Three-Coat Stucco Over Foam

Q. My clients like the look and durability of a traditional cement-based three-coat stucco finish, but they want it installed over a continuous layer of rigid foam. Can I simply install a double layer of Grade-D 60-minute paper over the foam before nailing up the lath, or are there other steps I should take?

A. Barry Jenkins of Southern Stucco in Knoxville, Tenn., responds: Normally a designer would specify EIFS cladding when both exterior insulation and a stucco-like finish are desired. The mixture of exterior insulation and traditional stucco you describe is possible, but it’s not a conventional installation with standard details. Here is the approach my company — which installs both stucco and EIFS — would use if we were to tackle a project like this.

Install the rigid foam over plywood sheathing (not OSB) that has been properly nailed to the studs, with 1/8-inch gaps between the panel edges so the sheets can expand without compressing each other. This will help prevent the sheathing from developing joint lines that could telegraph through the foam and crack the stucco.

With EIFS claddings, the IRC now requires a fluid-applied weather-resistive barrier (WRB) over the sheathing prior to installation of the rigid foam, an approach I would recommend with this system. A liquid WRB (StoGuard is a good example; stocorp.com) also provides a terrific air barrier, and can actually be extended down over the above-grade part of the foundation if the sheathing is flush with the foundation wall.

I recommend XPS foam, and would glue it to the sheathing with a trowel-applied adhesive like Sto BTS plus, orienting the U-notched trowel so that the grooves are vertical and continuous from top to bottom to enhance drainage. Since fastener deflection is a critical concern whenever a heavy exterior cladding is attached over continuous foam insulation, I wouldn’t use rigid foam that is more than an inch thick. If the lath fasteners are numerous and penetrate at least 3/4 inch into the studs (rather than into just the sheathing), the lath assembly should be strong enough to support the stucco.

Despite the presence of the liquid-applied WRB, some local codes may still require a double layer of building paper underneath the stucco. If not, I would use only a single layer of Grade-D 60-minute paper, which should satisfy your code official. The paper (along with the liquid-applied WRB under the foam) provides a secondary belt-and-suspenders drain plane. Even more important, it acts as a bond break that allows the stucco to behave monolithically, independent of the underlying foam and sheathing. We’ve found that cement-based scratch coats applied directly to foam tend to bubble, creating lots of craters and voids in the scratch coat.

When installing the metal lath (I recommend using galvanized 2.5-pound self-furring diamond mesh lath), you may find that the rigid foam complicates the application of the accessory beads. I can’t make any specific recommendations without seeing actual details, but I can tell you that when the available profiles don’t work, we have a metal shop fabricate exactly what we need. By the way, don’t forget the screed. Standard stucco screed won’t be deep enough to accommodate both the foam and the stucco, but vinyl screeds typically used with EIFS should work fine.