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Benefits  > Thermal Mass
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thermal masses concrete green designThermal mass is a property that enables building materials to absorb, store, and later release significant amounts of heat. Buildings constructed of concrete and masonry have a unique energy-saving advantage because of their inherent thermal mass. These materials absorb energy slowly and hold it for much longer periods of time than do less massive materials. This delays and reduces heat transfer through a thermal mass building component, leading to three important results.

  1. There are fewer spikes in the heating and cooling requirements, since mass slows the response time and moderates indoor temperature fluctuations.
  2. A massive building uses less energy than a similar low mass building due to the reduced heat transfer through the massive elements.
  3. Thermal mass can shift energy demand to off-peak time periods when utility rates are lower. Since power plants are designed to provide power at peak loads, shifting the peak load can reduce the number of power plants required.

 

The thermal mass of concrete has the following benefits and characteristics:

  • Delays peak loads
  • Reduces peak loads
  • Reduces total loads in many climates and locations
  • Works best in commercial building applications
  • Works well in residential applications
  • Works best when mass is exposed on the inside surface
  • Works well regardless of the placement of mass
Mass works well in commercial applications by delaying the peak summer load, which generally occurs around 3:00 pm to later when offices begin to close. As a case in point, the blackout in the Northeast in August 2003 occurred at 3:05 pm. A shift in peak load would have helped alleviate the demand and possibly alleviated this peak power problem.

Damping and lag effect of thermal mass

Damping and lag effect of thermal mass

The ASHRAE Standard 90.1Energy Standard for Buildings Except Low-Rise Residential Buildings, the International Energy Conservation Code, and most other energy codes recognize the benefits of thermal mass and require less insulation for mass walls.

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. The most energy is saved when significant reversals in heat flow occur within a wall during the day. So, 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). For these conditions, the mass can be cooled by natural ventilation during the night, and then be allowed to absorb heat or "float" during the warmer day. When outdoor temperatures are at their peak, the inside of the building remains cool, because the heat has not yet penetrated the mass. Although few climates are this ideal, thermal mass in building envelopes will still improve the performance in most climates. Often, the benefits are greater during spring and fall, when conditions most closely approximate the "ideal" climate described above. In heating-dominated climates, thermal mass can be used to effectively collect and store solar gains or to store heat provided by the mechanical system to allow it to operate at off-peak hours.

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 DOE-2 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. The heat flow through the wall is dependent on the materials’ unit weight (density), thermal conductivity, and specific heat.

Where Can I Find Thermal Properties of Concrete?

Specific heat is defined as the amount of heat energy (in Btu) required to raise the temperature of one pound of a material by one degree Fahrenheit. Specific heat describes a material's ability to store heat energy. The specific heat of concrete and masonry can generally be assumed to be 0.2Btu/lb·°F. (ASHRAE Handbook of Fundamentals, 2005)

Heat Capacity (HC) is the amount of heat energy required to raise the temperature of a mass one degree Fahrenheit. Heat capacity is per square foot of wall area (Btu/ft2·°F) and includes all layers in a wall. For a single layer wall, HC is calculated by multiplying the density of the material times its thickness (in ft) times the specific heat of the material. HC for a multilayered wall is the sum of the heat capacities for each layer.

Values of heat capacity, thermal resistance, and thermal transmittance for concrete and masonry are presented in Appendix A of ASHRAE Standard 90.1-2004. Thermal conductivities are presented in the ASHRAE Handbook of Fundamentals.

Thermal Mass modeling studies are highlighted under "Energy Models" Under the Tools menu. Full reports are listed here as resources as well.
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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 Bookstore2005 ASHRAE Handbook - Fundamentals (2005)
ASHRAE
The 2005 volume of the ASHRAE Handbook covers basic principles and provides essential data for HVAC&R design. In all, the Fundamentals volume includes more than 1,000 pages and 40 chapters on a variety of HVAC&R topics, covering general engineering information, basic materials, load and energy calculations and duct and pipe design. Available for $155
Located at BookstoreCast In Place Walls (2000)
Portland Cement Association. Item Code: LT117
Available for $28.50. A training aid for apprentices, journeymen, and foreman in the area of cast in place walls. Not a design manual, but a guide to good practice.
Located at BookstoreConcrete in Practice
National Ready Mixed Concrete Association
Available for $25. Free to download nonprinting PDF A series of 38 one page information sheets on important technical topics, written in a non-technical format.
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 BookstoreStandard 90.1-2001 - Energy Standard for Buildings Except Low-Rise Residential Buildings (2001)
ASHRAE. ISBN/ISSN: 1041-2336
Available for $88 member, $110 non-member. Incorporates 34 new addenda covering a wide range of topics, as well as editorial changes and updates to the body of the standard. The new addenda contain information on minimum energy efficiency standards, building envelope requirements, zone isolation, floor, ceiling and roof insulation, and power allowance calculation.
Download DocumentAn Engineers Guide to: Economical Concrete Floor Systems (2005)
This 6-page bulletin presents information on cast-in-place reinforced concrete floor systems. The publication includes guidelines for selecting different floor systems for virtually any span and loading condition. The emphasis is on selecting cost-effective slab system for different situations. Also included are design aids for preliminary thickness estimation. The floor systems covered are; flat plate, flat slab, one-way joist, wide-module joist, two-way joist, and banded-beam. In addition information on drop panel, form work details, standard form dimensions for one-way and two-way joist construction is also included.
Download DocumentComfort and Quiet with Concrete Homes (2005)
Portland Cement Association. Item Code IS305.
Available for free. This document highlights the benefits derived from combining the mass of concrete with the insulating value of insulating forms. Together they provide a home that lessens the intrusion of outside noise, while improving the thermal performance of the home.
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.
Download DocumentResidential Technology Brief: Building a Better House with Concrete (2005)
Portland Cement Association. Item Code IS301.
Available for free. This document provides an overview that describes the various types and features of ICFs as well as the many benefits of building with ICFs. *Due to the setup of the PCI website, you must perform a search for this title at their bookstore.
Download DocumentResidential Technology Brief: Fire Resistance of Concrete Homes (2005)
Portland Cement Association. # IS302.
Available for free. This document provides a comparison of the fire ratings of ICF exterior walls versus conventional residential walls.
Download DocumentResidential Technology Brief: Plastic Foams for Concrete Homes (2005)
Portland Cement Association. # IS303.
Available for free. In this document the two types of foams used in ICFs are identified and the various physical properties of each are compared.
Download DocumentResidential Technology Brief: Side By Side Comparison: A Quality Concrete Home for Only 3% More (2005)
Portland Cement Association. Item Code IS308.
Available for free. This document summarizes the results of research involving the construction of three identical homes, two ICF, one wood frame.
Download DocumentResidential Technology Brief: The Quality of Concrete Costs Little More (2005)
Portland Cement Association. # IS304.
Available for free. This document compares the cost of building with insulting concrete forms with the cost of conventional wood frame residential construction. It discusses how the cost of building with concrete declines as a crew becomes more familiar with the materials and methods. It briefly touches on the big advantages of paying slightly more for a concrete home.
Download DocumentThe Art of Concrete (2001)
Portland Cement Association. Item Code: PL721
Available for free. White and colored concrete made with white cement have numerous applications, from cast-in-place to precast to tilt-up. This attractive brochure highlights the benefits of this versatile material, which can be used for decorative and structural purposes.
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 SiteInsulating Concrete Form Association
An industry resource website.
Located at External Web SiteNational Ready Mixed Concrete Association
Industry resource for ready mixed concrete.
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.