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Community and Q&A

Best ceiling air barrier for house addition?

Michael Tuso | Posted in Energy Efficiency and Durability on

I live in the mountains of Virginia, climate zone 4 bordering on climate zone 5. We typically have summer high temperatures of 90-100 deg F and winter low temperatures of 10-20 deg F.

The ceiling joists in my single level house addition are 16” deep I-Joists 24” OC with 1-1/4” LSL rim board perimeter. Above the I-Joists and rim board is a 5/8” plywood attic floor. The attic floor panels are nailed and glued around the perimeter and tacked down with deck screws everywhere else. My plan is to temporarily remove the screwed down panels so the 16” deep I-Joist cavities can be filled from above with blown in fluffy stuff – cellulose or fiberglass.

The space above the attic floor is well ventilated with continuous soffit and ridge vents. Because the roof rafter lower ends rest on a 2×6 top plate sitting on the attic floor perimeter, the ceiling insulation in the I-Joists will be full depth all the way to the perimeter. (Rafter upper ends rest on a structural ridge beam to eliminate outward force on the house side walls).

There are no interior walls yet so the entire ceiling area under the I-Joists is open with no obstructions. I want to figure out the best ceiling air barrier strategy before building interior partitions that might compromise the air sealing. I am considering a couple of ceiling air barrier approaches:

1. Air Tight Drywall – apply continuous ceiling drywall to the I-Joist bottom flanges before building any interior partitions.

2. Attach a continuous poly air/vapor barrier membrane to the I-Joist bottom flanges, folding the membrane down onto the wall top plate at the perimeter.

10 ft wide sheets of ceiling air barrier membrane can probably be sealed more thoroughly than drywall panels. I have an unused 10 ft wide roll of Bilar sheeting. Bilar is a 15 mil laminated material typically used for foundation waterproofing and basement slab vapor barriers. If I use it in the ceiling it will be an air barrier and class A (rated 0.04 perm) vapor barrier.

I know that a Class A vapor barrier should not be used in my 2×6 walls (planning to use MemBrain smart vapor retarder there). I am wondering if it would be OK to have a 0.04 perm vapor barrier in my ceiling since the space above the blown-in insulation and attic floor is well ventilated and drying can occur to the attic space ?

An additional question is the effect of the plywood attic floor on attic insulation and condensation. The attic floor is not sealed: there are 1/16” – 1/8” gaps along the plywood 8 ft edges. Is it possible that outside air will cause condensation on the underside of the attic floor ?

All comments and ideas are welcome.



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  1. Stephen Sheehy | | #1

    Mike- in my recently completed house we put an air barrier (Siga Majpel) stapled to the underside of the roof trusses, with the joints taped. The same membrane ran up the wall and overlapped the ceiling membrane. Then we installed 1x4 strapping and then drywall. This was all before any interior partitions went in. We then blew cellulose on top of the membrane. It was pretty easy to do. 2 feet of cellulose is pretty heavy, so the strapping and drywall were necessary to keep the cellulose from blowing through the membrane.

  2. Michael Tuso | | #2

    Hi Stephen,

    Siga Majpel has a perm rating of 0.68 which qualifies it as a Class 2 vapor retarder. I am wondering if a Class 1 vapor retarder (Bilar perm rating is 0.04 perms) will be OK in my ceiling application. A Class 1 vapor retarder would prevent any inward drying of fluffy attic insulation but I expect all drying to be outward to my vented attic space.


  3. GBA Editor
    Martin Holladay | | #3

    In your climate zone, you don't want an interior vapor barrier (especially if there is any chance that your house might be air conditioned in the future). So, no poly.

    Stephen's suggestion (using Siga Majpel) will work, but it's a bit pricey.

    By far the most common air barrier material for ceilings is gypsum drywall. Drywall is an excellent air barrier, and it's affordable.

  4. Michael Tuso | | #4

    Hi Martin,

    My house will be air conditioned so I will forget about using the Bilar vapor barrier in the ceiling. I am planning to use Membrain smart retarder in the walls and will probably use it in the ceiling as well. Do you know if an unsealed plywood attic floor above the attic insulation would cause any problems (such as condensation on the underside of the plywood) ?



  5. GBA Editor
    Martin Holladay | | #5

    No, the plywood floor in the attic won't cause any problems.

    Most moisture problems in attics are caused by a leaky ceiling. If you do a good job of sealing air leaks in your ceiling, you've limited the chance of any problems.

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