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Community and Q&A

Vapor Retarder Between Ceiling and Vented Attic

mrkawfey | Posted in Energy Efficiency and Durability on

This may sound like a dumb question, but I want to make sure I get it right. I have not seen much discussion about a vapor barrier between the ceiling and a vented attic.  So:

1) Clearly understand the need for an air barrier, but I just want to confirm that in a typical vented attic with something like blown cellulose that a vapor barrier is not needed.

2) Separately, in the case of a ceiling service cavity, would that change the requirement for a vapor barrier? Per the codes requiring a vapor barrier for an un-vented space, would this apply? Does a lack of air sealing on the interior side of the service cavity make it vented?

3) In the summer, why doesn’t condensation in a vented attic become a problem? If you have unlimited moisture from outside at a high temp/humidity and air permeable insulation like loose cellulose, combined with AC cooling the interior, wouldn’t it be common to have a spot in the insulation or exterior surface of the ceiling that is at the dew point?


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  1. mrkawfey | | #1

    I should clarify: Northern NY heating climate.
    Regarding ceiling vapor barrier, I've read or been told: "Don't use one", "Ok in your climate, if you want", "Not needed" and "Yes you should have one".

  2. GBA Editor
    Kiley Jacques | | #2

    Hi Christopher,

    Martin Holladay answers that question here: Vented Attic Vapor Retarder. Regarding vapor barrier vs. vapor retarder and understanding vapor diffusion, you will likely find this article interesting, too: Vapor Retarders and Vapor Barriers.

  3. Jon_R | | #3

    Also note that an interior side vapor barrier isn't a good idea where there is significant AC use.

  4. mrkawfey | | #4

    So Martin's response was

    "N1102.5 Moisture control.
    The building design shall not create conditions of accelerated deterioration from moisture condensation. Above-grade frame walls, floors and ceilings not ventilated to allow moisture to escape shall be provided with an approved vapor retarder. The vapor retarder shall be installed on the warm-in-winter side of the thermal insulation.
    1. In construction where moisture or its freezing will not damage the materials.
    2. Frame walls, floors and ceilings in jurisdictions in Zones 1, 2, 3, 4A, and 4B. (Crawl space floor vapor retarders are not exempted.)
    3. Where other approved means to avoid condensation are provided."

    In the 2009 codes and later, well ventilated attics are not required to have interior vapor retarders. However, note these provisions in the 2012 IRC -- provisions which apply to Zones 6, 7, and 8, but not Zone 5:

    "R806.1 Ventilation required. Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. ...

    R806.2 Minimum vent area. The minimum net free ventilating area shall be 1/150 of the area of the vented space.
    Exception: The minimum net free ventilation area shall be 1/300 of the vented space provided one or more of the following conditions are met:
    1. In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling. ..."

    If I am interpreting that correctly, ANY ceiling service cavity needs to be vented. But does it need to be vented to the outside? Can it be vented to the inside? Obviously you can't have a vapor retarder on the warm-in-winter side if you are venting to the interior, but it appears that you don't need it if you have large enough vents.

    Or, is that implying that a ceiling service cavity is ok to be unvented as long as it is built below the rafters (as opposed to using the rafters as the service bay).

  5. Expert Member


    It's the latter. Those code provisions are for roof assemblies. What happens to the interior, whether it be service cavities or bulkheads, isn't covered by those requirements.

    The one thing you should be careful of is that service cavities can sometime short-circuit the fire-blocking between walls and roofs. They can't form a path for flames to spread between the two.

  6. mrkawfey | | #6

    Once again, thank you Malcolm.
    So is the inverse of that true? That a service cavity built between the rafters would need to be vented?

    For example, if I boxed in the top of a rafter bay (bottom cords on a truss) to run a duct for an HRV from it's exit at the top of an interior wall, over to a ceiling register in a bathroom, would that bay need to be vented?


    1. mrkawfey | | #7

      Ok, I think my terminology is failing me. I just realized that when the code says "enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafter"
      they mean like a cathedral ceiling with drywall affixed to the actual rafters of the roof deck.

      So, no VC control layer required as long as the attic meets the ventilation area requirements.

      So, in the OP, #2, if I boxed in a lower truss chord as a service bay and air sealed the top and ends, would that enclosed space require a VC layer?

    2. Expert Member
      MALCOLM TAYLOR | | #8


      No, neither the bottom of any roof assembly or service cavity needs venting.

      Venting always occurs on the top side of roof assemblies, either below or above the roof sheathing. Venting is required unless you have sufficient foam insulation either above or below the sheathing to keep moist air from reaching its dew p0int. The requirement for a vapour retarder or venting flows from which assembly is used, not whether you run services through them.

  7. mrkawfey | | #9

    Anybody have any insight into question #3?
    In a vented attic on a hot humid summer day, where ambient is already close to the dew point, why don't you create excessive condensation in loose fill insulation?

    It would seem inevitable that with the temperature gradient through the insulation to a conditioned space that you would hit the dew point somewhere. And then, with no air barrier between the insulation and the vented attic air, wouldn't you have an unlimited amount of moisture available?

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