Inflated R-values?

Dome, sweet dome
I recently came across the Web site of a manufacturer that sells materials and equipment for building a monolithic dome home. It’s an interesting construction technique. First, a fabric “airform” is inflated into a dome shape over a slab foundation. Next, polyurethane foam is sprayed onto the airform from the inside. Concrete is then “shot” under pressure from a gun (shotcrete) onto a steel rebar grid and troweled smooth as the finished inside wall.

I don’t particularly like the style of a dome home. To me, it looks like a home for Hobbits or something from a fairytale book. What really caught my attention as an engineer was the claim that the wall of the dome home has a measured “effective R-value” of R-80! The steady-state R-value of polyurethane foam is about R-6 per inch. But these dome walls have only about 5 inches of polyurethane, which yields an R-value of R-30 at best.

Heating up
The higher R-value is attributed to the thermal mass of the concrete. Even though the literal definition of this term is misleading (i.e., heat has no mass!), it conveys the important effect that envelope materials can store heat due to their mass and a property known as specific heat—the amount of heat that can be stored in a material per unit mass per unit rise in temperature.

The specific heat of concrete is actually less than most building materials; but because concrete is much more dense, it can store more heat per unit volume and so its “thermal mass” is greater than that of any wood or insulation product.

Day and night
High-thermal-mass walls have an advantage because heat storage can be used to “buffer” the effects of a harsh outdoor environment on the thermal comfort inside of a home. The thermal mass benefit is greatest in climates where there is a large difference in outdoor temperature versus indoor temperature.

In the summer, the heat absorbed by a massive wall during the day is stored and can be partially discharged to the outside during the night. In the winter, heat absorbed by the wall during a sunny day can supplement heating during the evening. Nearly all areas with significant cooling loads and sufficiently low night-time temperatures can benefit from thermal mass in exterior walls. The heating load benefit is restricted to climates where there is significant winter sun.

I’ve experienced the good and bad of thermal mass from owning a log cabin near Mount Shasta. In the winter, it takes forever to heat the place up due to the massive cold logs. However, once the logs are warm, very little heat is required to maintain comfort. Furthermore, on sunny winter days we get the bonus of some passive solar heating. In the summer, we benefit from the cool evenings that discharge some of the heat stored in the walls before it conducts into the living space.

Check it out
Unfortunately, many concrete-wall and log-home salespeople like to combine the thermal mass benefit with the steady-state R-value to yield a higher “effective R-value,” or “mass-enhanced R-value,” or “dynamic R-value.” Because the modified R-value depends upon location and season, the combined effect cannot be expressed by a single number. The number quoted often corresponds to the most favorable locations, such as the desert Southwest.

The more honest manufacturers will often quote modified R-values for certain cities during the cooling season, but even this is misleading because it depends on the house design as well. My suggestion is to ignore R-value claims that are modified by thermal mass unless they have been calculated for your home and location.