Masonry Magazine March 1977 Page. 18
other relative humidities.
A discussion of the reason for this relationship may be found in the ASHRAE Guide and Data Book, Fundamentals and Equipment Volume, 1963, published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.
In Fig. 1, the difference in temperature between the air and the dew point (temperature drop) is plotted for relative humidities from 50 per cent to 100 per cent. It will be noted from this curve that for relative humidities above 80 per cent a drop in temperature of 6.8 deg or over will cause condensation. These high humidities usually occur during the summer when the difference in temperature between the air on opposite sides of a wall is small, probably 10 deg or less. For walls below grade, the temperature difference of the two sides of the wall may amount to 20 deg or more.
If condensation occurs, it may be eliminated by:
1. Reducing the humidity of the air. This may be accomplished by adequate ventilation if the high humidity is caused by conditions inside the building.
2. Increasing the temperature of the surface upon which the condensation occurs. Probably the simplest means of increasing the surface temperature is to increase the movement of air over the surface.
3. Increasing the heat resistance of the wall. This is usually done by the addition of an air space or insulation back of the interior finish.
The temperature gradient or temperature drop through a composite wall is directly proportional to the resistance of the various elements, including surface resistances; that is, if the total resistance of a wall is 8.0 and the resistance of one element is 2.0, the temperature drop across this element will be 2/8 of the air temperature difference on the warm and cold sides of the wall.
Table 1 gives the difference in temperature between the inside surface of various types of walls and the inside air temperature for differences between inside and outside air, ranging from 10 deg to 70 deg Fahr. These figures are based upon the conductivities and conductances of materials listed in Tables 1, 3 and 4 of Technical Notes, 4, "Heat Transmission Coefficients of Brick and Tile Walls".
From Table 1, it may be noted that, for a difference in temperature between inside and outside air of 70 deg, the inside surface temperature of an exposed brick-and-brick cavity wall is 16 deg below the temperature of the inside air. Referring to the curve, Fig. 1, a temperature drop of 16 deg will cause condensation for relative humidities of 58 per cent or over. The temperature drop for a vermiculite insulated brick-and-brick cavity wall, exposed, is 61% deg for which relative humidities
TABLE 1
Difference in Temperature Between Inside Air and Inside Wall Surface for Various Total Differences In Temperature Between Inside Air and Outside Air
| Wall Construction | Difference in Temperature Between Inside and Outside Air, deg Fahr |
| :-------------------------------------------------------------------------------- | :------------------------------------------------------------------- |
| | 10 | 20 | 30 | 40 | 50 | 60 | 70 |
| 4-in, brick and 4-in. file, furred and plastered. | 2 | 3% | 5 | 7 | 9 | 10 | 12 |
| "SCR brick", furred and plastered | 2 | 5 | 7 | 10 | 12 | 14 | 17 |
| 8-in. tile (2-cell). furred and plastered. | 1% | 3 | 4% | 6 | 7 | 8% | 10 |
| 10-in. brick and brick cavity, exposed | 2% | 4% | 7 | 9 | 11 | 13% | 16 |
| 10-in. brick and tile cavity, plastered | 1% | 3 | 4% | 6 | 7 | 8% | 10 |
| 10-in. brick and brick cavity, vermiculite insulated, exposed | 1 | 2 | 3 | 4 | 5 | 6 | 6% |
| 10-in. brick and tile cavity, vermiculite insulated, plastered | 1 | 2 | 2% | 3% | 4 | 5 | 6 |
Note: Ploster: gypsum and aggregate
Lath: % in. gypsum.
Reg. U.S. Pat. Off., SCPI
must be 81 per cent or over to cause condensation. Table 1 may be used in connection with the "temperature drop" curve to select a type of wall construction which will be free from condensation.
CONDENSATION WITHIN WALLS
The National Bureau of Standards Report, BMS63, Moisture Condensation in Building Walls, contains a method for calculating potential condensation if the temperature and vapor-resistant gradients of the wall are known. The following summarizes the discussion and method outlined in the report:
A definite volume of air held at a fixed temperature can contain permanently no more than a definite amount of water in the form of vapor. This limiting quantity of water per given volume is termed "moisture content at saturation". If the air contains a greater proportion of moisture than this at the particular temperature, the water will start condensing on the surfaces of the container or even on the dust particles in the air which then fall out in a fine mist. The ratio of the actual moisture content to the saturation moisture content for the particular temperature is termed "relative humidity". It is customarily expressed in per cent.
The concentration of water vapor may also be stated by giving its pressure. If water vapor is present, part of the atmospheric pressure is maintained by the water vapor and the remainder of the pressure by the other constituents of the atmosphere. At a particular temperature and at saturation, the water vapor exerts a definite pressure.
Data on saturated vapor pressures are listed in tables of the ASHRAE Guide and Data Book and Table 2 gives saturated vapor pressures for various temperatures as included in the 1963 edition.