Masonry Magazine October 1986 Page. 21
NCMA-TEK
An Information series from National Concrete Masonry Association
Tables of R-Values for Concrete Masonry Walls
Introduction
This TEK is a tabulation of calculated thermal resistance values for concrete masonry composed of hollow units. It includes units manufactured of concrete having densities between 60 and 140 pounds per cubic foot, with cores empty and filled with loose insulation, and with various insulations added to the outer surface of the walls. Tables 2 through 8 were compiled using the following:
1. Standard two-core hollow units.
2. Average face shell and web thickness based on ASTM minimum requirements assuming "" taper of face shells and webs.
3. Calculation method is the combined series-parallel heat flow method described in ASHRAE FUNDAMENTALS Handbook.
4. Thermal properties of the various materials listed in Table 1.
The combined series-parallel heat flow method accounts for thermal bridging. Figure I illustrates a hollow block treated as a series of layers of which one layer contains differential parallel paths. The face shells form continuous outer layers in series with the parallel combination of webs and empty or filled cores.
67A
(32B)
The equation for thermal bridging may be expressed as:
R=1+ (Lw/Kc) (Lw/Kf)
Where:
kc aw (Lw/Kf) + ac (Lw/Kc) fi fo
fs
Average thickness of both face shells
Lw= Length of web between face shells
kc = Thermal conductivity of block concrete
kf Thermal conductivity of insulating material
aw Fractional face area of webs
ac
Fractional face area of cores
fi
Inside surface air film coefficient
fo Outside surface air film coefficient
Note: (Lw/Kf) = 0.97 if cores are not filled.
Example:
Calculate the R-values of CMU wall composed of 8"x8" x 16" hollow units with the following properties having cores filled with vermiculite insulation.
average face shell thickness = 1.31"
average web thickness = 1.125"
100 lb./cu. ft. density: kc = 3.7
loose fill insulation, kc = 0.45
fs = (1.31) x 2 = 2.62";
Lw= 7.62"-2.62" = 5.0"
aw =
References:
Calculating The Steady State U-Value of Concrete Masonry Walls-Expanded Shale, Clay and Slate Institute
ASHRAE Guide