High performance buildings must be designed with more energy conserving features add to the sustainable environment of a community by reducing demand on and adding complexity to required utility delivery systems. Building operation and space conditioning equates to nearly 1/3 of all energy use in the United States. The significant contribution of reduced energy consumption and shifted peak demand is less environmental impact from non-renewable resources used to generate power. This also requires less expenditure of materials and resources for construction of utility infrastructure to meet increased utility demand to serve new buildings.
What is Energy Performance?
Energy is the first “E” in
LEED, and one of the tenets of sustainability. The
operational energy use and associated emissions to air during the life of the building account for 85-95 percent of the total energy and emissions (including that embodied in construction materials, transportation, installation and used for maintenance). Saving energy reduces the use of natural resources as fuel, the need for additional power plants to generate electricity, and the energy and emissions associated with obtaining and producing these fuel sources.
Buildings are designed to meet occupants’ needs for thermally comfortable, well-lit, well ventilated spaces. Energy in buildings provides for lighting, appliances and equipment, and service hot water as well as condition interior spaces with ventilation and temperature control. Designing for energy performance also includes the optimal sizing and efficiency of the heating, ventilating, and air-conditioning (HVAC) equipment. The predominant fuel source for heating is natural gas, although fuel oil and electricity including heat pumps are also common. The predominant method of cooling is with electricity.
The impact of thermal mass on building envelope performance varies with several interrelated factors. The most important of these are the climate at the building site,the building design and occupancy, and the position of the wall insulation relative to the mass.
How Does Concrete Help Save Energy?
Buildings with exterior concrete walls, also called mass walls, utilize less energy to heat and cool than similarly insulated buildings with wood or steel frame walls. Over the course of a day, building systems respond to changing conditions in outside air temperature, occupant and equipment activity, and incident solar energy. The impact of thermal mass on building envelope performance varies with several interrelated factors. The most important of these are the climate at the building site, the building design and occupancy, and the position of the wall insulation relative to the mass.
A building with thermal mass has the capacity to store warmth or cold and:
- Moderate indoor temperature fluctuations
- Slow the transfer of heat through the building envelope
- Store energy and shift peak energy requirements
The amount of energy savings due to thermal mass is dependent on climate. In some climates, thermal mass buildings have better thermal performance than low mass buildings, regardless of the level of insulation in the low mass building. Mass has the greatest benefit in climates with large daily temperature fluctuations above and below the balance point of the building (55-65 degrees Fahrenheit).
Comparison of heating and cooling energy an costs for identical houses with mass and frame walls in Boulder, Colorado (from PCA CD026)
Building design and occupancy significantly impact the energy savings due to thermal mass. In low-rise residential buildings (houses and apartments), for example, heating and cooling loads are primarily determined by the thermal performance of the building envelope. In commercial buildings, loads are influenced more by internal heat gains from occupants, lights, and equipment. For optimal benefit, the thermal mass should be exposed to the interior, conditioned air, and insulated from outdoor temperature variations. However, thermal mass is effective in many climates regardless of placement or building type.
Thermal resistance (R-values) and thermal transmittance (U-factors) do not take into account the effects of thermal mass, and by themselves, are inadequate in describing the heat transfer properties of construction assemblies with significant amounts of thermal mass. Only computer programs such as DOE2 and EnergyPlus that take into account hourly heat transfer over an annual basis are adequate in determining energy demand in buildings with mass walls and roofs.
Why Does Peak Load and System Size Matter?
Most utility companies charge higher rates during peak demand periods. By lowering peak loads, energy dollars can be saved. Peak cooling loads in office buildings are often in mid-afternoon. Properly designed thermal mass can shift a portion of the load from mid-afternoon until later when the building is unoccupied or when peak load electricity costs are less.
Small equipment that runs continuously uses less energy than large equipment that is run intermittently as it responds to peak loads. Smaller equipment is also less expensive to purchase, and uses less material and energy to manufacture and transport.
