Thermal Mass - Energy Savings Potential in Residential Buildings

INTRODUCTION:

Several massive building envelope technologies (masonry and concrete systems) are gaining acceptance by U. S. builders today. It is believed that building envelopes made of concrete, earth, insulating concrete forms (ICFs), and solid wood (log) may be helpful in lowering building heating and cooling loads. For centuries, the vast majority of European and Mid East residential buildings have been built using massive wall technologies. They have made life without air conditioners relatively comfortable even in countries with hot climates such as Spain, Italy, or Greece.

Numerous historic and current field studies have demonstrated that in some U.S. locations, heating and cooling energy demands in buildings containing massive walls of high R-value could be lower than those in similar buildings constructed using lightweight wall technologies. This better performance results from the thermal mass encapsulated in the building reducing temperature swings and absorbing energy surpluses both from solar gains and from heat produced by internal energy sources such as lighting, computers, and appliances. In addition, massive building envelope components delay and flatten thermal waves caused by exterior temperature swings.

Since all U.S. thermal building standards including ASHRAE 90.1 and 90.2 and the Model Energy Code are linked primarily to the steady-state clear wall R-value, calculating heating and cooling needs of a house built with high-mass walls is not straightforward. The steady-state R-value traditionally used to measure energy performance does not accurately reflect the dynamic thermal behavior of massive building envelope systems. This makes it difficult to convince builders, investors, code officials, etc...about the improved energy performance of massive building envelope systems. Such a situation opens the door for many companies to claim unrealistically high energy performance data for their wall technologies.

The main objective of this work is to provide a comparative study of the energy performance of massive wall technologies. Since the majority of U.S. residential buildings are built using light-weight wood-framed technologies, all energy performance comparisons in this paper are made against light-weight wood-framed buildings. An overview of several historic and current U.S. field experiments are discussed. These experiments were performed in a wide range of U.S. climates utilizing several building sizes and shapes. Theoretical energy performance analysis is presented for a series of four wall assemblies. The wall material configurations of these assemblies represent most of massive wall systems utilized in U.S. residential buildings. Theoretical and experimental results presented in this paper should enable approximate energy performance evaluations for the most popular massive wall configurations.