Masonry Magazine March 1977 Page. 17
BIA Technical Notes
on Brick Construction
Brick Institute of America 1750 Old Meadow Road, McLean, Virginia 22101
FEB. 1965
MOISTURE CONTROL IN BRICK AND TILE WALLS
CONDENSATION
INTRODUCTION
It is generally agreed that the durability of masonry depends primarily on its resistance to the penetration of moisture into the body of the masonry. The source of this moisture may be wind-driven rains or it may be from interior exposures resulting from various occupancies which create high humidities. These include, among others, air conditioning with humidity control, food processing and unventilated space heaters. Differences in humidity between inside and outside air resulting from these occupancies will cause vapor flow within the wall and, unless controlled, either by the use of properly placed vapor barriers or by ventilating, this vapor may condense within the wall under certain temperature conditions.
When wall surface temperatures are substantially below air temperatures, condensation may occur on the wall surface.
CONDENSATION ON WALL SURFACES
Atmospheric air is a mixture of dry air and water vapor. At a given temperature air is saturated when the space occupied by the mixture holds the maximum possible weight of water vapor at that temperature. The amount of water vapor necessary to saturate the air at constant pressure depends upon the temperature-the higher the temperature the more water vapor will be required. If saturated air at a temperature of 50 deg, for instance, is warmed to a temperature of 70 deg, the mixture is no longer saturated but will absorb additional water vapor. However, if unsaturated air is cooled at constant pressure, a temperature will be reached at which the air is saturated. This temperature is called the dew point and, if the mixture is cooled below the dew point, water will condense from the air. Dew which occurs in the early mornings during the warmer months in many localities is one of the most common examples of the effect of cooling unsaturated air to a temperature below the dew point or to a point where the water vapor which the air contains begins to condense.
The water vapor in air is called humidity, and relative humidity is the ratio of the amount of water vapor which a mixture contains to the amount required for saturation at a given temperature. Obviously, for a fixed amount of water vapor, the relative humidity will vary with the temperature, increasing as the temperature is lowered and decreasing as the temperature rises.
While the dew point depends upon the amount of water vapor in the air and is the temperature at which the water vapor present is sufficient for saturation, there is also a practically constant relation between dew point and relative humidity for a considerable range of temperatures; that is, for a relative humidity of 50 per cent, the difference between air temperature and dew point is approximately 20 deg for any air temperature from 60 deg to 90 deg. Similar relations hold for