EIFS and sustainable design

Thermal barrier
By incorporating a thermal barrier within the building envelope, it is possible to minimize heat flow between the inside and the outside, allowing interior conditions to be maintained at a comfortable temperature.

In conventional frame wall construction, batt insulation is placed within the stud space to provide the thermal barrier. Improved thermal resistance is achieved by increasing the studs’ thickness, allowing thicker insulation to be used. However, as the studs are exposed to both the inside and outside environments, they conduct heat at a much higher rate than the insulation, creating multiple thermal bridges.

As the first two letters in the EIFS acronym suggest, insulation is applied to the exterior of the building frame, eliminating the multiple thermal bridges. The thickness of the insulation can be significantly increased to make even more dramatic increases to the R-value and the wall’s thermal efficiency.

Air barrier
Required by the National Building Code of Canada (NBC) since 1985, air barriers prevent air leakage through the building envelope, helping to stop exfiltration of heated air to the cold exterior and the infiltration of cold, untreated air to the interior. The result is a much more thermally controlled environment requiring less energy input to maintain.

Stopping air leakage prevents the second most common source of moisture in a wall assembly—condensation. In a cold climate like Canada, as warm humid air from inside a building moves through the envelope, air cools until it is no longer able to hold its moisture. The moisture condenses on the first cold surface within the wall assembly below the dewpoint of the air (often the back of the exterior sheathing).

A structural, durable, continuous, and air-impermeable barrier throughout the building envelope can prevent air movement across the envelope. The fluid-applied membrane of some newer EIFS assemblies meets all these requirements. Being fluid-applied makes certain any joints or gaps are filled, helping to ensure continuity. Where necessary, compatible sheet materials or sprayed foams can bridge larger gaps between different materials within the wall (e.g. junctions with windows or doors). As it is fully adhered to the substrate, the fluid-applied membrane becomes an integral part of it, assuming the strength of the substrate and the ability to resist pressure differences caused by wind, stack effect, or mechanical systems. The membrane is located on the interior side of the insulation in an EIFS wall, so it is protected from the exterior elements, helping ensure longevity.

Avoiding the double vapour barrier
For conventional frame-wall construction in a cold climate, a polyethylene vapour barrier is typically installed behind the interior gypsum wallboard. Alternatively, special vapour barrier paint can be applied to the drywall surface. Some design professionals may be concerned the aforementioned air barrier/waterproofing membrane found in some newer EIFS assemblies could also function as a vapour barrier, creating a ‘double-vapour-barrier’ situation. While this may be a problem with some waterproofing and air barrier materials, the products used in EIFS are generally vapour-permeable and thus permit diffusion and drying. (It is always important for the design professional to discuss system properties with its manufacturer.)

The accepted definition of a vapour-retarding material is one with a water vapour permeance of 57.4 ng/(Pa•s•m²) or 1 perm. A 0.1-mm (4-mil) polyethylene product is 4.6 ng/(Pa•s•m²) or 0.08 perms, and No. 15 building felt is at about 325 ng/(Pa•s•m²) or 5.7 perms. Some EIFS water-resistive barriers have a vapour permeance equivalent to building felt, while others can be slightly higher or lower, depending on the manufacturer. As long as the EIFS water-resistive barrier has vapour permeability significantly higher than the interior vapour retarder, the ‘double vapour barrier’ does not exist. Consequently, it should not be a concern in a cold climate, provided bulk water is kept out of the stud cavity (which should always be the case in any durable wall design).

Conclusion
Exterior insulation and finish systems contribute to sustainable design in several ways:

• longevity and lifecycle analysis (LCA);
• reuse of existing building shells;
• optimization of energy performance;
• reduced carbon emissions; and
• components low in volatile organic compound (VOC) emissions. (The importance of a low-VOC product on the exterior is not only the health and safety of the applicators, but also the low carbon dioxide emissions.)

Further, as illustrated throughout this article, EIFS assemblies can be multifunctional and highly effective systems. As demonstrated by the effective R-value of these systems and the moisture protection they provide, EIFS can be an important component of an overall sustainable design strategy effective in achieving building envelope and operating efficiency.

Dale D. Kerr, M.Eng., P.Eng., is a principal at engineering firm, GRG Building Consultants. She has more than 20 years of experience in building science research, testing, failure investigation, and building repair. Kerr was the first to be recognized as a Building Science Specialist of Ontario (BSSO) by the Ontario Building Envelope Council (OBEC) and is a regular contributor to technical publications across North America. She can be contacted via e-mail at dkerr@grgbuilding.com.

Tom Remmele, CSI, is the director of technical services/R&D for exterior insulation and finish systems (EIFS) producer, Sto Corp. He has held technical management positions in the construction industry for more than 20 years. Remmele is a past Technical Committee chair of the EIFS Industry Members Association (EIMA) and is a member of the Construction Specifications Institute (CSI) and the ASTM Committee E6 on Building Performance. He has published numerous technical articles on EIFS, air barriers, stucco, and related topics in technical journals. Remmele can be contacted via e-mail at tremmele@stocorp.com.

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