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These energy study results are from a 2005 study that demonstrates how to model thermal mass in mid-rise concrete commercial buildings that are pursuing LEED-NC Credit EA 1 The energy modeling results are based on the criteria in American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) Standard 90.1-2004, the national energy standard invoked by the LEED Rating System.

The buildings in this study are five-story commercial buildings with plan dimensions 105 by 105 ft. and are based on the prototype building used by ASHRAE committees and other building industry groups to model the effects of materials and energy use. The baseline building comprises an exterior insulation finishing system (EIFS) with steel stud walls, structural steel frame, and metal deck floors with concrete topping slab. In addition to the baseline buildings, there are nine proposed buildings, all variations of the structure and building envelope of the baseline building.  Further detail on the building is available in the full report (link below).

In most scenarios, the effect of thermal mass in concrete frame buildings was shown to lower energy use, and the overall effect of thermal mass in concrete framed buildings is to lower energy cost relative to the baseline steel framed EIFS buildings. Read the entire report (400kB pdf will open in a new window).

Building Types

The table summarizes the designations and construction of the baseline and nine proposed buildings. Of the designation, the first letter refers to the exterior wall system: “E” for EIFS, “C” for curtain wall, or “M” (“mass”) for precast concrete. The second letter refers to the structural framing system and interior walls and floors: “L” for light and “M” for mass. The light materials are structural steel framing and metal deck floors with concrete topping slab. The mass materials are reinforced concrete framing and 12-in. concrete floors. An “X” indicates that the building envelope exceeds code requirements and an “I” indicates that the internal loads are clustered near the central core of the building.

Buildings EM, CM, and MM are like EL, CL, and ML, respectively, except they have more concrete in interior floors and walls. Buildings MLX and MMX are like ML and MM, respectively, except their building envelopes exceed code. Buildings MMI and MMXI are like MM and MMX, respectively, except that internal loads are clustered near the central core of the building, where most of the interior concrete is located.

Summary of Results

In all cities except Miami, reinforced concrete frame buildings with precast concrete walls and building envelopes that exceed code will most likely qualify for points under EA Credit 1. In the cold climate category (Denver and Chicago), these buildings will likely qualify for 2 points, that is, at least 20% energy cost savings. In the cool climate category (Salem), these buildings will likely qualify for 3 points, that is, at least 25% energy cost savings. In the mild climate category (Memphis), these buildings will likely qualify for 1 point, that is, at least 15% cost savings. Note that these energy savings and qualifications towards LEED points were calculated according to LEED-NC v2.1. Updates have not yet been made to LEED-NCv2.2, which requires a different method of calculating energy savings, and a different threshold for obtaining points.

In Memphis, Salem, Denver, and Chicago, significant energy cost savings are indicated for the three concrete frame buildings meeting code compared to the three steel frame buildings meeting code: 6 to 11% when receptacle loads are not included. This energy cost savings is due to the concrete shear walls and increased thickness of the concrete floors in the concrete frame building.

Other findings include:

  • Thermal mass in or near the building envelope helps offset internal loads more than thermal mass in the core.
  • Energy simulation with VisualDOE shows significant energy cost savings for precast concrete walls exceeding code. When receptacle loads are excluded, energy cost savings are in the range of 10 to 27% for all cities except Miami, where the energy cost savings are about 5%.
  • The amount of insulation chosen to make the walls exceed code is not unusual. For example, in Denver and Chicago, the added insulation in the concrete wall exceeding code is about the same as the amount of insulation in the EIFS and curtain walls meeting code.

The Relationship Between Annual Energy Use and Cost, by City