Taped sheathing joints with primary air barrier on the interior
Hi folks! Long time lurker here but first time poster. I’m hoping to get a little feedback on air control for a wall design in PEI, Canada (Climate zone 6). The wall stack up is
Intello Plus Smart Membrane (interior vapour control is required by code here)
2×6 wall cavity with dense packed cellulose
1/2 plywood sheathing
WRB
3″ Rockwool Comfortboard
3/4″ rainscreen
Cladding
With a vapour control membrane required on the inside, Intello seemed like a logical approach since it would also act as my barrier with proper detailing. I’m wondering if it would be worth also taping the seams of my sheathing to form a secondary air barrier on the exterior side of the wall. It wouldn’t be a huge expense and I’m leaning toward doing it, but if this is simply redundant with an interior air barrier already, then maybe my efforts are better placed elsewhere. My WRB will be mechanically fastened and will really only be relied upon for water control.
Any input would be greatly appreciated!
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Replies
Hey Stuart,
Over the years I talked to designers and builders who believe an air barrier should always be on the exterior, should always be on the interior, or that you really need both. I'll leave those opinions to others. What I will offer is that your air barrier needs to be continuous around the building, so you may consider which option is easier to detail as it transitions to the foundation below the wall, ceiling or roof structure above, etc. The less complex these details are, the better. It seems a lot of builders have become comfortable with these details with exterior air barriers and that is why it is becoming the common location, among other benefits.
I look forward to reading what others have to say about this...
Brian, your link isn't working--here's the article: https://www.greenbuildingadvisor.com/article/one-air-barrier-or-two.
I agree that an air barrier can be anywhere within the wall assembly, and that having two air barriers is marginally better than having one, but the important thing is to have one really good one. I usually locate it at the sheathing layer, often with taped sheathing but sometimes with a separate membrane, but that's mainly for overall ease of doing a really good job.
That sounds like a great wall assembly,
I would vote on having two myself, counting the primary air barrier as the one on the exterior, and the backup, if you want to call it that, on the interior. Eventually the interior will be degraded by new electrical, plumbing, picture mounting, and all kinds of small penetrations, and it's nice to have while it's intact in most the areas, I wouldn't count on it as a durable barrier, no matter the brand, for the entire duration of the home.
Thanks for the feedback! Yes I think it will be a good wall assembly. I see the logic behind moving my air barrier focus to the exterior (although keeping the interior one and detailing it as best I can). The requirement for a vapour barrier made me think that controlling air at that plane would be the most simple, but I do see how it can get complicated with partition walls, transitions between floors, etc.
Taping the sheathing seams will be a minor investment in time and money to make for a more efficient home. I've been really happy with the Tescon Vana tape I have for the Intello so I'll likely use it for the seams. It has a a perm rating of 8 which makes sense for the exterior. This almost seems (no pun intended!) to be too simple of a solution though. Are taped sheathing seams with a basic WRB (likely residential Tyvek) generally viewed as adequate if attention is given to window openings, foundation connections, etc? The Intello was a bit of an investment that was justified by thinking it could be my primary air barrier, allowing me to use a more basic WRB. Having said that, If upgrading to a more expensive WRB and taping the sheathing would provide significantly better results, I'd be open to that too. I have easy access to Vaprosheild and ProClima products.
Thanks again for the input! The wealth of knowledge on this site is fantastic.
Stuart_Wight,
I haven't been to PEI in over 30 years (we used to own the old lighthouse at Cousin's Shore). Do people use air-conditioning there now? If not, another option is to just use poly as your interior vapour-barrier, and detail both it and the sheathing as air-barriers. Your walls should have good drying to the exterior.