Designing Masonry Buildings to the 2012 Energy Code: Thermal Mass Basics

A material’s thermal mass denotes its ability to store heat within a cycle of time. K-values, generally calculated on a 24-hour cycle, are important because they give general references to a material’s capabilities for storing heat. All materials may be considered for use in a thermal mass calculation, but steel, aluminum, and other metal claddings tend to cycle too quickly, while wood tends to cycle too slowly to offer desirable design values.

Masonry—such as concrete masonry unit (CMU), stone, and brick—offers a good blend of characteristics for the thermal mass design based on several values. Storing heat well, the dense material can be designed with wall thicknesses that allow for normal window and door jamb details with reasonable per-area costs to construct.

In most cases, thermal mass should be measured on a cycle representative of both a typical heating and cooling cycle or a variable daily winter cold temperature cycle. While this is done for either season with the same principals, external factors contribute to the winter wall calculations in a more direct way. Building orientation, ceiling heights, lighting, solar heating, soffits, wall finishes, number of occupants, and usage round out a general list for design.

In colder climates, thermal mass is based on the function of interior heating cycling through the core of the wall. As the evening temperatures fall and the interior begins to feel cooler, warmth that was gained and stored during the daylight hours can then reverse the heat path and move back to the interior space of the building.

Summer cycles seem a bit clearer when explained, as the heat of the day penetrates toward the core of the wall. The term ‘decrement property’ takes into account the wall’s material density (e.g. concrete mix), final façade finishes, and exposure. The decrement factor dictates the speed at which the heat can be absorbed into the building. The design should stop the absorption of the heat before it alters the interior of the building’s cooler temperature and cycles the heat to the exterior of the structure as the afternoon temperatures begin to fall.

‘R-value’ has become a term familiar to even consumers, as it is listed on every insulation package in the home improvement stores. The general thought often reduces this metric’s significance to ‘the higher the R-value, the better the product when placed in the wall.’ However, as a unit of thermal resistance, R-value is the conduction rate of heat flow through a combination of materials comprising a wall. Mass-enhanced R-value walls are a combination of thermal mass walls and use of materials that offer high resistance to heat flow. They are extremely useful in climates where the external building temperatures rise well above and fall well below the interior space daily temperatures.

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