Masonry Magazine August 1988 Page. 18
Procedure
An 11-column table is set up, similar to that shown in Table 3.
The wall components, including the inside air, inside air film and exterior air film, are listed in Column 1.
The thermal resistance (deg. F* sq. ft.hr/Btu) of each wall component is entered in the rows of Column 2 respectively. Values of thermal resistance may be found in Technical Notes 4 Revised, Table 1.
The component thermal resistances are totaled and entered at the bottom of Column 2.
The component thermal resistance percentage is calculated by dividing the component thermal resistance by the total thermal resistance of the wall assembly and multiplying this quotient by 100. The results are entered in the rows of Column 3, respectively. Check the total of the component percentages to make sure that they equal 100.
The temperature drop across each component is calculated by multiplying the component thermal resistance percentage by the total temperature drop across the wall section (Ti To), and dividing this product by 100. The results are entered in the rows of Column 4 respectively. A quick check is to total the component temperature drops. They must equal the total temperature drop across the wall section.
The temperature of the inside air, Ti, is entered in Row 2 of Column 5.
The component temperatures (the temperature of the component's exterior face) are calculated by subtracting the component temperature drop from the temperature of the component preceding it. The results are entered in the rows of Column 5 respectively.
Check the calculated temperature of the outside air film. It must equal the temperature of the outside air, To.
The component saturated vapor pressures are taken from Table 1 for the temperature of each component, and entered in the rows of Column 6 respectively.
The component vapor resistances (in Hg sq. ft. * hr/gr), taken from Table 2, are entered in the rows of Column 7 respectively.
The total vapor resistance of the wall section is calculated by totaling the component vapor resistances. This total is entered at the bottom of Column 7.
The component vapor resistance percentage is calculated by dividing the component vapor resistance by the total vapor resistance of the wall section and multiplying this quotient by 100. The results are entered in the rows of Column 8 respectively. Check the total of the component vapor resistance percentages to make sure that they equal 100.
The actual vapor pressure of the interior and exterior air is calculated by multiplying their saturated vapor pressures by their respective relative humidities. The results are entered in the rows of Column 10 respectively.
The total vapor pressure difference is calculated by subtracting the exterior actual vapor pressure from the interior actual vapor pressure. The result is entered at the bottom of Column 9.
The component vapor pressure difference is calculated by multiplying the component vapor resistance percentage by the total vapor pressure difference and dividing this product by 100. The result is entered in the rows of Column 9 respectively. Total the component vapor pressure differences. This total must equal the total vapor pressure difference, entered at the bottom of Column 9.
The component actual vapor pressure is calculated by subtracting the component vapor pressure difference from the actual vapor pressure of the component preceding it. The results are entered in the rows of Column 10 respectively.
The component actual vapor pressures (Column 10) are checked against the component saturated vapor pressures (Column 6). If any wall component has an actual vapor pressure which is larger than its saturated vapor pressure, condensation is likely to occur in that area of the wall section, as indicated by the asterisks in Column 11.
Table 3 shows an example of this condensation analysis procedure. The wall being considered is an insulated brick and block cavity wall system. It is assumed that the wall is located in Washington, DC. The analysis is for a winter day with an exterior temperature of 17 deg. F@73% relative humidity, and an interior temperature of 72 deg. F@ 50% relative humidity.
The temperature gradient, as well as the saturated vapor pressure and actual vapor pressure gradients, can be plotted across the wall section, as shown in Figures 2 and 3 respectively. When plotting the saturated and actual vapor pressure gradients, the areas where the actual vapor pressure gradient is above the saturated vapor pressure gradient are where condensation is likely to occur.