In the midst of framing his new house, Joe Norm has switched gears and opted out of Zip System sheathing in favor of CDX plywood. The question he faces now is how he should air seal the exterior side of the walls—tape the seams between sheets of plywood, or tape the seams of the water-resistive barrier (WRB) he installs over the sheathing? More to the point, will a common WRB like DuPont’s Tyvek be good enough, or should he be prepared to spend more on an “exotic” product?
“I priced out VaproShield IT and it looks to be 3x the cost of Tyvek,” Joe writes in a Q&A post. “Solitex Mento looks to be about 2x the cost. Why are these products so much more and how are they justified over Tyvek?”
That’s where we start this Q&A Spotlight.
Staple-up WRBs are not the best air barriers
GBA Editor Brian Pontolilo has just finished a series of blogs on WRBs and has come to at least one conclusion: “There is no easy answer when it comes to the performance of individual products.”
In order to win code approval, a WRB must pass certain criteria of the International Code Council’s Evaluation Service. Reports on test results offer only limited information, but Pontolilo adds that there are better choices for an air barrier than a staple-up housewrap.
“It is well known that staple-up housewraps like Tyvek don’t make the best air barriers,” he says. “At least they are challenging, at best, to detail as an air barrier. If you choose this type of product, taping the plywood seams is a more straightforward approach to air sealing.”
If Norm wants his WRB to double duty as an air barrier, Pontolilo would recommend the Zip System sheathing, peel-and-stick membranes, or liquid-applied products instead.
Beyond the question of which WRB to use, Pontolilo adds, is the importance of including a ventilated rainscreen in the design.
Tighter houses need better WRBs
Houses with drafty exteriors can tolerate water more readily than tight houses, AlexPoi writes. If Norm is building a tight house, he’ll be better off with a high-quality WRB.
“A little bit of water in a leaky house is not a problem because air movement will dry the sheathing pretty quickly if it gets wet,” AlexPoi says. But when the house is tight, and air movement through the walls is limited, is takes longer for moisture to dissipate.
“So, if you are building a tight house with OSB sheathing in a wet climate, I would personally not take the risk to cheap out on the WRB just for the peace of mind,” AlexPoi continues. “After all, a good WRB will cost you just a couple of thousand more but could save you big time.”
AlexPoi suggests Norm pay particular attention to flashing details to keep bulk water out.
Sheathing makes a robust air barrier
Andrew C believes that taping the seams of the plywood sheathing is Norm’s best bet for a durable air barrier.
“Properly detailed, the WRB can be a belt-and-suspenders backup air barrier to the taped sheathing, but the sheathing remains primary,” Andrew C writes. “As others have pointed out, flashing design details and attention to these details, along with avoiding dumb designs like dead-ending a roof valley into a wall, are critical to durability.”
To Andrew C, a wall design with taped plywood and Tyvek CommercialWrap (a suggestion from Malcolm Taylor as a more durable alternative than Tyvek) makes the most sense.
Why opt out of Zip System sheathing?
Norm opened his original question by noting that he had decided against using Zip System sheathing, an OSB product with an integral WRB from Huber Engineered Woods that has become increasingly common on both walls and roofs.
How come? Patrick O’Sullivan wants to know.
“No good reason really,” Norm replies. “I’m not fond of OSB in general and I couldn’t totally get past the idea of hundreds of feet of reverse laps for water to get caught up in. I”m sure Zip is great and in the end I may regret not trying it out.”
Even though Taylor thinks taped plywood sheathing and a WRB in sheet form would be a good route, he also thinks Zip Sheathing is worth considering. “Zip-R [a version of the sheathing bonded to a layer of foam insulation] makes some sense to me, and I’m pretty sure Zip will turn out to be a good long-term sheathing/WRB,” he says.
But when it comes to choosing a particular WRB, Taylor says that as long as it is installed “diligently,” he hasn’t seen anything suggesting one WRB is significantly better than another.
As to the long-term durability of plywood vs. OSB, a question that Zerphyr7 raises, Alex P refers him to an Instagram photo posted by architect Steve Baczek showing two small pieces of Zip sheathing taped together with Zip tape that have been left outside for a decade.
“He says the tape is perfectly fine and the edges have swollen a little with full exposure to rain, ice and snow,” Alex P says.
Our expert’s opinion
GBA Technical Director Peter Yost adds this:
I like to keep the air barrier and the water-resistive barrier together so that at penetrations I can seal both in the same plane or location.
On the topic of ventilated rainscreens, for all but the driest of climates, I strongly prefer a free-draining and airflow-promoting gap (open top and bottom) between the cladding and the WRB. The one situation that no code-approved test for WRBs requires is water held in tension between the claddings and the WRBs (no gap between them). I have seen this water held in tension result in wet sheathing with a variety of different WRBs.
Quite some time ago now, Paul Fisette—now retired, but then head of the Wood Technology and Construction program at the University of Massachusetts, Amherst—worked up a test for water held in tension. You can read that report here.
On a side note, does WRB stand for “water-resistive barrier,” “water-resistant barrier,” “weather-resistant barrier,” or “weather-resistive barrier?” In the 2018 International Residential Code (IRC), these terms are used interchangeably: water-resistive barrier 30 times, water-resistant barrier six times, weather-resistant barrier three times, and weather-resistive barrier twice. That is just odd.
Even more odd is to use the terms “resistive” and “resistant”—both meaning some level of opposition—with the term “barrier,” which means an absolute stop or block. In our industry, we certainly struggle with water and weather, even in our terminology.