Masonry Magazine March 1969 Page. 43
FIGURE 1. Resistance Values Shown in Table 1 are Compared with Published and Unpublished Test Results on 8-inch Hollow Concrete Block Walls.
RESISTANCE, R = 1/C
6.0
5.0
• Unpublished Tests
Published Tests
4.0
Statistical Correlation
3.0
2.0
Table 1 Values
1.0
50
60
70
80 90
100
110 120
130
140
UNIT WEIGHT OF CONCRETE MASONRY UNITS, Ib/cf
The reciprocal of the total resistance is the overall heat transmission coefficient, or U-factor.
Thermal resistance values, 1/C, that can be used to predict the conductance of concrete masonry of various nominal thicknesses and unit weights, are contained in Table 1. Values are shown for both uninsulated concrete masonry walls and walls in which the hollow cells of the concrete masonry unit are filled with loose-fill insulation such as Perlite, Vermiculite, and others. Table 1 values do not include individual resistance to heat flow due to surface conductance (air film and treatment of masonry wall surface such as paint). For example, the total resistance of an 8-inch hollow concrete masonry wall composed of units weighing 80 lbs/cf would be compared to one constructed of units weighing 140 lb lbs/cf as shown in table at right.
In general, total resistance values calculated from the data in Tables 1 and 2 will be similar to results from the ASHRAE Guide. However, it
Resistance Value
Outside Surface* (1/fo)
Concrete Unit** (1/c)
Inside Surface* (1/fi)
Total Resistance, Rt = 1/U
U-Factor=1/Rt =
Unit Weight of Masonry Units
80 lbs/cf
140 lbs/cf
0.17
0.17
1.94
1.00
0.68
0.68
2.79
1.85
0.36
0.54
*15 MPH wind for outside surface and still air inside surface (see Table 2).
**Resistance value from Table I.
should be kept in mind that estimating thermal resistance of building components which contain air spaces of unconventional shape, such as concrete masonry units, is not as accurate as would be obtained by a laboratory test. Where laboratory test values of thermal conductance are available, they should be employed.
Values shown in Table 1 are based upon published test data only, and are generally conservative. Figure 1 compares test data from a large number of unpublished thermal conductance tests made on 8-inch concrete block walls with published data, and suggests that considering all known test data, the values listed herein are quite conservative. Note in Figure 1 that the statistical correlation based upon all data (published and unpublished tests) would yield resistance values considerably greater than those contained in Table 1.