Failures have been documented in association with acrylic-based synthetic stucco applied over fiber cement panels. Depending on local code requirements, the fiber panels are generally installed over an approved sheathing such as OSB or exterior gypsum. A number of these systems have been patented and marketed as ‘synthetic stucco finish system’ or as high build finishes – neither should be confused with Exterior Insulation Finish Systems (EIFS) or acrylic-finished stucco applied over metal lath.
These synthetic finishes also vary from what the industry knows as ‘Direct-Applied Exterior Finish Systems’ as follows: 1) the synthetic stucco finish systems are applied over fiber cement panels on approved sheathing; 2) the synthetic stucco finish systems generally lack a continuous base coat with embedded reinforcing mesh; 3) the synthetic stucco finish systems generally rely on elastomeric tape at fiber cement panel joints.
- The elastomeric tape impedes drying of incidental moisture that enters through, within, or behind the fiber cement panels. This results in premature degradation of the finish, fiber cement panel, and underlying sheathing.
- The system is vulnerable to finish cracking caused by normal movement of the fiber cement panels. This results in cracking, which serves as additional routes for moisture intrusion.
- Panel stabilization and performance necessitates proper edge support, which in some constructed assemblies require the use of double studding to accommodate required fastening. Failures are often associated with inadequate edge support.
- Aside from James Hardie’s recent requirement for a minimum 3/8” rainscreen behind their fiber cement panels (Technical Bulletin #18), manufacturers of the synthetic stucco finish systems have not required a rainscreen. The system as a whole is inherently prone to high water burdens. The lack of a rainscreen simply compounds the problem.
Caution is advised when using these systems in locations subject to moderate or high temperature changes or moderate to high rain. Design professionals are urged to reconsider the basis for performance as an acceptable exterior finish system. The described systems are component-based and therefore do not fit the bill for a simple high-build finish. Likewise, acceptance criteria, standards, and codes governing conventional stucco do not apply. Although these synthetic finish systems are best described as a modified Direct-Applied Exterior Finish System (DEFS), they are generally not installed in accordance with recognized DEFS standards (ASTM C 1516). Demonstrated compliance with acceptance criteria per AC59, if known, should be compared to the intended application and design conditions.