Masonry Magazine June 1975 Page. 12
TERMINOLOGY
In order to more fully understand and to be consistent, some of the more commonly used terms relating to heat transmission are defined. Most of these definitions are given as they appear in the 1972 edition of the ASHRAE Handbook of Fundamentals.
U Overall coefficient of heat transmission or thermal transmittance (air-to-air); the time rate of heat flow usually expressed in Btu per (hour) (square foot) (Fahrenheit degree temperature difference between air on the inside and air on the outside of a wall, floor, roof or ceiling). The term is applied to the usual combinations of materials and also to single materials, such as window glass, and includes the surface conductance on both sides. This term is frequently called the U value.
k - Thermal conductivity; the time rate of heat flow through a homogeneous material under steady-state conditions, through unit area, per unit temperature gradient in the direction perpendicular to an isothermal surface. Its unit is Btu per (hour) (square foot) (Fahrenheit degree per inch of thickness). Materials are considered homogeneous when the value of the thermal conductivity is not affected by a change in thickness or in area within the range normally used in construction. Some materials are not isotropic with respect to thermal conductivity. Care should be taken that the test method used is suitable for the particular material and gives a value of conductivity applicable to the intended use.
C - Thermal conductance; the time rate of heat flow expressed in Btu per (hour) (square foot) (Fahrenheit degree average temperature difference between two surfaces). The heating, air-conditioning and refrigerating engineer, however, dealing largely with compound walls with parallel surfaces, makes considerable use of the term unit conductance, or conductance per unit area. The average temperature is one which adequately approximates that obtained by integrating the temperature of the entire surface. The term is applied to specific materials as used, either homogeneous or heterogeneous, solid or gaseous, for the thickness of construction stated, not per inch of thickness. The conductance of an air space is dependent on the temperature difference, the height, the depth, the position, character and temperature of the boundary surfaces. Since the relationships are not linear, accurate values must be obtained by test and not by computation. The space must be fully described if the values are to be meaningful.
f = Film or surface conductance; the time rate of
R - Thermal resistance; the reciprocal of a heat transfer coefficient, as expressed by U, C or f. Its unit is Fahrenheit degrees per Btu/(hour) (square foot). For example, a wall with a U value of 0.25 would have a resistance value of R1/U 1/0.25 4.0.
Btu British thermal unit; approximately it is the heat required to raise 1 lb of water from 59 F to 60 F.
Capacity Insulation The ability of massive materials to store heat and delay transmission of heat as a result of its mass, density and specific heat.
CALCULATING OVERALL COEFFICIENTS
Conductance and resistance coefficients of various elements of walls are listed in Table 1. These coefficients are taken from the ASHRAE Handbook of Fundamentals. Conductances and resistances of homogeneous materials of any thickness may be obtained by the following formulas:
Cxk/x
and Rx = x/k
where:
x thickness of material in inches.
In computing the heat transmission coefficients of a composite wall, the paths of heat flow should first be determined. If these are in series, the resistances are additive; thus, the total thermal resistance of the wall is obtained by the formulas:
RRR...
and U = 1/R
However, if paths of heat flow are parallel, transmittances are additive and average transmittances may be obtained from the formula:
Uavga(Ua)+b(U)+...
where:
a, b, etc. area of heat flow path in square feet per square foot of wall,
Ua. Ub. etc. transmission coefficients of the respective paths.
Such an analysis is important for wall constructions with parallel paths of heat flow when one path has a