New Mexico Bureau of Mines & Mineral Resources
Traditionally, materials are evaluated for thermal performance based on measurements known as R- and U-values. The R-value is an indicator of the ability of the wall to insulate effectively. Insulation is nothing more than the resistance of a material to the transfer of heat, and naturally the higher that resistance, or R-value, the better insulator a 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 ft2 per hour flowing from the hotter to the cooler side of the wall.
The U-value, sometimes referred to as the value of conductance, is represented by the reciprocal of the R-value (U=1/R) and reflects the rate at which heat is conducted through a material. Total R- and U-values may be calculated for a given wall by adding up the sum of the values of each of the individual components of the wall structure; for example, all insulation, interior sheathing, framing, or masonry must be taken into consideration.
R- and U-values do not, however, tell the full story in determining what constitutes a high-quality, thermally efficient wall. 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 what is of critical importance in masonry-mass walls such as adobe, is the heat capacity of the wall, which determines the length of time that passes before 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 real situations, external and internal temperature are changing constantly so that a true steady-state condition is rarely achieved. What does occur, in the case of a high-capacity wall such as adobe, is outlined below.
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 of both the specific heat capacity of adobe and the thickness), the heat takes a certain length of time to reach the cooler, interior side of the wall and to be released into the surrounding air. In adobe walls of sufficient thickness and sufficient R-values, the normal daily fluctuations of temperature never 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 in 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 flywheel effect and is responsible for the comfort well known to those who inhabit properly designed adobe homes.
Taking into account these principles, clearly the thermal properties of masonry materials in general, and adobe in particular, have often been unjustly maligned because only R- and U-values have been considered when evaluating these properties. For mass materials such as adobe, a more accurate representation of thermal performance than R- and U-values is given in what is known as the effective U-value. This value is determined as a factor both of the resistance of the wall to the transfer of heat and of its capacity to hold heat. Therefore, in actual home use, optimum comfort may be achieved by a mass wall with a moderate R-value and high heat capacity (adobe), as well as by a highly resistant wall with little or no heat capacity (traditional highly insulated frame wall).