April 27, 2007

Building Materials

Index assesses moisture rates

PETER KENTER

correspondent

The increasingly globalized economy is presenting Canadian builders with a wider choice in building materials than ever before.

But in some cases, there’s simply not enough local experience with a particular material to determine its long-term suitability to the Canadian climate.

In a study appearing in the December, 2006 issue of the Journal of Architectural Engineering, Phalguni Mukhopadhyaya — associate research officer, performance of roof systems and insulation with the National Research Council’s (NRC’s) Institute for Research in Construction — and his associates, propose a new indicator of the moisture performance of exterior walls and their components, the Relative Humidity and Temperature (RHT) Index.

“Serious consideration for the moisture design of building envelopes and various material components is something we only started doing after the energy crisis of the 1980s,” says Mukhopadhyaya.

“In Canada, we began to insulate without thinking about the ways in which this would affect the moisture response of our buildings. Although we now understand more about managing building moisture, we’re also seeing a whole new range of building envelope materials for which we don’t have hygrothermal performance data.

“An insulating material imported from China or any other country may look the same as a material with which we have experience, but may perform differently, especially in the diverse range of Canadian climates. A different species of wood may have similar strength characteristics, but different moisture performance characteristics from wood we are used to working with.”

Mukhopadhyaya argues that builders require a design-oriented methodology to compare the relative moisture responses of various building materials before exposing them to the Canadian elements. Using the RHT assessment tool, the researchers entered data describing the initial moisture content and temperature of a building material, likely accidental moisture entry, and climatological conditions. The result was a single RHT Index number that assessed the moisture performance of the material.

“A higher value of RHT Index indicates a greater potential for moisture-related deterioration,” says Mukhopadhyaya. “The higher the RHT, the more likely that moisture will cause chemical and biological decay.”

In order to test the validity of the RHT, the researchers simulated the effects of moisture on four different wall systems — stucco, exterior insulation and finish systems (EIFS), masonry, and hardboard or vinyl siding — in five locations: Phoenix, Winnipeg, Ottawa, Seattle and Wilmington, NC.

“You don’t build the same way in Ottawa as you would in Phoenix,” says Mukhopadhyaya. “Each location has a unique Moisture Index based on Wetting and Drying Indexes derived from many years of data collection.”

What makes the RHT unique is that it doesn’t attempt to force any particular outcome on the builder or end user, says Mukhopadhyaya. “You don’t want all buildings to look the same and different cultures have different expectations of moisture conditions they’re more comfortable with,” he says. “Similarly, you would expect different moisture conditions in a kitchen and an office. When sufficient information is available from laboratory and field experiments, the RHT has proved itself to be both accurate and useful in predicting moisture performance.”

The RHT Index is already being used by the NRC’s Canadian Construction Materials Centre to assess various materials and systems. As the index develops, Mukhopadhyaya would like to see it adopted more widely by stakeholders of the building envelope design community.

“Using the RHT would help builders avoid a lot of unfortunate consequences.”

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