Principles of Heat Transmission
Heat flows through a barrier of building materials (e.g., walls, ceil- ings, and floors) by means of conduction and radiation. When it has passed to the other side of this barrier, it continues its movement away from the source by means of conduction, radiation, and convection. The direction of heat flow depends on the relationship of outdoor and indoor temperatures because warmed air will always move toward a colder space. Thus, in the summer months,
Insulating and Ventilating Structures
the warmer outdoor air will move toward the cooler indoor spaces. In the winter months, on the other hand, the reverse is true: The warmer indoor air moves to the cooler outdoors. The purpose of insulation is to reduce the rate of heat flow to an acceptable level. This, in turn, will reduce the amount of energy required to heat or cool the structure.
Heat Transfer Values
The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) has conducted considerable research into the thermal properties of building materials, and these data are used in the calculation of heat loss and heat gain. Tables of computed values for many construction materials and combi- nations of construction materials are made available through ASHRAE publications. One table gives the design values (i.e., conductivity values, conductance values, and resistance values) of vari- ous building and insulating materials. These values are used to calculate the coefficients of heat transmission (U-values) for different types of construction (e.g., masonry walls and frame partitions). Examples of these two tables have been taken from the 1960 ASHRAE Guide (see Tables 3-1 and 3-2).
The four heat transfer values contained in these tables are:
(1) the k-value (thermal conductivity); (2) the C-value (thermal con- ductance); (3) the R-value (thermal resistance); and (4) the U-value (overall coefficient of heat transmission). Each of these heat trans- fer values will be examined before their use in heating and cooling calculations is explained.