Traditional and modern formulas for stucco
In traditional stucco, a mix of portland cement, sand, water, and lime is applied in four coats over expanded metal lath attached to the sheathing. Synthetic stucco looks like traditional stucco, but it is installed in a single coat over a layer of rigid foam insulation sheathing.
TWO LAYERS OF FELT PAPER ARE BETTER THAN ONE. Over traditional stucco, the second sacrificial layer of felt paper bonds to the stucco and pulls away as the stucco dries. The gap between the two layers allows for drainage. To upgrade, add a mesh drainage mat.
One-coat, three-coat, EIFS
Traditional modern stucco is a portland-cement plaster applied in three coats to a building exterior. Stucco is more expensive than some types of cladding — vinyl and some types of wood, for example — but the surface is durable, vapour permeable, fire-resistant, and suitable for any climate, with proper detailing. As long as the surface has been installed with details that address inward solar vapour drive and wind-driven rain, stucco should require little maintenance unless the surface is damaged.
There are many types of stucco claddings, but they can best be split into three primary types: traditional three-coat stucco; newer one-coat stucco; and exterior insulating and finish systems (EIFS). Each system has its advantages and drawbacks. Each system can be a durable, protective cladding, but each requires a different set of installation details to achieve these results. Each can create an exterior air barrier, depending, again, on details at penetrations, transitions, and the edges of the wall cladding.
Three-coat stucco is a longstanding, nonproprietary cladding system that has a scratch, then base, then finish coat, resulting in a 7/8-inch- to 1-inch-thick cladding. This system is the most time and labor intensive.
One-coat stucco systems have just one base coat about 1/2-inch thick with a thin finish coat, so these claddings are sometimes called “two-coat.” The base coat is a blend of portland cement, fibers, and proprietary additives, with each system carrying its own International Code Council (ICC) Evaluation Service (ES) report that dictates the installation details. There is less labor and time required for this system than three-coat stucco, but custom installation standards must be followed.
Exterior insulation and finish systems (EIFS) are essentially one-coat systems, but the marriage of a stucco finish to exterior rigid insulation brings with it different water-management details than the other two types. EIFS claddings consist of synthetic stucco applied over an insulating layer of rigid polystyrene insulation. Insulation can be up to 4 inches thick. EIFS has many energy advantages over conventional stucco.
Keeping water out
Stucco installations require a minimum of two layers of water-resistive barrier material beneath the lath to protect the sheathing. Water driven into the permeable stucco surface dries to the outside. Unfortunately, modern stucco finishes have the potential to cause moisture problems, as building scientists has pointed out. A variety of seemingly unrelated factors, including modern sheathing and weather barriers, appears to cause problems that traditional materials and methods did not.
Well-publicized moisture problems have also occurred in EIFS walls. Moisture that found its way into walls, particularly around windows and other wall penetrations, had no way to get out. In some instances, the result was extensive sheathing and framing rot. Once the problem was uncovered, lawsuits and lots of repairs soon followed.
The resulting publicity gave the industry a black eye, but new designs (moisture-managed EIFS) allow water to escape, and installers now recognize the importance of careful detailing.
Lime stucco was long ago replaced in the mainstream by portland cement-based stuccos, which are cheaper and faster to install. But lime has been kept alive by preservationists who insist on using it on historic structures, natural builders who apply it to straw-bale buildings, and other adherents who appreciate lime’s aesthetic, durability, and arguably smaller carbon footprint.