Continuing the Debate on Solar-driven Moisture

September 2, 2013

Our January 2013 article, “Ensuring Moisture Protection for Manufactured Stone,” written by Jeff Diqui, Arch. Eng., CSI, resulted in a letter to the editor from another frequent contributor to The Construction Specifier.

In the spring, Maria Spinu, PhD, LEED AP, e-mailed her comments:

I want to compliment Mr. Diqui on a generally well-written and informative article, but I wish to address the statements made around solar-driven moisture. He wrote:

A related effect in adhered masonry veneer cladding is inward vapor drive, which can occur in warm weather. It forces moisture stored in the masonry through vapor-permeable housewraps and building papers and into the sheathing and stud bay.

While recognizing that vapor diffusion is not the only source of moisture in buildings (i.e. “bulk water is not the only enemy”), the author goes on to say that “another challenge is inward water vapor drive … Bulk water intrusion and vapor drive have led to damage and rot in sheathing and structural members.” Bulk water intrusion and solar-driven moisture in the same sentence? That’s what got to me!

I fully agree adhered masonry veneer is not the best practice—incorporating a drainage plane and a ventilation gap in this wall assembly is much better. However, attributing condensation problems to solar-driven moisture is not supported by field research or advanced moisture analysis simulations. Multi-year field tests have unsuccessfully tried to reproduce solar-driven moisture effects in wall assemblies using weather-resistant barriers (WRBs) of different vapor permeance, but instead have found those materials’ vapor permeance has little or no effect on solar-driven moisture.(See American Society of Heating, Refrigerating, and Air-conditioning Engineers [ASHRAE’s] 2010 report, “Evaluation of Cladding and Water-Resistive Barrier Performance in Hot-Humid Climates Using a Real-Weather, Real-Time Test Facility,” by Theresa A. Weston et al.)

ASHRAE TRP 1235, “Research Project on Solar-driven Moisture,” found under conditions solar-driven moisture occurred, the critical wall component was not the WRB’s vapor permeance, but rather the interior wall’s vapor permeability—in other words, use of interior vapor barriers in hot/humid climates. (Synthesis of research project RP-1235, ASHRAE OR-10-061, “The Nature, Significance, and Control of Solar-driven Water Vapor Diffusion in Wall Systems,” by D. Derome et al.)

Condensation problems are very unlikely to be caused by inward vapor diffusion. Building scientists have long agreed more than 90 percent of moisture vapor transported through the building enclosure is due to air leakage, and only less than 10 percent can be attributed to vapor diffusion. The major source of water vapor in the building enclosure is infiltration of humid, exterior air that could lead to condensation if the exterior air reaches the cooler interior surfaces

Advanced moisture simulations—such as Wärme und Feuchte instationär (WUFI) or Transient Heat and Moisture—can estimate the condensation potential due to air infiltration. It has been shown if the humid exterior air reaches the cooler interior surfaces with temperatures below the exterior air dewpoint, it will deposit excess moisture and lead to condensations. (See the 2006 report, “Assessing the Durability Impacts of Energy Efficient Enclosure Upgrades Using Hygrothermal Modeling: Research Report–0605,” by John Straube and Christopher Schumacher.) In fact, most condensation problems attributed to vapor diffusion are likely due to moisture transported by air leakage.

So, let’s see the forest for the trees and focus on good design and execution. Vapor-permeable WRBs do not contribute to wetting in a properly designed building envelope. However, WRB vapor permeance is critical for efficient drying of any incidental moisture that makes its way behind the WRB where neither drainage nor ventilation is available for drying.

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