Thermal and water management are crucial in buildings; no building can be durable, or long-lived, if design and construction fail to protect the building’s frame from moisture damage. Repeated failures of exterior insulation and finish systems (EIFS) in coastal British Columbia (B.C.) are a reminder that EIFS designed with only a single moisture barrier have short-lived lives and make for expensive repairs. Shoddy construction has only added to the woes of condominium owners.
Building scientists with the CMHC are concerned with better design and installation of EIFS. Their case studies demonstrate that poor design and construction ultimately undermine the durability of building components. The CMHC has sound advice for building designers who aspire to creating more durable buildings.
Redundancy is Essential in Building Envelope Design
The CMHC’s Best Practice Guide for EIFS, released in 2004, educates designers and builders in EIFS detailing. As the guide points out, EIFS differ from other claddings because the insulation is part of a composite cladding. This improves thermal performance if properly installed. Detailing at the interfaces, for instance where windows meet walls, or roofs connect to walls, is especially critical.
Two rules of thumb apply.
- Plan for redundancy. Redundancy refers to anticipating the failure of wall and roof systems by building in extra barriers or sealants for a rainy day.
- Ensure the water-resistive barrier coating is continuous. Breaks in the coating permit moisture to get in and degrade the substructure.
Until recently, EIFS installed in the residential market were expected to provide the total barrier for a building envelope. Stucco or other surface materials were applied over a single moisture barrier. The barrier covered common construction materials like plywood or exterior gypboard sheathing, with wood stud frame and insulation behind these layers.
There was no secondary line of defence if the wall or roof system performed badly. As the B.C. condominium debacle has demonstrated, when systems fail because of a rainy climate or poor construction practices, moisture accumulates within the structure and permanently damages the building envelope.
Engineers with Halsall Associates, specialists in dealing with building failures in Canada, suggest the practice of value engineering has opposed durable design by eliminating redundancy in building envelopes. Value engineering recommends single-stage sealants and air barrier connections. In B.C., the combination of these practices, plus a wet climate and poorly-supervised and rushed construction, has been especially destructive.
Improving the Design of Building Envelopes
EIFS must be fully drained with weather-resistive barriers (WRBs). The best advice is to use tested materials, ensure subtrades are qualified to perform the installation, and watch contractors carefully, so that corners are not cut during the installation of system components.
Insurance Canada recommends proper detailing of system penetrations and terminations, especially at points where dissimilar materials come together. Flashings and sealants are critical to the durability of a building envelope. Poor detailing can result in moisture penetration, leading to subsequent mould growth and envelope degradation.
In times past, traditional stucco systems installed over wood frames and sheathing incorporated a two-ply weather resistive building paper behind the stucco and lath. The EIFS industry has been slow to adopt a similarly redundant approach. The recent rash of building failures has had a salutary result: the design of building envelopes is finally improving.
Andree Iffrig - Andrée Iffrig LEED AP is a writer and award-winning graduate architect. She uses her broad background in environmental design and ...
