation is over rated --- instead making the space a conditioned space ---- not much different to wall cavities.

    The one thing I have to nit pick on your posts --- you keep mentioning just spraying the underside of the roof and failing to mention the entire exterior surfaces of the attic need to be insulated. This is the only way of making a sealed envelope.

    I don't believe the temperature will indeed be the same as the interior conditioned space but it will be significantly increased ---- lessoning the Delta T.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  5. #5
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    Aug 2007
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    Default Re: No vapor barrier in ceiling

    Quote Originally Posted by tcfarm View Post
    I know now that I should have put a vapor barrier in the ceiling when I redid it a couple years ago, but hindsight is always 20/20. That is what happens when you ask the question to the wrong people. Now I have the dilemma of frost buildup on the roof decking when it gets below zero. What are some of my options? Will a couple coats of oil based paint help retard this, or that vapor primer I have read about help. Should I blow in some more fiberglass insulation or cellulose? How about laying some unfaced R19 on top? What is the best route to take. I spent a lot of time making a race track ceiling and do not want to redo it.

    TIA

    Kirk
    The main area that is often not done or simply over looked is sealing any and all penetrations into the attic space. Penetrations from electrical , plumbing vent stacks , exhaust fans , etc. need to be air tight preventing warm moist air entering the cold attic space --- this is the majority of the source of moisture.

    Vapour diffusion is another componenet that can be retarded by a minimum of 2 coats of oil paints and there are vapour retarder paints. These are not vapour barriers which have a perm. value of 1 or less ---- retarders slow vapour diffusion they don't block it.

    Air infiltration accounts for 100 times more vapour than diffusion --- seal up penetrations air tight.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  6. #6
    Join Date
    Feb 2010
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    Eastern MA
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    75

    Default Re: No vapor barrier in ceiling

    Canuk,
    You are absolutely correct about needing to cover the entire exterior surface. This is referred to as an "unvented attic assembly" and there are a number of explanations on the web. The attic has to be completely sealed from air infiltration. I am glad you brought it up as others not familiar with the method may not have understood. If the attic isn't completely sealed, you won't get much benefit from the foam insulation. In particular, it is necessary to bring the foam all the way to the eave and seal to the top of the wall. All penetrations in the roof for venting must be sealed.

    I have measured the temperature in a number of the houses I have worked on and found the attic temperature to be within a few degrees of the house below. Certainly, if the ceiling had no leaks and was well insulated the temperature difference would be greater than what I am seeing. If that was the case it probably would not make any sense to foam the rafters.
    Regards

    is one of the pro

  7. #7
    Join Date
    Mar 2009
    Posts
    1,387

    Default Re: No vapor barrier in ceiling

    Howdy, consider having cellulose added on top of the fiberglass it stops air infiltration. It is easy to blow into the space and not costly. Foam is great but cost is too

  8. #8
    Join Date
    Feb 2010
    Location
    Eastern MA
    Posts
    75

    Default Re: No vapor barrier in ceiling

    Cellulose doesn't stop air infiltration from below. You must seall all penetrations into the attic before adding it.

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