Masonry Magazine June 1972 Page. 25
Design Techniques for Controlling Moisture in Building Structures
Every builder and contractor knows the damage to a building and its contents that can be caused by excessive moisture. Moisture control should therefore be an important consideration in the construction of any building. But in order to control moisture, it is first necessary to understand the nature of the problem, to know where moisture comes from and what it does.
Basically, moisture in this connection can be defined as water in three forms, as a liquid, a solid and a gas. The gaseous form is the one which causes most of the problems-water vapor. Almost every building is sufficiently protected against water in its liquid form, but adequate vapor protection is frequently lacking, primarily because many of the materials used as vapor barriers do not really stop vapor which will penetrate anything air can pass through.
Water vapor exists both in the air around the building and in the ground under it as it goes through the various phases of the hydrologic cycle, evaporating into the atmosphere from the soil, from rivers and lakes and from all kinds of vegetation, and returning to the ground in the form of rain or snow.
Like any gas, vapor follows the line of least resistance, moving from areas of higher vapor pressure to areas of lower vapor pressure. This pressure depends on temperature and humidty. Warm air, for instance, at a given percentage of relative humidity, has a higher vapor pressure than cold air at the same relative humidity. This means that the air can absorb more moisture as the temperature rises. Air at any given temperature reaches its dew point when it has a relative humidity of 100 percent. The vapor pressure of the air therefore varies with the seasons, being considerably higher during the summer than during the winter. (See illustration #1.)
Vapor pressure in the ground, on the other hand, is constant except for the few feet directly below the surface. The reason for this is the constant ambiency, which exists just a few feet below the surface. This remains the same, regardless of the seasons. The ground ambiency ranges from 65°F (Continued on page 26)
WESTERN
the complete
mortar and
plaster mixer line
No matter what you need in a mortar-plaster mixer, Western has just the machine. Size range from 3% to 11 cubic feet for high or low speed towing gasoline or electric powered belt or gear driven. Best mixer-engine combination automatically power-matched at the factory ..simplifies ordering. Get the complete story on Western's line and exclusive money-saving features. Contact: