Thermal Mass and Wall Insulation

The insulation in your home’s ceiling, walls, and floor helps to slow heat movement, keeping your home warmer in the winter and cooler in the summer. Massive exterior and interior walls have an averaging effect on the indoor temperature as the outdoor temperature varies.

The R-value of insulation describes only a wall’s resistance to heat flow. If a building’s walls are massive enough, they can also store heat, which has an averaging effect on the outdoor temperature variations. Heavy masonry walls without insulation, for example, will maintain a temperature of around 70 degrees as the daytime temperature fluctuates between 50 degrees and 90 degrees without any indoor space conditioning. When you insulate the massive wall on the exterior, you both reduce heat flow and maintain the averaging effect of the mass.

Recent studies performed by the Oak Ridge National Laboratory have confirmed the effect that massive walls have on home energy consumption. These studies show a “mass effect” for walls that increases the effective R-value of a home’s walls by a factor of 1.2 to 2.1 times. This improves the effectiveness of a heavy concrete or brick wall that measures R-11 to an effective rating of between R-15 and R-22. It’s not a large difference, and it’s still a good idea to install as much insulation as you can afford in your home. This mass factor could figure in when architects or builders design the homes of the future or recommend insulation retrofits for massive homes.

High-mass masonry walls are effective only in warm climates where the temperature varies widely between day and night. The best way to employ thermal mass is to maximize the surface area of the thermal mass facing the interior of a home by making the interior walls massive in addition to the exterior walls. The right combination of insulation to slow heat flow, and thermal mass to moderate temperature variations, can create a home that is both energy efficient and comfortable.

Insulated concrete forms may be the most practical way of combining insulation and thermal mass in new homes.

For more information, see Exterior Insulation and Finish Systems (EIFS) and visit the Oak Ridge National Laboratory’s Building Technology Center. Saturn’s book Residential Energy: Cost Savings and Comfort for Existing Buildings summarizes ORNL research on insulation and thermal mass in Appendix 5.