Masonry Magazine March 1969 Page. 42
Estimating Thermal Coefficients (U-Factors)
The measure of the quantity of heat transmitted through a building wall is the BTU (British Thermal Unit). In figuring heat loss, it is always accompanied by a definite time factor, generally one hour, and a unit of area, generally one square foot. Heat loss is stated as the number of BTU's lost per degree difference in temperature, the temperature on the warmer side of the building wall being at least one degree higher than on the cooler side. Thus, the coefficient of heat transmission is based on the number of BTU's lost per hour, per square foot, per degree difference in temperature (BTU/Hr/Sq.Ft/°F).
There is a thin film of air at the surface of all walls, floors, or roofs, which in itself acts as insulation. For this reason, the heat loss from the air on one side of the wall to the air on the other side will be less than that due solely to the resistance of the material. Also, the temperature of the air and the temperature of the surface will always be different due to the resistance of these surface films of air. The insulating value of the air film (f) will be greater in still air than in moving air such as caused by wind or forced circulation.
Air spaces are insulators against heat loss. Thus, the air spaces within concrete masonry walls are insulators, their value depending upon their location, width, air flow in them, and other factors.
The resistance of a homogeneous material to heat transmission is directly proportionate to its thickness. With the same temperature differences, half as much heat is lost from surface to surface through a 2-inch thickness of a given material as lost through one-inch in thickness.
The most common source of information on the U-factor of various wall construction is the ASHRAE Guide published by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. In addition to values of conductivity, conductance, and resistance, the ASHRAE Guide also gives a procedure for calculating overall coefficients (U-factors) for composite constructions. Using the procedure outlined in the Guide, building designers and engineers can estimate U-factors for various types of construction, such as, cavity walls, veneer walls, and masonry walls with added insulation.
A limitation to the ASHRAE Guide is that it contains few data on concrete masonry units of different unit weight (density). Conductance values are listed for: (1) sand and gravel, (2) cinder, and (3) lightweight aggregate concrete block of different thicknesses ranging between 3 and 12 inches, but no relationship is established between conductance and unit weight beyond that implied by aggregate name. Since the unit weight of lightweight aggregate concrete can vary over a wide range, 60 to 115 lb/cf, values of conductance for concrete masonry of lightweight aggregate could differ considerably from the Guide values. Thermal conductance varies with the unit weight- directly and significantly. Conductance values shown in the ASHRAE Guide for lightweight concrete indicate an increase in conductivity greater than twofold between 60 and 100 lb/cf density and threefold between 60 and 120 lb/cf.
The total resistance (1/U) to heat flow through a building section numerically equals the sum of the thermal resistances of the various components of the building section. These components consist of air films, materials of construction, and air.
TABLE 1-Resistance Values (R = 1/C) of Single-Wythe Concrete Masonry Walls with Hollow Cells
Empty and Filled with Bulk Insulation.
| CM Units | Insulation in Cells | Unit Weight, Ib/CF |
|---|---|---|---|---|---|
| | | 60 | 80 | 100 | 120 | 140 |
| 4": | Filled | 3.36 | 2.79 | 2.33 | 1.92 | 1.14 |
| | Empty | 2.07 | 1.68 | 1.40 | 1.17 | 0.77 |
| 6": | Filled | 5.59 | 4.59 | 3.72 | 2.95 | 1.59 |
| | Empty | 2.25 | 1.83 | 1.53 | 1.29 | 0.86 |
| 8": | Filled | 7.46 | 6.06 | 4.85 | 3.79 | 1.98 |
| | Empty | 2.30 | 2.12 | 1.75 | 1.46 | 0.98 |
| 10": | Filled | 9.35 | 7.46 | 5.92 | 4.59 | 2.35 |
| | Empty | 3.0 | 2.40 | 1.97 | 1.63 | 1.08 |
| 12": | Filled | 10.98 | 8.70 | 6.80 | 5.18 | 2.59 |
| | Empty | 3.29 | 2.62 | 2.14 | 1.81 | 1.16 |