Operation of a cooling system is more significant in warm and humid climates than any other climate. Since the latent load (that required to remove moisture) is often greater than the sensible load (that required to bring down the temperature), the system needs to be designed to remove the latent load without cycling off before it has reached the desired temperature set point. Oversized air-conditioning equipment may cycle off before the latent load is removed, leaving the interior air cooler but humid. This is known as short cycling. Ideally, the system will run longer with a slightly warmer temperature rather than short bursts of very cool temperatures. This also moderates the temperature swings within the space, leading to greater comfort.
How Does Concrete Reduce System Size?
Analytical and experimental studies show that the use of materials with thermal mass in buildings reduces heating and cooling peak loads, resulting in reduced heating, ventilation and air-conditioning (HVAC) system capacity requirements. Studies show that the HVAC system in a house with mass walls can be downsized from that of a house constructed with frame walls, even when the mass walls have less insulation.
Some designers choose not to reduce heating system size due to thermal mass. The excess capacity can help heat the buildings after nighttime or weekend temperature setback. However, the same effect can be achieved by starting the reheating period earlier with smaller, less expensive equipment.
For cooling loads, the reduced peak load due to thermal mass should be taken into account to prevent short cycling and moisture problems, especially in humid climates (the eastern and central portions of the U.S. and Canada, and the Pacific Northwest).
References
Energy Use of Single-Family Houses With Various Exterior Walls, (2001), J. Gajda, Portland Cement Association, 50 pages.
A typical 2,450 square-foot single-family house with a current design was modeled for energy consumption in twenty-five locations across the United States and Canada. Locations were selected to represent a range of climates. Energy simulation software utilizing the DOE 2.1E calculation engine was used to perform the modeling. PDF.
HVAC Sizing for Concrete Homes, CD044, (2009), Portland Cement Association
This software provides an alternative means of estimating heating and cooling system capacities for single-family concrete homes. The software calculates the system capacities based on the house dimensions, construction materials, location (U.S. and Canada) and thermostat set point.
2014 Radiant Flooring Guide, Radiant Professionals Alliance
Available for download for $1. This publication is designed to help homeowners and building designers understand their choices. It includes information on how radiant floors work, how to include radiant floor in your design, hydronic (hot water) and/or electric, product directory, gallery of radiant systems, resource guide, selecting floor coverings for radiant floors: wood, decorative concrete, tile, stone, marble, carpet, laminate flooring, resilient flooring.
Energy Use in Residential Housing: A Comparison of Insulating Concrete Form and Wood Frame Walls, SN2415, (2000) J. Gajda, M. VanGeem, CTLGroup, 17 pages
Free for download. A typical 2,450-square-foot house with a contemporary design was modeled for energy consumption in five locations. Locations were selected to represent a range of climates across the United States. Energy simulation software utilizing the DOE 2.1E calculation engine was used to perform the modeling. In each location, three variations of the house were modeled. The first variation utilized conventional wood framed exterior walls constructed with typical materials. The second variation utilized insulating concrete form (ICF) walls. The third variation had non-mass exterior walls that met minimum energy code requirements. For all variations, all other assemblies such as the roof, floors, windows, and interior partitions were identical. In all locations, the house variations were insulated to meet the minimum levels required in the 1998 International Energy Conservation Code (IECC). Due to the inherent insulating properties of the ICFs, total energy use (including heating and cooling, cooking, laundry, and other occupant energy) for houses with ICF walls ranged from 8-19 percent below that of the houses with walls that met IECC requirements. Houses with wood frame walls constructed with standard materials also showed total energy saving over that of houses with walls that met IECC requirements. In all locations, houses with ICF walls had total energy requirements that ranged from 5-9 percent below those of houses with wood frame walls. Houses with ICF walls also showed additional savings resulting from a reduction in the required heating, ventilation, and cooling (HVAC) system capacity. Total system capacity for houses with ICF walls ranged from 16-30 percent less than that of houses with walls meeting IECC requirements and 14-21 percent less than that of houses with wood frame walls.