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Is taped plywood “good enough” air barrier for T&G ceiling with flash and batt?

Rob603 | Posted in General Questions on

Zone 5 (NH)

Nearing the end of an extensive to the studs remodel of about 1/3 of my home and now it’s finally time for the ceiling. My hat is off to GBA for the amazing knowledge I’ve used throughout.

Is taped 1/2″ plywood a “good enough” air barrier before I put up my pine T&G ceiling? To the best if my abilities I taped the seams with 3M All Weather Flashing Tape 8067, but there are certainly spots that don’t have a 100% seal and I know the tape won’t hold up for all eternity. The house is old and nothing is even so the sheets are not a perfect snug fit, but pretty good. There are plenty of other areas that I simply cannot seal without gallons of caulking and years to apply it. I used plywood on the walls and ceiling for strength and to make nailing the T&G easy.

Unvented cathedral ceilings – flash and batt (3″ closed cell, R21 unfaced)
Careful attention paid to potential areas of thin foam and leaks to the outside with dozens upon dozens of cans
Lots of skylights
Lots of recessed lights
Lots of receptacles in ceiling

I read on GBA that CertainTeed Membrain could be used as my air seal, but with all the unavoidable penetrations in the envelope, is it worth chasing the plywood seams? Where should I focus my attention prior to T&G? The remodel is nearly complete so it’s literally JUST putting up the ceiling but I don’t want to miss a step.

Thanks GBA!

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  1. GBA Editor
    Martin Holladay | | #1

    The short answer is: plywood with taped seams is a good air barrier.

    The longer answer is: If you know areas where the ceiling will leak, you might want to reduce air leakage in those areas before you put up the ceiling. You wrote, "there are certainly spots that don't have a 100% seal and I know the tape won't hold up for all eternity." I'm not sure what this means -- but if you know that you have wrinkly tape on some seams, it makes sense to remove the wrinkles with a utility knife, and re-tape the bad seams.

  2. GBA Editor
    Martin Holladay | | #2

    A few more comments: You described your work as "to-the-studs" remodel. Was your work also a "to-the-rafters" remodel?

    You wrote, "My hat is off to GBA," which implies that you have read articles and taken advice from this web site. If that's so, why does your ceiling have "lots of recessed lights" and "lots of receptacles in the ceiling"? If you performed a "to-the-rafters" remodel, you would have had an excellent opportunity to replace your recessed lights with better options.

  3. GBA Editor
    Martin Holladay | | #3

    Finally, it needs to be pointed out that flash-and-batt ceilings have less of a need for an interior air barrier then vented ceiling assemblies insulated with fiberglass alone. The fact that your ceiling assembly probably has a pretty good exterior air barrier -- the spray foam layer -- means that there won't be much exfiltration, even if the plywood and T&G are leaky.

  4. Rob603 | | #4

    Thank you for the quick reply. My apologies for the length, but it addresses your comments with added info.

    Yes I do plan on going through again to make sure any obvious defects are addressed. I was not haphazard in my taping. It was carefully cut, placed and smoothed, but I've already seen evidence that the tape is lifting on areas that had any surface disruption like screw holes with slightly lifting wood, slightly ragged edges, unlevel plywood joints, etc On a side note, I used tens of thousands of screws for the whole construction (mostly GRK - general purpose and structural) and very few nails. I hate nails. Tne house is strong.

    My secondary question is, is there a 'better' tape I should use to perhaps go over the existing tape? Something say, 4" wide as opposed to the 2" already on? I had read that the 3M tape was among the best for plywood adhesion, but like I said, I doubt it will hold for decades. As much as I would have liked to put a layer of taped and mudded gypsum throughout, the way the house is, it would not have been possible. You'll just have to go with me on this. It's only the ceiling left.

    Yes it was the complete destruction of that portion of the house. Inside and outside wall sheathing and siding were replaced, doors, windows, to the rafters and down (or up) to the roof sheathing (EPDM roof). There was also an addition, changing the interior configuration of the house, removing walls, removing an unused chimney, adding LVLs, electrical, plumbing.. It was a lot of work. I did everything aside from the actual application of the primary spray foam. Cathedral ceiling is only to indicate that it isn't flat as it only has a 7 degree pitch. And the ceilings are low, 6'6" at the lowest to 9' at the highest. Yes, I almost hit my head. All the time. It's a converted summer camp that is now my year-round home. Skylights were existing and there was no choice but to use recessed cans. I added them, not replaced them. The way the house is configured, there were literally NO better options. The ceilings were too low and the house too dark despite all the sliding doors and windows. Plus any mention of surface lights were immediately nuked by my significant other, and I'd like to keep my marriage. Receptacles were a requirement for future...things. It's all done now, so no going back.

    Yes I thank GBA for the dozens of articles I read about "better" construction, sealing, and all the little things most people miss or dismiss. I'd like to think I did a good if not very good job with the renovation and addition.

    Also, I do realize that a foamed and unvented ceiling has a pretty good air barrier. I made sure to go completely over the top with foaming in and sealing any access points to the outside in the walls, ceiling, sill, top plate, windows, doors...People were looking at me like I was insane with yet another can of foam, tube of caulk, or roll of flashing tape. That part of the house is as tight as a renovaton on an old house can be.

    Last winter, during the coldest week, I went around and used a smoke pen and my Flir thermal camera looking for anything suspect. The house then was virtually the same as it is now since I have been focusing on other projects now that primary construction was done and the house finally livable. I found only one leaky spot around one window and addressed it. Little air movement aside from some whisps generated by the unrenovated part of the house and currents from my baseboards which were off but still a bit warm. The Flir found no cold spots in the ceiling or walls, meaning completely ambient, and only the usual slightly cooler than normal corner temp fluctuations. Perhaps this was the most telling test aside from a blower?

    So aside from some more due diligence to the tape, should I bother with the Membrain? Lowes has it and I could have some more fun this weekend...

  5. GBA Editor
    Martin Holladay | | #5

    If you have taped plywood, you won't get any further benefit by adding MemBrain.

    In my article on tapes, I mentioned several tapes that work well on plywood, including Zip System tape; Siga Wigluv; Siga Sicrall; and Pro Clima Tescon No. 1. If you want something wider than your 2-inch wide 3M tape, you have lots of choices. Many of these brands of tape are available in widths that exceed 2 inches.

  6. Rob603 | | #6

    I forgot you wrote the article. It had been some time since I read it. Excellent work by the way.

    I have several leftover rolls of wider Zip tape. I will add it over my existing 3M tape as an added layer and call it a day. Maybe the pressure of the T&G will give it some added hold.

    Thank you. I was in the forest for the trees mindset, but sometimes engineers get that way.

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