R-value claims for ICF buildings can be confusing, mainly because the thermal effects of the wall’s concrete core vary depending on climate. In the case of Rastra, for example, tests at the Oak Ridge National Laboratory have found the “steady state” R-value of a 10-inch-thick wall is between 7.7 and 8.2. But Rastra says its walls behave as if they had higher R-values because of the mass of the concrete—what it calls the “thermal mass multiplier.” This, the company says, varies by climate zone—a 10-inch panel (which contains 6 1/2 inches of composite material) has a claimed R-23.8 in climate zone 2, a mild part of the country, which includes the Gulf Coast and southern Arizona, but only R-11 in Alaska.
In writing about “mass-enhanced R-value,” Environmental Building News says that it is “only significant when the outdoor temperatures cycle above and below the indoor temperatures within a 24-hour period. Thus, high-mass walls are most beneficial in moderate climates that have high diurnal (daily) temperature swings around the desired indoor set point.”
In very cold climates, where outdoor temperatures stay below the indoor temperature (the “set point”) for days, weeks, or even months, the mass of the concrete wall doesn’t provide a thermal benefit. Where the mass of the concrete does help is in regions where there’s a big swing in daily outdoor temperature, or during the spring and fall “shoulder seasons” in colder climates.