Feature Article

Thermal Mass and R-value: Making Sense of a Confusing Issue

Background on the physics of heat transfer to help understand the relationship between thermal storage and heat flow, and when this information is relevant and how it should be used in building design. 

April 1, 1998

The advertisement promises R-30 from a lightweight masonry block wall system. Log home product literature claims that log walls insulate as well as fiberglass because of the thermal mass. Salesmen at a trade show argue that a new fiber-cement building system achieves R-28 even though the “tested” R-value comes in at only R-16.

What’s going on here? Do these claims of “effective R-values” that greatly exceed the widely published R-values for high-mass materials hold up? Just what effect does thermal mass have on the energy performance of an exterior wall system? The issue of thermal mass and its effect on the energy performance of buildings is one of the most confusing issues facing designers, builders, and buyers of buildings today. This article tries to sort out these mysteries, providing enough background on the physics of heat transfer to understand the relationship between thermal storage and heat flow, and then explaining when this information is relevant and how it should be used in building design. This article does not address the use of thermal mass inside a building, where it can store heat (or coolth) and even out temperature fluctuations.