Masonry Magazine March 1977 Page. 20
The vapor resistances are:
4-in. tile 0.333
Insulation 0.008
4-in. brick 0.480
0.821
The vapor pressure differential is 50 per cent of saturated vapor pressure at 75 deg (0.4375 in. Hg) minus 90 per cent of saturated vapor pressure at 20 deg (0.0924 in. Hg) equals 0.345 in. Hg. The temperature difference is 75 deg minus 20 deg equals 55 deg.
From the above resistances, the temperature gradient and vapor pressure gradient may be plotted as indicated in Fig. 2. As may be noted, the vapor pressure at the inside face of the brick wythe is 0.29 and the temperature at this point is 24 deg.
Referring to Table 2, the saturated vapor pressure at 24 deg is 0.124. Since the actual vapor pressure exceeds this amount, condensation might be expected to occur in the insulation just back of the outer wythe.
Vapor transmission tests, sponsored by the Structural Clay Products Research Foundation, a Division of Structural Clay Products Institute, were conducted on a wall similar to the above at Pennsylvania State University. The temperatures and humidities maintained during these tests were approximately the same as those assumed. Data obtained from the tests are included in the paper, Review of Recent Research, by C. B. Monk, Jr., published in the Proceedings of the BRI Conference on Insulated Masonry Cavity Walls, which was held in New York City in April 1960.
The results of the tests confirm the above calculations, since frost formed on the cavity face of the brick wythe. However, Mr. Monk states: "The major observation of the heat transfer data is the relative constant air-to-air conductance of the wall during the 18 days of steady state conditions, indicating no change in the thermal characteristics of the wall due to the frost observed at the end of the test."
From the vapor transmission tests, it is apparent that, insofar as condensation affects heat transfer, a vapor barrier is not required on the warm side of an insulated cavity wall, whose permeance is approximately equivalent to the wall tested when vapor pressure differentials are of the order of 1 in. of mercury. However, as previously indicated, water in a masonry wall may contribute to disintegration of masonry units and, where soluble salts are present in the masonry, it also contributes to efflorescence. For this reason, vapor barriers are recommended on the warm side of cavity walls with exterior wythes of glazed brick.
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R10M-3-77