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While mass transfer of moisture with air, as infiltration and exfiltration, is now a concern in building performance, when these correctable errors are minimized, migration of moisture by the laws of physics will still occur. The tendency is to presume that moisture migrates in straight lines, as flow perpendicularly through thin materials. However, migration will occur in all materials or constructions in any direction that vapor pressures dictate, including migration laterally in sheet materials if interstitial paths exist. As example, lateral migration through felts was demonstrated by use of tritiated water vapor as a tracer. Strips of felts immersed in water for 100 days did expand across the fibers, as expected, but they shrank in the longitudinal direction. Uncoated felts responded promptly to changes in relative humidity, but coated felts, with their partially protected fibers, persisted to expand for a while even after the exposure relative humidity had been decreased. Felts even at sub-zero temperature expanded and contracted with changes in relative humidity induced by silica gel and by ice. Field examples of moisture migration and persistence are given, and attention is called to overlooked freezing induced by radiative cooling to the sky at night that decreases surface temperatures as much as 5.5°C (10°F) even at freezing temperatures.
relative humidity, moisture migration, interstitial, intracellular, tritiated water, shrinkage, expansion
Consulting engineer (retired), State College, Pa.