Defining Mass Walls through Heat Capacity
There are significant differences in the prescriptive requirements of mass walls versus other above-grade walls. Using ASHRAE 90.1-2007 as an example, above-grade walls for a steel-framed nonresidential building in Climate Zone 5 will need a minimum of R-13 insulation AND a continuous insulation layer of at least R-7.5. However, the building's mass wall assemblies would only be required to exhibit a continuous layer of R-11.4 insulation.
What Qualifies as a Mass Wall?
ASHRAE's definition of a mass wall is as follows:
"Any concrete or masonry wall with a heat capacity exceeding 7 Btu/ft2●°F. If the mass elements are constructed with lightweight materials with a unit weight not greater than 120 lbs/ft3 then the HC must be greater than 5 Btu/ft2●°F in order to qualify as a mass wall."
A mass wall is a heavyweight wall, typically exhibiting a unit weight greater than 15 lb/ft2.
While many professionals (and even code reviewers!) presume that any wall system containing concrete and/or masonry qualifies as a 'mass wall' with regard to energy codes and standards, it might not necessarily be true. At least per ASHRAE, it depends on the assembly's heat capacity.
Defining Heat Capacity
Heat capacity (HC) is defined as the amount of heat (in Btu) necessary to raise the temperature of a given mass 1°F. Numerically, the HC per unit area of a building envelope assembly (Btu/ft2●°F) is the sum of the products of the mass per unit area of each individual component of the assembly multiplied by its individual specific heat.
Stated differently:
Heat capacity (Btu/ft2●°F) = (mass per unit area, lb/ft2) x (specific heat, Btu/lb●°F)
Heat Capacity per ASHRAE 90.1
In practice, it can be difficult to find helpful resources to determine the heat capacity of an assembly. Standard 90.1 does provide a few helpful tables in the appendix.
For instance, ASHRAE 90.1-2007 provides two tables—one for determining the heat capacity of poured concrete (Table A3.1B); another for concrete masonry units (Table A3.1C).
As an example, according to Table A3.1B, an assembly exhibiting a 4-inch thick concrete layer consisting of lightweight aggregate that has a density of 85 lb/ft3 would achieve a heat capacity of 5.7—thus, qualifying as a mass wall.
Fair warning: the appendix of Standard 90.1 does outline some important qualifiers regarding the determination of mass wall U-factors, so please consult the standard and exercise caution.
Heat Capacity of Common Building Materials
For reference, the table below exhibits the mass, specific heat, and heat capacity for several common building materials.
For more information regarding ANSI/ASHRAE/IES Standard 90.1-2013—Energy Standard for Buildings Except Low-Rise Residential Buildings:
www.ashrae.org/resources--publications/bookstore/standard-90-1.
ASHRAE also provides a Standard 90.1 User's Manual, which contains helpful clarifications:
www.ashrae.org/resources--publications/bookstore/standard-90-1-user-s-manual.
