A Heated Debate: Thermal Mass vs. R-Value

The debate on whether the thermal mass of concrete improves the r-value of ICFs can evoke a harsh response from the most mild-mannered building scientist. We try to break down this complicated issue.

Materials with high mass, such as concrete, can deliver better energy performance than materials with lower masses. This seems to hold true in cooling dominated climates with large outdoor temperature swings. Heat always moves from warmer to colder, so if the outside temperature is 90 degrees during the day and the interior of the dwelling is cooled to 70 degrees, in theory an ICF wall will absorb the heat trying to transfer to the colder exterior. When the external temperature drops below the internal temperature, the heat absorbed in the ICF wall will transfer back towards the cooler outside air. This is referred to as the mass effect. Comparably, an insulated stick framed house, with a low mass, is only as good as its resistance to the outside heat flow (r-value). There is no mass effect benefit, simply the insulation preventing heat transfer. This principle is far less evident in heating dominated climates, such as the Pacific Northwest. In winter, the interior temperature is always warmer than the outside temperature, so heat always travels in only one direction: out.

Additionally, materials with a high mass, even if they have poor r-values, will still slow the transfer of heat from the outdoors to the indoors (called thermal lag). Massive walls have slower thermal lag times. When consumers see ICF manufacturers stating that their products test up to values of R-58, these companies are combining the actual R-value with the mass effect and thermal lag times. This term is called the mass enhanced R-value. Some industry professionals feel that mass effect and resistance to heat loss are separate processes, so they should not be combined into a single unit figure.

While the industry has seen mass effect play a role in green building, specifically in passive solar homes and radiant heated slabs, builders and homeowners must be careful of mass enhanced r-values. There is little evidence supporting the idea that these concepts actually enhance R-value. Many different variables may influence mass-enhanced r-value, including climate, solar orientation, and HVAC systems. For instance, the warmer climate example used above is an ideal situation for the concept of mass effect but runs into problems when you try to apply it to a home in a colder climate. Thus far, there is no one conclusive study proving that high-mass materials consistently demonstrate higher R-values.