Understanding thermal mass

By Gary Brown and Steven H. Miller, CDT
Thermal mass is a substance’s ability to hold heat energy. It is related to density. A substance with high thermal mass stores a greater quantity of heat energy than one with low thermal mass, even if both are heated to the same temperature.

High thermal mass takes longer to heat up because it is storing more energy. It also takes longer to cool, as there is more energy to dissipate. This is why cast-iron pans (high thermal mass) are better for some types of cooking than thin stainless steel or aluminum (low thermal mass). Cast iron heats more consistently—a cold burger does not cool off the metal so quickly  cooking stops.

Stone, brick, and concrete are classic high-thermal-mass building materials. Heat slowly builds in the wall before it can move through and begin dissipating to the cold side. The effect of thermal mass is recognized in the concept of “mass-enhanced R-value,” developed by the U.S. Oak Ridge National Laboratories (ORNL), and included in the International Code Council’s (ICC) International Energy Conservation Code (IECC) for the United States. While the National Building Code of Canada (NBC) does not explicitly reference thermal mass, it is acknowledged in the approval of ICF assemblies of R-22 in applications where other building products (e.g. wood and light-steel framing) would require R-27.*

Thermal mass is most effective in conjunction with good insulation on the exterior side of the thermal mass layer of the wall assembly, as in an ICF wall. (This fact is often overlooked in designing brick structures, where the thermal mass—brick—is the exterior finish. In this respect, performance can be severely compromised by the way brick is almost universally used.) However, numerous other factors can contribute to or detract from performance.

One manufacturer estimates an ICF wall assembly, including interior drywall and exterior finish, with an insulation R-value of R-30 can deliver the same performance as a wood-framed structure with R-50 insulation, due in part to the thermal mass effect. Inherent airtightness of the cast-in-place concrete in the ICF assembly also contributes to this increased performance.  

* The Ontario Building Code (OBC) supplementary standard SB-12 provides several compliance packages. The packages with ICFs have a standard R-value requirement of R-22 where other packages require from R-24 to R-29. NBC is updating its Section 9.32, which will include similar allowance for ICF based on the system’s performance versus just R-value.

For more on Enhancing ICF performance, click here or here.

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