Masonry Magazine June 1972 Page. 26
Controlling Moisture
(Continued from page 25)
In the southern part of the United States to 50°F in the north. (See illustration #2.) For the Chicago area, for instance, the ground ambiency in 55°F. At this temperature, the ground has 100 percent relative humidity and a corresponding vapor pressure of 0.214 pounds per square inch.
If the temperature of the air above the ground is 80°F, with a relative humidity of 80 percent, the vapor pressure of the air would be 0.405 pounds per square inch, almost twice as much as in the ground. Vapor movement would therefore be from the air to the ground. If the air temperature is 0°F, with a relative humidity of 60 percent, the vapor pressure would be 0.010, much less than in the ground, and the vapor would move from the ground into the air.
However, moisture moving up through the ground at such low temperature would start freezing between the ground ambiency level and the surface, and effectively stop any moisture from escaping into the atmosphere. In other words, during cold weather, moisture normally cannot escape from the ground. However, a building prevents the ground under it from freezing, and moisture is free to continue its migration.