Besides being a durable structural element, concrete also provides insulating benefits and acts as a continuous barrier to reduce infiltration and temperature differential between the exterior and interior. The concept of "thermal mass" applies to concrete, stone or masonry – building materials that can absorb heat, store it for a period and then gradually release it. Concrete's thermal mass can help achieve points under Energy and Atmosphere (EA) Credit 1, Optimize Energy Performance, in the U.S. Green Building Council's (USGBC's) Leadership in Energy and Environmental Design (LEED-NC 2.2) program.
Up to 19 points can be obtained under EA Credit 1 using the "Performance" path, which means modeling with whole-building energy simulation software. This simulation must demonstrate an improvement in the proposed building performance rating compared to the baseline building defined in ASHRAE 90.1-2007 Appendix G, "Performance Rating Method."
A CTLGroup research team examined the actual energy savings – and LEED points – obtainable in mid-rise commercial buildings due to thermal mass of materials like concrete. To determine the impact of thermal mass on energy efficiency, 5-story prototype buildings (with the same plan dimensions and window-to-wall ratios) were modeled using the software program VisualDOE 4.0.
Since effects of thermal mass vary with climate, the buildings were modeled in 6 cities, compensating for the range of conditions found across the U.S.:
- Miami, FL – very hot and humid
- Phoenix, AZ – hot and dry with large daily temperature swings
- Memphis, TN – warm
- Salem, OR – mixed
- Denver, CO – cold with large daily temperature swings
- Chicago, IL – cold
The team found that in the 4 cities representing warm, mixed and cold climates, reinforced concrete frame buildings with building envelopes exceeding the standard will most likely qualify for points under LEED's EA Credit 1. In the cold climate category, buildings will likely qualify for three points – at least 17.5% energy cost savings (actual in Denver is 21%; in Chicago, 18%). In the mixed climate category, buildings should qualify for 4 points – at least 21% energy cost savings (actual is 23%). In warm climates, buildings will likely qualify for 2 points – at least 14% energy cost savings (actual is 16%).