Why do we need interior thermal mass if we’ve already insulated a building?
This commonly asked question is best answered by explaining that insulation works to prevent thermal loss through the enclosure but insulation does not work to recover excess heat that results from people, lighting, and equipment that occupy the interior of the building. Thermal mass, on the other hands, works in conjunction with insulation and reduces thermal transfer through conduction and heat absorption characteristics that most benefits the interior.
The use of thermal mass also provides a path to code compliance that includes a reduction in insulation levels. Because the International Energy Conservation Code (IECC) as well as ASHRAE/IES 90.1 Energy Standard for Building Except Low-Rise Residential Buildings require less insulation for mass walls than non-mass walls such as metal or wood frame, the use of thermal mass provides a path to code compliance that includes lower material costs. We therefore use interior thermal mass to address thermal performance, code compliance and costs.
Designing a high performing thermal envelope is only the first step toward the creation of a high performing building. The second step, addressing the interior, opens the door to numerous solutions including maximizing energy performance through the selection of thermal mass materials – materials that are capable of absorbing and retaining heat. But isn’t interior thermal comfort the job of HVAC – heating, ventilation, and air conditioning?
HVAC obviously plays a critical role in our buildings yet by absorbing excess heat, thermal mass moves the heat buildup away from the occupant and into the surface material creating 1) a change in air temperature surrounding the occupant and 2) a lag in the time that peak performance is required by HVAC systems. The possibility that we have been most focused on thermal loss through the enclosure may explain why we have become overly dependent on the HVAC system as the sole solution to interior thermal comfort.
A more sustainable and resilient design solution is found by integrating passive design strategies such as the use of interior thermal mass with active design strategies such as the HVAC system. The passive design strategy offered by thermal mass provides a valuable complement to the goals of the HVAC system resulting in improved comfort levels together with lower operating cost.
Passive Design and Interior Thermal Mass
Passive design strategies such as the incorporation of thermal mass allow buildings to use alternate energy sources instead of always purchasing energy for building operation.
Passive measures are paid for once, yet perform repeatedly over the life of the building. They also support resilient design strategies and allow a building to address occupant needs when in full service as well as in the event of a utility disturbance or interruption. Lacking passive measures buildings become 100% dependent on the aid and cost of operating HVAC systems for both heating and cooling conditions. We therefore want to select materials to work to the benefit of building performance on the interior just as we expect similar performance in the thermal envelope.
To best use thermal mass on the interior begin by using block, tile, terrazzo, glass block, and other masonry materials inboard of the insulation. These masonry materials are available in a range of thickness that can easily provide the right amount of thermal mass in the design of vertical and horizontal surfaces. Consider using higher levels of thermal mass in functional areas of the building such as assembly areas, gymnasium, and circulation areas that are not occupied 100% of the time yet are on the HVAC schedule throughout the course of the day.
On the interior, the benefit of thermal mass is most appreciated as a complementary strategy to cooling requirements therefore consideration can also be given to areas of the building that receive a greater level of solar gain. Thin mass systems such as tile are just as valuable as thick mass found in concrete block walls. Interior locations such as single wythe block walls within a building are just as effective for heat absorption as external walls. Remember we want to be able to visually see the thermal mass. Unlike insulation that is kept concealed behind finishes, thermal mass should be used as finishes and needs to be exposed to the occupied space to be most effective.
Building performance is a multi-faceted requirement that calls for a design solution capable of efficiently meeting the expectations and intended purposes of a building occupants, owners, designers and builders. As we continue to explore ways to deliver buildings that address benchmarks for energy, operating, and maintenance costs it is incumbent on the design team to introduce options that result in synergistic solutions. Why do we need interior thermal mass if we’ve already insulated a building?
Because without interior thermal mass, the job is only half done.
Author: Maria Viteri, AIA, LEED AP BD+C
