My company has been installing synthetic and traditional stucco cladding on homes and businesses in the Southeast for more than 30 years. We’ve also installed a lot of acrylic-based exterior insulation and finish systems — or EIFS — but there was a period when I stopped recommending them to our clients. Although we hadn’t had any problems with our own installations, I had seen too much rotted OSB siding on other EIFS jobs to be confident that these systems were performing as well as they should (see “Installing Water-Managed Synthetic Stucco,” 9/98, for more on EIFS moisture problems). And that’s unfortunate, because EIFS is a great cladding option when done properly; it installs quickly and economically and puts the insulation where it provides the most benefit.
Then, a few years ago, I discovered Teifs WeatherTight VNT EIFS (Parex USA, 866/516-0061, teifs.com). This product makes it easy to get the details right, even over a potentially moisture-absorbent sheathing like OSB, so I’m once again enthusiastically recommending EIFS to my clients. The system features an acrylic water-resistant membrane called WeatherSeal that we trowel right onto the sheathing before installing the EPS foam. Polymer-based coatings over the foam still act as the primary water-resistive barrier, but the WeatherSeal coating creates a secondary barrier that protects the sheathing from moisture damage. Since there’s no housewrap under the foam, we can glue the foam directly to the WeatherSeal.
The Teifs system also provides drainage, thanks to vertical channels left by the notched trowels we use to apply the adhesive. Water that gets behind the stucco finish and foam drains down these channels between the foam and the waterproofing membrane. It exits the wall assembly through a special perforated fabric embedded in the adhesive over door and window openings and at the bottom of walls.
Sheathing Options
EIF systems are available for different types of sheathings and climates. WeatherTight VNT was originally designed for commercial construction, where steel framing and gypsum-board sheathing are common, but it’s recently been approved for wood-framed construction and OSB sheathing in certain regions, including mine. Even so, I’m still reluctant to install any type of EIFS over OSB — even WeatherTight VNT — since virtually all of the EIFS problems I’ve seen have involved OSB. CDX plywood is in my opinion a more reliable substrate, and it’s also an approved sheathing with the WeatherTight system.
Treating cut edges. For both plywood and OSB sheathing, Parex recommends a water-resistive coating on all cut edges. This makes sense along roof rakes and the bottom corners of door and window openings, which is where I’ve seen most OSB damage occur. However, we don’t try to force WeatherSeal into joints in the field where the sheathing has been trimmed to fit. Not only would this be hard to do, but it also would be unusual to find water damage here even with older versions of EIFS.
On the project shown here, most of the house was enclosed with DensGlass Gold gypsum sheathing (Georgia-Pacific, 800/225-6119, gp.com), which has fiberglass-mesh faces over a moisture-resistant core. We didn’t install the sheathing, but when we arrived on the job, we inspected it carefully to verify that there were no gaps and that all of the vertical seams fell on studs (horizontal blocking at seams isn’t generally necessary). We also checked that enough noncorrosive fasteners had been used to secure the sheathing properly to the framing.
