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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 is 85 to 95% of the total energy and emissions (including that embodied in construction materials 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 using 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. It is also used to condition interior spaces with ventilation and temperature control. Energy used for temperature control provides heating, cooling, and circulation. Energy performance also includes the 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. 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
     
     
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 to 65°F).
Comparison of Heating and Cooling Energy and 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. Because exposed thermal mass can absorb intermittent internal heat gains, thermal mass is generally more effective in commercial buildings than in low-rise residential. To best moderate indoor temperatures, 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 on an annual basis are adequate in determining energy loss in buildings with mass walls and roofs.
 
See the Energy Model section (click here) for more information and examples of simulations of high thermal mass energy performance.

Why Does Peak Load and System Size Matter?
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 because it has reached the desired temperature set point. Oversized air-conditioners may cycle off before the latent load is removed. Setting the chilled water supply temperature too high will have the same effect of not being able to remove the latent load. Also, many people erroneously think that setting the thermostat lower will remove moisture problems. Low thermostat settings on hot humid days has the opposite effect; it makes surfaces colder and more prone to condensation.
 
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 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.

For cooling loads, the reduced peak load due to thermal mass should be taken into account to prevent moisture problems, especially in humid climates (the eastern and central portions of the U.S. and Canada, and the Pacific Northwest).

 



BOOKMARK
Resources
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 Energy Use of Single-Family Houses With Various Exterior Walls (2001)
Gajda, John, R&D Serial No. 2518, Portland Cement Association, Item Code CD026, 50 pages.
Available for free. 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.
Located at BookstoreHVAC Sizing for Concrete Homes (2009)
Portland Cement Association. Item Code: CD044
Available for $60. 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.
Located at BookstorePortland Cement Concrete Overlays - State of the Technology Synthesis (2002)
American Concrete Pavement Association, Product Code SP045P, 188 pages
Available for $5. Link goes to bookstore page, search by product code. This report, produced by the Federal Highway Administration (FHWA), presents the latest information on the design, construction, and performance of portland cement concrete (PCC) overlays. Bonded, unbonded, whitetopping, and ultra-thin whitetopping overlays are covered in this synthesis of the current state of the technology. This comprehensive book is a must-have for those interested in concrete pavement overlay design and construction.
Located at BookstoreRadiant Flooring Guide
Radient Panel Association.
Available for download for free. 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.
Located at BookstoreThermal Mass Comparison of Wall Systems (2001)
Portland Cement Association. Item Code: CD026
Available for $35. This 49-page report provides the thermal performance of eleven different structural wall systems: concrete masonry, insulated cast-in-place, insulated concrete forms, and AAC as well as wood and steel frame. The results illustrate the benefits of thermal mass, depending on climatic conditions for most of North America.
Located at BookstoreTilt-Up Construction and Engineering Manual, Sixth Edition (2004)
Tilt-Up Concrete Association
Available for $145. A comprehensive manual dealing with all aspects of tilt-up construction. Includes information about practices and compliance with ACI 318-03 and IBC 2003, comprehensive insulation systems discussion, revised and expanded section on details and connections, expanded information on unique project applications, and updated easy-to-use supplier and product section.
Download DocumentIntegrated Energy Systems: Productivity and Building Science (2005)
New Buildings Institute, Inc. Item Number P500-03-082
Available for free. The Buildings Program Area within the PIER Program produced this document as part of a multiproject programmatic contract. The Buildings Program includes new and existing buildings in both the residential and the non-residential sectors. The program seeks to decrease building energy use through research that will develop or improve energy efficient technologies, strategies, tools, and building performance evaluation methods.
Download DocumentModeling Energy Performance of Concrete Buildings for LEED-NC v2.1 EA Credit 1 (2005)
Marceau, Medgar L. and Martha G. VanGeem, Portland Cement Association. Item Code: SN2880, 54 pages
This project provides in-depth information on energy savings in mid-rise buildings due to additional thermal mass and for exceeding building envelope thermal performance requirements.
Located at External Web SiteA Sustainable Approach to Outdoor Lighting Utilizing Concrete Pavement
By Lawrence C. Novak, SE, SECB, LEED® AP David N. Bilow, PE, SE
Located at External Web SiteConcrete's Contrubition to Sustainable Development
Concrete is the most widely used building material on earth. It has a 2, 000 year track record ofhelping build the Roman Empire to building today's modern societies. As a result ofits versatility, beauty, strength,·and durability, concrete is used in most types ofconstruction, including homes, buildings, roads, bridges, airports, subways, and water resource structures. And with today's heightened awareness and demandfor sustainable construction, concrete performs well when compared to other building materials. Concrete is a sustainable building material due to its many eco{riendly features. The production ofconcrete is resource efficient and the ingredients require little processing. Most materials for concrete are acquired and manufactured locally which minimizes transportation energy. Concrete building systems combine insulation with high thermal mass and low air infiltration to make homes and buildings more energy efficient. Concrete has a long service life for buildings and transportation infrastructure, thereby increasing the period between reconstruction, repair, and maintenance and the associated environmental impact. Concrete, when used as pavement or exterior cladding, helps minimize the urban heat island effect, thus reducing the energy required to heat and cool our homes and buildings. Concrete incorporates recycled industrial byproducts such as fly ash, slag, and silica fume that helps reduce embodied energy, carbon footprint, and waste.
Located at External Web SiteEnergy Use in Residential Housing: A Comparison of Insulating Concrete Form and Wood Frame Walls (2000)
Gajda, J. and VanGeem, M. CTLGroup, SN2415, 17 pages
Free for download. A typical 228-square-meter (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% to 19% 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% to 9% 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% to 30% less than that of houses with walls meeting IECC requirements and 14% to 21% less than that of houses with wood frame walls.
Located at External Web SiteEnvironmental and Cost Benefits of High Albedo Concrete
By Erin Ashley, PhD, LEED AP, Director of Codes and Sustainability, NRMCA
Located at External Web SiteICF Points to LEED (2008)
Insulating Concrete Form Systems contribute to LEED credits
This two page .pdf summarizes the credits available to designers and building owners when using high performing insulating concrete forms in wall construction. Documents available for download to ICFA members.
Located at External Web SiteInsulating Concrete Form Association
An industry resource website.
Located at External Web SitePolished Concrete Can Be Green (2007)
L&M Concretenews, January, 2007: Volume 7, Number 1
A durable, long lasting, attractive polished concrete floor is a value-loaded option within the reach of almost any facility today.
Located at External Web SiteRadiant Panel Association (2006)
An industry resource website.
Located at External Web SiteWhole Building Design Guide (2006)
National Institute of Building Sciences
Up-to-date information on integrated 'whole building' design techniques and technologies