Incorporating Thermal Mass to Save Energy and Improve Thermal Comfort

Incorporating thermal mass into the design of a building is a completely passive way of reducing annual heating and cooling energy use and shifting the summer peak demand to later in the day. In architectural terms, thermal mass refers to the incorporation of solid or liquid materials into the building design to absorb heat or cold and then release it later to moderate building temperature swings. Typical thermal mass elements include concrete, masonry, or stone in a building's walls or floor, or water stored in tanks within the building. Thermal mass can be utilized in a wide variety of facility types, including commercial and residential buildings. And designs can incorporate active strategies such as night flush ventilation to improve the effectiveness of thermal mass elements.

Not only can thermal mass strategies save energy costs, they can also provide improved interior thermal comfort - without requiring additional heating and cooling equipment. This issue of e-News details the benefits of using thermal mass, highlights important design considerations, and provides tips to get the most out of your building's massive elements.

History of Thermal Mass in Vernacular Architecture
Thermal mass is not a new technology. In fact, the very earliest human shelters and buildings were constructed with earth-based materials, and such materials can be seen in the vernacular architecture of nearly every region of the world. Construction based on stone, brick, and concrete has been continuously influenced by material availability, technology, and social factors, which include considerations of sustainability. Likewise, the passive heating and cooling properties of thermal mass are not revolutionary; they have been used for centuries to maintain comfortable living environments.	Traditional adobe houses in Mexico and the Southwest U.S. have thermal mass walls typically constructed of thick sun-dried clay, sand, and straw bricks. Similarly, concrete and stone houses in the Mediterranean and in many other parts of the world continuously moderate indoor air temperatures with little or no input of extra energy.   Thermal mass is an age-old technology that is once again gaining popularity in modern construction, and like many rediscovered technologies, it is taking on new forms in order to integrate effectively into modern buildings and lifestyles.
Courtesy National Renewable Energy Laboratory

The Benefits of Thermal Mass

Well-designed thermal mass elements can provide a wide range of benefits, including:

• Lower Peak Loads. Storing heat in massive building elements contributes to lower peak loads, by allowing cool massive elements to soak up and then gradually release heat gains throughout the day. This reduces the diurnal fluctuation in temperature and results in more moderate heating and cooling spikes. Figure 1 provides a graphical representation of this effect on a summer day. The wintertime effect is similar.

Figure 1. Thermal Mass Moderates Interior Temperature Swings

• Energy Savings. In addition to lowering peak loads, incorporating thermal mass into the building structure can contribute to savings in total space conditioning energy consumed year round. Thermal mass effects vary with climate, and a 2007 study entitled "Modeling Energy Performance of Concrete Buildings for LEED-NC v. 2.2, Energy and Atmosphere Credit 1" demonstrated an energy cost savings of 6 to 11% in a code-compliant concrete office building, as compared to a similar code-compliant steel office building simulated in several different climate zones.
  
  
• Thermal comfort. Proper application of thermal mass can delay heat flow through the envelope of a building by as much as 10 to 12 hours, providing a buffer against the weather during cold winters and hot summers. Additionally, thermal mass works well in commercial applications because it delays the peak summer cooling load - which generally occurs around 3 pm for buildings with less thermal mass - to later in the day when offices begin to close.
  
  
• Energy Demand Savings. Shifting the peak cooling load to later in the day can pay big dividends for a facility's energy bills, as demand charges may be avoided. In cases where demand charges are a significant component of the overall utility cost, the time of day that a building uses energy can be a major consideration. (See sidebar, On-Peak vs. Off-Peak Utility Costs.)
  
  
• Low Implementation Cost. Thermal mass is a relatively inexpensive, passive system that can be easily integrated into the building structure. Many buildings already incorporate concrete, bricks, and other massive elements. Designing for proper orientation and exposure of the building's mass elements can provide for significant energy savings with little increased building cost.

Design Considerations

• Climate. Thermal mass is most appropriate in climates with a large diurnal temperature swing. As a rule of thumb:
  • Diurnal ranges of less than 10°F are usually insufficient to provide significant savings potential.
  • Ranges of 12 to 18°F can be useful, depending upon other factors, particularly humidity.
  • Ranges of 18°F or more are well suited to massive construction. Exceptions to these rules occur in more extreme climates.
  In arid climates where both winter heating and summer cooling are required, high mass construction combined with sound passive heating and cooling principles and adequate insulation is the most effective and economical means of maintaining thermal comfort. Diurnal ranges in such climates are generally quite significant and can be extreme.
  
  Figure 2 shows the summertime diurnal temperature range for selected California climate zones. Even in the more temperate Los Angeles and San Diego areas, diurnal temperature swings are sufficiently large that massing structures can make sense.
  
  Figure 2. Diurnal Temperature Range for Selected California Climate Zones
  
• Implementation and Building Integration. Thermal mass can work to stabilize interior temperatures and minimize the need for active space conditioning, but a design that correctly links thermal mass to a heat source and air distribution system will improve the system performance.
  
  Additionally, without a well-planned heating and cooling control strategy, the energy saving benefits of thermal mass construction may be negated. For example, employing a heating season setback can cause huge morning warm up demand spikes as the HVAC system must compensate for the overnight cooling of the massive elements.
  
  
• Type and Amount of Thermal Mass. Generally speaking, the thermal capacitance of a material is roughly related to its density. Water has more thermal mass than nearly any other material, but is not terribly practical as an architectural material. Brick, stone, earth, and concrete are certainly more common, and should be chosen based upon thermal capacitance, functional performance, appearance, and other factors. For example, high-density concrete provides more thermal mass than low-density concrete, and they can be used interchangeably in many applications. Regarding the amount of thermal mass to incorporate, beyond a 4-inch thick slab, diminishing returns begin to occur. For example, a 3-inch slab provides 95% of the performance of a 4 -in. slab.

Strategies for Effectively Using Thermal Mass

The following tips and strategies can help to ensure that the massive elements in your building are successfully reaching their full energy-saving potential:

• Always use thermal mass in conjunction with good climate-adaptive design (see Design for Your Climate).
  
  
• Thermal mass works best when the massive elements are exposed on the interior surfaces of the building. Avoid covering important mass elements with carpet or other interior finishing elements. If using insulation, place it on the exterior side of thermal mass.
  
  
• The underside of floor slabs should be exposed to the occupied spaces. Avoid using suspended ceilings, which may form insulating barriers. Attempt to ensure "thermal transparency" in the ceiling if such a structure is required to provide a corridor for wiring, ductwork, or other connections. For example, consider using perforated or open-grid ceiling tile. Even 15% open area can allow for significant air circulation.
  
  
• Thermal mass works particularly well when coupled with natural ventilation systems (see e-News #61). For example, a night-flush cooling strategy introduces cooler exterior air into the building during nighttime hours to remove heat from massive elements.
  
  
• If possible, cool night breezes should be directed to flow over the thermal mass, flushing out all of the heat stored within the material. Night-flush ventilation can be improved by locating mass near diffusers, fans, or operable windows to facilitate removing heat.
  
  
• During the day, this mass should be sheltered from excess solar gains with shading and insulation.
  
  
• Thermal mass can be incorporated into lightweight structures through steel-framed concrete floors or isolated masonry walls. The underside and edges of suspended thermal mass floors that are exposed to the exterior should always be insulated, however.
  
  
• Internal or enclosed water features, such as fountains or pools, can also provide thermal mass, provided evaporation and condensation issues are carefully considered and controlled.

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