Frankensteinian Design: Energy Efficiency At The Expense Of Durability

Energy efficient initiatives, coupled with the IECC’s requirements for continuous insulation over exterior surfaces, have prompted a vast array of ‘Frankensteinian’ energy assemblies.  From three-coat stucco over outbound EPS to EIFS on structural insulated panels (SIPs), the combinations of such hybrid approaches appear endless.  While the professed energy efficiencies can be impressive, moisture management may suffer drastically.  We previously expressed these concerns in a blog posting in February 2012:  (Doctrines for Moisture Control) – our closing remarks are worth repeating here:

“Now we are in a new era of ultra-efficient buildings involving LEED, net-zero energy, and passive house – all noble in intent but rife with designs that often ignore the basic and enduring principles of moisture management.”

We are renewing our call for designers and contractors to re-examine current practices, which, at present, show no signs of slowing in their breakneck pace for greater energy efficiency.  In this article, we show the implications of overemphasizing energy ratings at the expense of moisture performance and building durability.

The graph below illustrates predicted moisture performance based on one-dimensional hygrothermal modeling using climate data for Toronto, Canada.  The modeled R-54 energy wall consists of adhered 3” EPS over OSB sheathing and 7” foil-faced SIPs containing a closed cell polyurethane core.  We have included a liquid-applied weather-resistive barrier that is frequently employed with adhered EPS in conventional EIFS applications.  The exterior finish is a typical EIFS base and finish coat applied to 1/8” thickness.  Per ASHRAE 160, we have applied a 1% rain penetration load against the exterior face of the WRB.  The model assumed no appreciable contribution from moisture exfiltration.  Details of this assembly and its modeled outcomes are further outlined in our recently-published Hygrothermal Snapshot No. 5 (Nov. 2013).

Hygrothermal Modeling of Energy Walls

Without a ventilated rainscreen, the sheathing’s exterior face is exposed to high humidity for most of the year.  Likewise, the impermeable SIP panels impede inward diffusion from the sheathing’s interior face.  The results are highly unfavorable.  No single component fails here.  It is the assembly’s overall configuration coupled with its lack of drainage and ventilation that undermines performance.

It is not the intent of this article to question the efficacy of EIFS or SIPs.  These systems, when designed with adequate moisture management, perform very favorably in almost any climate. The above scenario simply reemphasizes the need for whole building design with sound, or even redundant, safeguards for moisture measurement. Tools for quality hygrothermal design have existed for more than a decade; and still we see these highly promoted designs fall prey to conditional performance or outright critical failures.  With the advent of integrated tools such as WUFI Passive, analysis is made even more accessible – but are these tools being used?  More importantly, are they used correctly?

Hygrothermal design is only part of the equation.  Hybrid, multi-layered assemblies are inherently complex and require special attention to design detailing, construction quality, and assumptions regarding realistic moisture burdens.  The industry must embrace the fact that quality is not measured solely by aesthetics, R-values, and LEED ratings.  The time has come where moisture management is seen as a fundamental element of true sustainability.