Thermal mass relates to the ability of a material to store and release heat. All materials have thermal mass. However, because concrete is used in large amounts in buildings, these buildings have greater total thermal mass than most buildings made with competitive materials. The concrete and masonry industries, based on research they have done, have claimed that thermal mass reduces annual heating and cooling requirements.

One way to take advantage of thermal mass would be to reduce the level of insulation in the building with concrete walls until it uses the same amount of energy as the building with metal walls. This can result in significant first-cost savings. Applying this approach for a six-city, shopping mall study we found that no insulation would be required for 8-inch-thick concrete walls in Washington, D.C. and points south to match the performance of metal building walls insulated according to the ANSI/ASHRAE/IES standard. In Chicago and Minneapolis reduced amounts of insulation would meet the performance of the ASHRAE-specified R-value if proper allowance were made for thermal mass effects. The greatest saving is possible when insulation is placed on the outside of the concrete walls.

Another important way to use thermal mass of concrete is as a storage reservoir for solar energy collected through south-facing glass. Concrete passive solar collectors may be either floors or trombe walls which later re-radiate the heat when surrounding temperatures have dropped.