GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Poly Sheeting for Air Barrier

KurtGranroth | Posted in Green Building Techniques on

The attic in the house I’m building is vented and entirely out of the conditioned space. As such, I need to be very careful about the ceiling and how well it is air-sealed.

The given part of the ceiling is that I will run 1×3 strapping 16″ O.C. perpendicular to the joists and attach 5/8″ Type X drywall to said strapping. I will minimize the number of protrusions through the drywall but will not eliminate them entirely. At a minimum, there will be several ceiling fans, a few select recessed lights (judiciously placed — most lights will be wall-mounted) and maybe an exhaust fan and each will be carefully sealed. I plan on using the 3/4″ gap between drywall and joists to run select electrical, where it makes sense.

What I’m also wanting to do is first staple 6mil poly sheeting (typically used as a vapor barrier) to the bottom of the joists before the strapping, sealing all of the edges with Tyvek tape equivalent. I see this concept being done in many photos of “smart vapor barriers” like Intello Plus and similar. I have zero need for a vapor barrier in my neck of the woods (Phoenix, AZ) so using inexpensive poly sheets would be every bit the air barrier without any “smarts” needed.

My thinking for adding this poly, first, is mostly as a “belt and suspenders” method of air sealing. That is, while the ceiling fan boxes are going all the way through regardless (and would thus be sealed twice), I might be able to reduce the intrusions of the recessed lighting through the poly to just a few individual wires (ultra slim canless lights), which is easier to seal.

As a side benefit, there will be an empty gap of 3/4″ between the drywall and attic insulation, making it easier in some potential future to fish more electrical through it, possibly. That’s not a super compelling reason at the moment but who knows later?

It’s just that I never see this done and when the equivalent is done with Intello Plus and such, the focus is on the vapor part and not the air sealing part, usually.

What are your thoughts on my “belt-and-suspenders” idea of poly + drywall?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. Expert Member
    Zephyr7 | | #1

    I wouldn't bother. The poly doesn't really add much if any benefit to the drywall itself as an air barrier, and it's tricky to seal any electrical penetrations to the poly (it's actually easier to seal them to the drywall). I think you're making a lot of extra work for yourself if you try this, for little if any real benefit.

    You won't be able to fish wire in the gap between the drywall and the poly without damaging the poly. Any wire fishing tools or the wire itself will snag the poly, probably in multiple places, and make big tears. This will snag your fished cable, making it a bear to complete the job, and destroy your redundant air barrier at the same time. I've had this happen myself (in walls), as I used to be a cable installer (datacom stuff, paid for college that way), and have fished untold thousands of runs and hundreds of miles of many kinds of cable.

    Bill

  2. Expert Member
    Malcolm Taylor | | #2

    Kurt,

    Unfortunately, although you may only want to use the poly as an air-barrier, but it will still act as a vapour-barrier too. You don't need one in Phoenix, but if there is one installed it will be on the wrong side of your assembly in a cooling climate, and may become a condensing surface.

  3. KurtGranroth | | #3

    Thanks, Bill and Malcom. I am sufficiently convinced that it's not only not worth the effort but will be counter-productive!

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |