Bill L. is planning a high-performance house in Massachusetts and is wrestling with options for the air barrier, that all-important building detail that enhances both energy efficiency and building durability.
Above-grade walls will consist of a 2×4 structural frame sheathed in 1/2-inch plywood, followed by I-joists packed with cellulose insulation, another layer of 1/2-inch plywood, a corrugated plastic product to provide an air space, and fiber-cement siding. The primary air-barrier plane will be at the plywood over the 2×4 studs.
“I don’t trust tapes or caulking to last long-term,” Bill writes in a Q&A post at Green Building Advisor, “and stapling a sheet material over everything seems a lot less labor-intensive than priming the plywood and taping or caulking all of the joints. It also seems considerably less expensive than Zip sheathing.”
Although Bill had considered using CertainTeed’s MemBrain, it’s proving hard to get where he lives, and he’s not convinced it will be durable enough.
Where does this leave him? That’s the topic for this Q&A Spotlight.
Consider a liquid-applied WRB or high-quality tape
If Bill doesn’t trust tapes to go the distance, suggests Albert Rooks, use a liquid-applied WRB at the sheathing layer. Specifically, he points Bill toward FastFlash, a product made by Prosoco. Rooks says that was used in a Seattle Passivhaus project described in a 2011 GBA blog by Richard Defendorf.
“The approach is not inexpensive, but the quality is exceptionally high,” says Rooks. “We sell it at The Small Planet Workshop, but I’m sure you can find a supplier in your local market at Prosoco’s website.”
But in the end, Rooks adds later, “good quality tape is still (in my opinion) the simplest ‘go to’ material for turning sheathing into an air barrier.”
“I would advise against re-inventing the wheel,” he says. “A lot of really earnest and passionate people have been working on the ‘quality and longevity of air barriers’ issue for a long time and have developed good results.”
“Tape gets a bad rap from the products that were never intended to be used as an air barrier,” Rooks says. “As most of us know, solvent-based adhesives loose their flexibility over time and the tape will fail because the backing material comes off. The acrylic adhesives don’t contain any VOCs to off-gas, and that’s why they remain flexible and don’t appear to fail in the 20+ years being used as air barrier tape so far. For longevity, I think they (tapes) have as good chance as any of the other methods, are simple to apply, and therefore are cost-effective.”
How long will building materials really last?
One of Bill’s chief concerns is the longevity of the air barrier he creates. “What does seem nuts to be is hoping that adhesive tape will maintain its grip for the hundreds of years I hope this house will stand,” he says.
And while GBA senior editor Martin Holladay understands his concerns, he says that most people in Bill’s situation have decided to trust tapes. “I can understand your skepticism concerning the durability of tapes,” he writes. “But frankly, what makes you think that membranes that come in a roll will last 100 years? There are plenty of examples of 30-year-old Tyvek that has disintegrated. When it comes to speculating about which materials will last 100 years, we are all just guessing.”
Jin Kazama complains that most manufacturers don’t offer any information on how their products will fare after 20 years, let alone a century. “And since [we] are relying heavily on membranes in our recent buildings and design,” he writes, “it would be nice to know the minimum expected lives of the products.”
“Of course it is important for building materials to last 50 or 100 years,” Holladay replies. “For those of us who want to be assured of 50-year or 100-year performance, the most certain way to proceed is to use materials that were used 50 or 100 years ago — and which have proved themselves to be long-lasting. Anyone who uses newly developed materials is taking a calculated risk.”
Should the sheathing be glued to the frame?
Part of Bill’s plan is to apply a bead of sealant on the studs as each sheet of plywood is put up.
“Certainly sealants can fail over time as well,” he says, “but at least it will be sandwiched between fastened pieces of wood, which may support its longevity and will at least prevent it from simply falling off.”
But Rooks points out using an adhesive between plywood sheathing and framing is not permitted by code in certain seismic areas, according to an engineer who has worked on a number of high-performance houses.
Ron Keagle ponders: What’s the point of that in the first place? “If the point is to accomplish air sealing, why place adhesive on the studs that fall in the field of the plywood?” he asks. “There is no joint there to seal. Adhesive could be used to seal the edge joints that fall on the stud lengthwise. Adhesive there between the plywood and the stud would prevent air from getting to the actual butt joint of the plywood at that location. You would not need any adhesive there where the edges of the plywood butt together.”
The real challenge is sealing seams perpendicular to the studs. Keagle is among those who wouldn’t trust tape. Instead, he would seal those seams with a 1/4-in. by 3/4-in. batten pressed into a bead of silicone caulk.
Our expert’s opinion
Here’s what GBA technical director Peter Yost had to say:
I have been spending a lot of time struggling with this topic of late, especially since just about every one of the more than 1,000 GBA construction details has these call-outs as part of a continuous air barrier: “Continuous bead of sealant” and “Tape all joints, horizontal and vetical.”
If you have the time and inclination, I have a blog series on this topic over at the BuildingGreen website.
But here are my overall recommendations:
- Mechanically trap your air barrier, especially at weak points, such as penetrations and margins. This way the chemical adhesion of your barrier is backed up by good old-fashioned physics.
- Stabilize your air barrier for the long haul by locating it on the interior side of your rigid insulation. If your air barrier sees more steady-state-like conditions (steady temperature, steady relative humidity, no UV exposure, etc.), it will last longer.
- Locate your air barrier where it can be inspected, repaired, and replaced. What? Isn’t the air barrier on the assembly exterior? And doesn’t this recommendation the recommendation directly above? Yes, and this recommendation is mainly for commercial assemblies that use liquid sealants in cladding designed for exposure and regular inspection (and maybe face-sealed stucco assemblies in dry climates).
NOTE: Bill mentioned the use of CertainTeed’s MemBrain as an air barrier. CertainTeed calls MemBrain an “air film” in its product info, and CertainTeed provides detailed instructions for the use of MemBrain as an air barrier. But as far as I know, CertainTeed does not provide the air barrier properties of the material. Sure, any plastic sheeting product — including MemBrain — can be detailed as an interior air barrier system, but MemBrain’s primary function is as a variable-vapor-permeance (“smart”) interior vapor retarder for cold climates, and I wanted to make sure we don’t portray MemBrain as a general-duty air barrier product.
P.S. At the Building Science Corporation’s Westford Summer Symposium, it was announced that Oak Ridge National Laboratory, working at the Tremco Air Barrier test lab in Ohio, is testing eight different air barrier systems (including liquid-applied barriers) and is likely to have results around the first of the year. (Think of the promised results as a sort of building science holiday gift).