Roy Schauffele, President and Founder of Division 7 Solutions, Inc., is an internationally published author and speaker in the fields of energy conservation and sustainable building envelopes. He shares his thoughts on permeance testing in an article about water vapor transmission through building envelope materials. He discusses “perm rating” testing often performed in accordance to ASTM E96. The specification itself states, in part, “A permeance value obtained under one set of conditions may not indicate the value in another set of conditions.” Based on a round-robin testing effort, ASTM reports E96 has about 20% lab-to-lab variability. Roy questions how data with that type of discrepancy can be reliable.
Today’s science tells us that unimpeded air infiltration can move 32 to 64 times more moisture into a building than can water vapor transmission alone and that is why the development of air barrier technology was so critical. Though, he cautions not to place too much importance on perm ratings with the ever-growing use of CI (continuous insulation) diminishing the significance of air barrier perm ratings.
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