“Self-Healing” Membranes & Air-Sealing Tongue & Groove
Zone 2A home with a tongue & groove roof deck (exposed to the interior for architectural reasons, can’t be changed). I’m sealing the *perimeter* in several ways suggested here (holes through seams filled with foam, placing a taped and foamed break above the gable-end rafters) — but am trying to decide how to seal the *seams* from above. I see 3 options:
1) Self-adhered membrane, like Solitex Adhero, which claims to be self-healing around penetrations. My concern: ther’s is 7″ of polyiso above the roof deck, topped with 5/8″ plywood, and fastened with 1/4″ diameter screws. The below (from a 2009 JLC thread) makes me think that this polyiso will act as a “cushion” that undermines the compression the membrane needs to self-seal.
JLC Q: “I also wonder if a bituminous flashing tape such as Vycor pre-applied to the sheathing would effectively seal nail penetrations.” JLC A: “This was lab tested in Boston a number of years ago… The result was, if the shank of the faster penetrated the membrane with no compression from the head, it very often failed. If you could achieve good compression, many times it would pass. We field tested it to ABBA standards, it was always a mixed bag.”
2) 1/2″ plywood, taped seams. This option is more expensive and more labor intensive. But plywood seems like a more reliable “self-healer” around screw penetrations. Of course, some screws will penetrate the tape over the seams too… but maybe I could snap chalklines to avoid the seams when I screw down the “upper” roof deck.
3) Tape the seams of the T&G itself, snap lines to avoid screwing through said seams and disrupting the air barrier as above. So, similar to the self-adhered membrane, but seems safer if I get the chalklines right.
Would really appreciate any advice on this! Thanks.
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Replies
Self healing only matters if you are looking to prevent water leaks. In this case the membrane is only for air sealing, having a couple of very small holes around screws will not matter.
With sealing the T&G you do have to be careful with the type of membrane you use, you don't want any modified bitumen/SBS as it can react with the sap in the wood and drip plus it has a very strong smell. Anything butyl or acrylic is fine.
Since the membrane is only air sealing, it doesn't have to be anything fancy. The cheapest self adhered one with the right adhesive will work just fine.
I'm not sure if there is any ROI for a high R value roof in zone 2. Since it is continuous exterior insulation you can comply based on whole assembly U factor, code min is probably R30 assembly in your case, so ~4.5" iso.
Thanks for the reply Akos, I've learned a lot from your posts here :) I'll definitely use acrylic. Could you elaborate on this: "In this case the membrane is only for air sealing, having a couple of very small holes around screws will not matter"? The entire roof will have something like 700 big screws. That feels like a lot of risk on the air seal. That's why I thought maybe Option 3, just avoid puncturing the adhesive altogether... but I'll probably miss sometimes, and I'd assume that the tape "self heals" worse than the SA membrane...
I might be overly paranoid, but there's no interior ceiling, so this air control layer is all there is to stop this infamous ceiling design from doing its leaky thing (the foam is fiber faced, and I've yet to see a tape that works well, long term, on that).
Since the screws are solid, you won't leak air through them. The only leak path is the small tear around the screw perimeter in the membrane when it is driven in. This is something but so small that it is nothing to worry about.
Most well sealed houses walls have rain screens built with strapping and a lot of screws, these regularly hit sub 0.5ACH, so not an issue I would loose any sleep over.
If you seal up the perimeter and take some care installing the peel and stick, you'll have a very nice well sealed ceiling.