Thermal Properties

Building materials are rated for thermal performance based on measurements we know as R- and U -values. The R-value indicates the ability of a wall to insulate efficiently. Insulation is nothing more than the resistance of a material to the transference of heat. It makes sense that the higher the R-value, or resistance, the better insulator the material is. The R-value is calculated by dividing the thickness of the wall by the wall's thermal conductivity, a value established by the amount of heat (per sq. ft. per hour) flowing from the hotter to the cooler side of the wall.

The U-value, or value of conductance, is represented by the reciprocal of the R-value and reflects the rate at which heat is conducted through material. Total R- and U- values may be calculated for a given wall by adding the sum of the values of each of the individual components of the wall structure (all insulation, interior sheathing, framing, or masonry must be taken into consideration).

Both of these values reflect the rate at which heat passes through a wall only after it has achieved the steady-state condition, or the state when heat energy is passing uninterrupted from one side of the wall to the other at a constant rate. What is not taken into consideration and is of critical importance in the case of masonry-mass walls like adobe, is the heat capacity of the wall, which determines the length of time which passes before a steady state of heat flow is achieved. The higher the heat capacity of the wall, the longer period of time it will take for heat flow to reach a steady state. In reality, external and internal temperatures are changing constantly so that a true steady state condition is rarely achieved. What does occur, in the high-capacity wall such as adobe, is the constant comfort zone found in adobe buildings.

For example, in the morning, when the sun rises, heat from the warmer, exterior side of the wall begins to move through the adobe mass. Depending not only on the resistance (R-value) of adobe, but also on the heat capacity of the wall (a factor both of the specific heat capacity and the thickness of the wall), the heat takes a certain length of time to reach the cooler, interior side of the wall and be released into the surrounding air. In adobe walls of sufficient thickness and of sufficient R-values, the normal daily fluctuations of temperatures never really allow much heat to pass through the wall at a steady state. At night, when the warmer side of the wall drops in temperature, heat already absorbed into the masonry-mass wall continues to flow, not just in one direction, but to both sides of the wall until a temperature equilibrium has been reached. This cycle is repeated in what is known as the fly-wheel effect. It is responsible for the comfort well known to those who live in properly designed adobe homes.

(Taken from the New Mexico Bureau of Mines and Mineral Resources)

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Earth Block, Inc.

P.O. Box 3605
Pagosa Springs, CO 81147
Jim or Nora Hallock
Phone: 970-883-2456
Email: earthblock@juno.com