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Community and Q&A

6 mil poly in the ceiling

ckfk | Posted in Energy Efficiency and Durability on

I am building a shop and garage that will eventually have in floor heating.  I need to put up a layer of 6 mil poly to “seal” the open trusses for temporary heat to pour concrete.  I will eventually put in R49 insulation and drywall the ceiling for the air barrier.  The question is should I leave the poly up on the ceiling.  I am in climate Zone 5.


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  1. Expert Member
    BILL WICHERS | | #1

    I doubt it would cause any problem to leave the poly in place as long as the insulation above it is unfaced and open on top so that it can dry to the exterior.


  2. BrianPontolilo | | #2


    A Bill pointed out, it is not likely to cause a problem, if for example you plan to blow cellulose insulation on top of the drywall or use un-faced fiberglass batts, but the answer does depend on how you plan to insulate and detail the assembly. The poly is creating a vapor barrier. Here are some articles to read with good information about when to and when not to install a vapor barrier and the implications of having a vapor barrier where you shouldn't.

  3. ckfk | | #3

    Thanks for the replies. I’m planning blown in insulation with great ventilation in attic space, so I know the attic should be good. I was somewhat concerned about trapping too much moisture in the garage from melting snow off the cars. What do you think?

    1. Jon_R | | #4

      Melting snow moisture is an issue - but you won't fix it with a little bit of vapor diffusion.

      The big problem with interior poly comes with either AC (?) or little external drying (not the case here).

  4. GBA Editor
    Martin Holladay | | #5

    Q. "I was somewhat concerned about trapping too much moisture in the garage from melting snow off the cars. What do you think?"

    A. I think you are laboring under the misapprehension that your ceiling assembly is a mechanism for removing moisture from your garage. It isn't. No one should ever design a ceiling assembly to allow for the flow of moisture through the assembly.

    I suggest that you read this article: "Worries About Trapping Moisture."

    In that article, I wrote, "Although it’s true that indoor air is warm and humid during the winter, while outdoor air is cold and dry, that doesn’t mean that indoor moisture needs to “escape” from your house. It’s perfectly OK if the indoor moisture stays where it is without “escaping.” Some old-time carpenters look at walls that include a layer of rigid foam and exclaim, “That won’t work! If you put in a layer of foam, the moisture will have nowhere to go!” Moisture in your home isn’t like your retired parents in Michigan on New Year’s Day, itching to go to Florida. The moisture can stay right where it is, all winter long."

    You may also want to read this article: "All About Attic Venting."

    In that article, I wrote:

    "My nickname for this [misguided] explanation is, 'Your ceiling is a safety valve.' This explanation is entirely divorced from any understanding of building science. Here’s how an old-time New England builder might explain the theory: 'You can’t put a poly vapor barrier in your ceiling because your ceiling has to breathe. If you put polyethylene up there, the moisture won’t have anywhere to go. It will be trapped. You want the moisture to be able to get out.'

    "The ceiling-is-a-safety-valve theory encompasses several misconceptions. Here are two of them:
    • The purpose of attic vents is to help lower indoor humidity levels. If you encourage moisture to flow through your ceiling assembly, you will improve conditions inside your house.
    • Because your attic is vented, you need to feed a continual stream of moisture towards the attic vents so that the vents have something to do.

    "Of course, these ideas are misguided. Ideally, your ceiling should include a thermal barrier that separates the warm, humid, interior air from the cold, dry, attic air. You don’t want to encourage any moisture flow through that assembly — whether by air leakage or by diffusion."

    One final point: If your indoor air has elevated relative humidity (RH) during the winter, the way to solve that problem is with ventilation -- not by encouraging moisture to enter your insulated ceiling assembly.

  5. ckfk | | #6

    Thank you all. Excellent information and references. Very much appreciated.

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