TABLE OF CONTENTS Heat or thermal energy is transferred through building components, such as walls and roofs, by a combination of conduction, convection, and radiation. A steady-state analysis is commonly used for design purposes to estimate an average thermal transmittance of building components. Since different modes of heat transfer are involved, the concept of an "overall" thermal transmittance allows one to determine the heat transfer from the temperature difference of the fluid (air) on either side of the component. The thermal transmittance coefficient is defined as the time rate of heat flow through a unit area under steady conditions from the fluid on the warm side of a body to the fluid on the cold side, per unit temperature difference between the two fluids. The mathematical expression is:
where
| q | = heat flux |
| U | = air-to-air thermal transmittance |
| R u | = total air-to-air thermal resistance through building component |
| A | = surface area perpendicular to direction of heat flow |
| t 1, t 2 | = mean fluid temperatures on either side of the building component. |
This concept also allows one to use an electrical-thermal analogy for "thermal circuits," that is, thermal resistances in series are additive, and thermal conductances ( C = 1/ R) are additive for resistances in parallel. This is the basis for one-dimensional methods as explained in Chapter 22 of 1993 ASHRAE Handbook-Fundamentals and ISO Standard 6946/1-1986(E) (ISO 1986a).
For many composite building sections (wall, floor, roof), assuming heat flow to be perpendicular to the...
