Moisture performance of EIFS is misunderstood and it continues to be a subject of consternation. Much of the confusion stems from the fact that design criteria for EIFS assemblies incorporating cavity insulation plus outbound insulation panels have not been adequately addressed by the EIFS industry. Manufacturers place the responsibility of assembly design squarely on the architect who may or may not understand the implications to transient heat and moisture movement. Throw in the added challenges posed by ASHRAE 160 and most architects would prefer to ignore the technical details, preferring instead a blissful state of denial. Our advice, take command and give your design the attention it deserves. Here, we explore the errors of conventional design and how you can reach a higher state of EIFS consciousness.
What we are presenting here is not new. Research conducted in 2006 by Oak Ridge National Laboratory and Achilles Karagiozis has demonstrated vast improvements with EIFS employing 4” EPS without cavity insulation and without interior vapor retarders. This configuration can accommodate 1% moisture infiltration loads for most climate conditions. Depending on the nuances of wall configuration and drainage, we’ve been able to demonstrate quality performance at 4% infiltration loads, which is our preferred design criterion. So the configuration works, but how? To find out let’s take a look at two framed wall assemblies.
Assembly A represents a conventional wall that we frequently see in cold climate commercial construction. It contains 2” thick expanded polystyrene (EPS) boards installed over a vapor-permeable liquid-applied weather-resistive barrier and glass-backed gypsum sheathing. It also employs 5-1/2” of fiberglass batt insulation within the wall cavity and an interior vapor retarder comprised of polyethylene sheeting. Assembly B varies by having 4” of EPS, but it lacks cavity fill insulation and an interior vapor retarder. Both assemblies represent drainable systems with a nominal 1/16” air space and incidental ventilation.
Hygrothermal simulations were performed using climate data for Minneapolis, Minnesota and a 1% moisture infiltration load as required for ASHRAE 160 compliance. Moisture content of wall sheathing reflects a three-year simulation commencing in January.
The results show a stark contrast in performance. Wall A has a high tendency for moisture accumulation for the modeled period and beyond. Only rarely does the sheathing dry to acceptable levels. In contrast, Wall B shows robust performance with the capability to accommodate even greater moisture loading either from exterior or interior sources.
The use of in-cavity insulation and an interior vapor retarder has two potentially detrimental effects which are exacerbated in systems having thinner insulation panels. First, by thermally conditioning the cavity from the interior space, the batt insulation pushes the dew point back into the wall cavity. Secondly, fiberglass batt insulation offers almost no resistance to moisture movement; exfiltrated vapor is therefore transported towards wall sheathing where the potential for condensation is higher. Such effects can still occur even in situations involving a polyethylene vapor retarded on the interior side. Our experience has shown that the vapor retarder must be installed in a perfect manner to achieve the design intent – but alas, here lies the second problem. During the summer cooling season, when the prevailing vapor movement is from exterior to interior, low permeability vapor retarders impede inward drying, resulting in higher potentials for elevated moisture and mold growth. This condition is compounded by moisture loading against the weather-resistive barrier.
Assembly B works for two primary reasons: 1) It does not create dew-point shifts to the interior side of wall sheathing – this is especially important for cold climates; and 2) Assembly B also facilitates bi-directional drying of incidental moisture, a necessary requirement for ASHRAE 160 compliance.
EIFS can be a highly effective cladding choice, but it deserves greater design attention than it routinely receives. Extended service life and quality performance depend on prudent design tailored to specific climates and micro-climates, building exposure/orientation, expected moisture loading, and insulation/energy requirements. If you are not considering all of these conditions, then your building is vulnerable to varied and undesirable outcomes. Consider the wisdom of this Chinese proverb: You cannot prevent the birds of sorrow from flying over your head, but you can prevent them from building nests in your hair.
Resources
- DOE (Department of Energy). 2006. The Hygrothermal Performance of Exterior Wall Systems: Key Points of the Oak Ridge National Laboratory NET Facilities Research Project. http://www.eima.com/pdfs/EIMA_Executive_Summary_new.pdf
- Karagiozis, A.; H. Künzel; and A. Holm. 2001. WUFI-ORNL/IBP – A North American Hygrothermal Model. American Society of Heating, Refrigerating and Air-Conditioning Engineers -ASHRAE-, Atlanta/Ga.: Performance of Exterior Envelopes of Whole Buildings VIII 2001. Conference Proceedings. Clearwater Beach, Florida. Atlanta, GA: American Society Heating, 2002, 10 pp.
- MGI (Moisture Group Inc.). 2008. Evaluation of the Moisture Performance of Walls Clad with Drvit EIFS Without Interior Vapor Barriers, July 5, 2008. Prepared for Dryvit Systems, Inc.
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