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Attic air barrier

CanadianExpy | Posted in Green Building Techniques on

So I have my plans for my new build complete, but looking for the best detail on how to air seal the ceiling between the main floor and unconditioned attic space.
The house is one story with walk-out basement, the attic will have at minimum R60 blown-in cellulose, the raised heel trust will allow for this or more.

Originally I was going to try and do a airtight drywall approach on the ceiling, but with potential of lights being mounted on the ceiling and pot-lights(LED hopefully low profile) I am worried this approach might not work, with the multiple penetrations.( my wife wants some pot lights in the kitchen)
I have looked at other builds that use taped OSB(not sure thickness) and drop the drywall below using 2x material to get a air barrier. Although I guess OSB might not even be an air barrier?!?!

The OSB walls will be taped for my air barrier and was thinking of trying to tighten the drywall also on the walls.Hopefully multiple air barriers won’t be a problem.

I am trying to get a tight house and heat with mini-splits. The main floor is 2200sq’ which I know isn’t small but still want to try and heat with a Mini-split.

Any advice on which way would be better recommend? or a different method…

Thanks Dave
Zone 6

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Dave,
    Here's my advice:

    1. Using drywall as your ceiling air barrier is usually the best approach. Verify air sealing details with a blower door.

    2. Buy airtight electrical boxes for any light fixtures in your insulated ceilings. Don't be tempted to install any recessed can lights. Make absolutely sure that your ceiling-mounted electrical boxes are airtight.

  2. CanadianExpy | | #2

    So with the ceiling drywall as my air barrier, I will do a blower door test after door and windows are in and ceiling only drywall is up?
    With penetrations in place (lights) and sealed.

    Just trying to make sure I get timing right as I am acting as General Contractor, and doing some of the work myself. Build starts in a month or when winter leaves!.

    As for the the can lights, I was hoping to use LED low profile, which still penetrates the drywall but I assume you can use the airtight electrical boxes. I'm trying to keep the lights to a minimum,

    Thanks again for all the advice.
    Well worth renewing my membership.

  3. GBA Editor
    Martin Holladay | | #3

    Dave,
    You can't conduct a blower door test until your air barrier is complete. When that occurs depends on the layers you will use for your air barrier. (For more information on blower door testing, see Blower Door Basics.)

    Let's say that your wall air barrier will be taped OSB sheathing. At a minimum, you would need to have the following steps finished before you could conduct a blower door test:
    Wall sheathing
    Windows and doors
    Ceiling Drywall

    Whether or not you decide to hang and tape the ceiling drywall before the rest of the drywall depends on your preferences and scheduling flexibility.

    You might want to buy your airtight electrical boxes from Allied Molded, a company that sells airtight boxes that are 2 3/8 inch deep:
    http://www.alliedmoulded.com/media/536222/Vapor-Seal-Box-Brochure.pdf

  4. CanadianExpy | | #4

    Alright sounds good, I just trying to get timing figured out.
    I have 6" of exterior Polyiso going on, so I just want to make sure I get things tighten up before it goes up.

    Thanks for the link..

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