Masonry Magazine March 1977 Page. 19
TABLE 2
Saturated Vapor Pressures
| Fahrenheit Temperature | Vapor Pressure, In. Hg |
|---|---|
| -10 | 0.0220 |
| 0 | 0.0376 |
| 10 | 0.0629 |
| 15 | 0.0806 |
| 20 | 0.1027 |
| 24 | 0.1243 |
| 30 | 0.1645 |
| 40 | 0.2477 |
| 50 | 0.3624 |
| 60 | 0.5216 |
| 70 | 0.7392 |
| 75 | 0.8750 |
| 80 | 1.0323 |
| 90 | 1.4219 |
| 100 | 1.9333 |
As may be noted, the variation of vapor pressure with temperature is not constant but increases rapidly with increased temperature.
The ratio of the actual pressure of the water vapor to the saturation pressure of the water vapor for the particular temperature is also termed relative humidity. Its value when defined thus is essentially the same as that given previously.
When a given mixture of air and water vapor is cooled without loss of moisture, a temperature is eventually reached where the air becomes saturated with water vapor and condensation can occur. The temperature then existing is called the dew point.
When a vapor-pressure differential exists, water vapor will move toward the lower pressure independently of air. This vapor movement through common building materials is at a relatively high rate for common pressure differentials. When vapor passes through pores of homogeneous walls, which are warm on one side and cold on the other, it may reach its dew point and condense into water within the wall; but, if the flow of vapor is impeded by a highly vapor-resistant material on the warm side of the wall, the vapor cannot reach that point in the wall at which the temperature is low enough to cause condensation.
Permeability to water vapor travel is known as permeance which is defined as "the ratio of water vapor flow to the vapor-pressure difference between the surfaces". Permeance is measured in perms. A perm is equal to 1 grain per sq ft per hr per in. of mercury vapor-pressure difference. A perm-inch is the permeance of 1-in. thickness of a homogeneous material. The reciprocal of permeance is called vapor resistance.
Differences in vapor pressure through different parts of the wall from inside to outside distribute themselves in proportion to the vapor resistance of the respective parts; that is, the fraction of the total vapor-pressure drop from inside to outside occurring between the air indoors and some chosen point within the wall is the same as that fraction of the total vapor resistance of the wall occurring between the air indoors and the chosen point.
In this it is analogous to heat flow, in which differences in temperature through different parts of the wall from warm to cold side are proportional to the thermal resistance of these parts.
As an example, assume a brick and tile, vermiculite insulated cavity wall, unplastered, exposed on the tile side to 75 deg Fahr, 50 per cent relative humidity, and on the brick side to 20 deg Fahr, 90 per cent relative humidity. The thermal resistances are:
Inside air 0.68
4-in. tile 1.11
2-in. vermiculite 5.56
4-in. brick 0.44
Outside air 0.17
R7.96 U = 0.125