Each strategy has its merits and downsides. The first strategy relies on complex heating and cooling systems that may not function during power outages or after mechanical failures. Additionally, a fully insulated and vapor sealed building does not “breathe” naturally, raising concerns of air quality. The indoor air quality of sealed buildings is such an issue the California Building Code now requires additional mechanical ventilation to bring in fresh air in all new construction.
The second strategy, while relying less on complex heating and cooling systems, relies instead on fluctuations between the nighttime and daytime temperatures, known as diurnal swing. Without these temperature fluctuations, the thermal flywheel mechanism doesn’t function and little energy savings are achieved.
With the above as background to the discussion, how do we answer the question when asked to put a number on the R-value of a Watershed Block or a Watershed Block wall, or a rammed earth wall? A Watershed Block, like rammed earth, has a relatively low R-value. That’s the point, after all, of materials with a high thermal mass. A Watershed Block, like a rammed earth wall, has to move heat in order to store heat. Even with a low R-value, a Watershed Block wall, like any earthen wall, can still contribute to maintaining a comfortable indoor air temperature. And, a low R-value does not mean that a Watershed Block or rammed earth wall needs to be insulated.
Let’s examine the two questions separately - how can a Watershed Block wall, like a rammed earth wall, provide insulative properties while having a low R-value, and, what is the effect of adding insulation to a Watershed Block wall or a rammed earth wall?
The first question - how can a Watershed Block wall, or a rammed earth wall, provide insulative properties while having a low R-value - examines the idea of “effective R-value”, otherwise (and more accurately) known as “mass enhanced R-value.” The difference between R-value and “effective R-value” / “mass enhanced R-value” can be understood by examining the difference between the lab environments in which R-value is measured versus the real world environments in which building materials are used.
R-value is measured technically in a lab, specifically in a machine called a guarded hot box. One side of a material - whether fiberglass, a Watershed Block, or a sheet of drywall - is maintained at a constant temperature. The other side of the material is kept at a different constant temperature. The amount of energy required to maintain this temperature difference is measured, and an R-value is determined.
This specific R-value is a “steady-state” R-value and reflects the material’s ability (or lack thereof) to insulate against two constantly different temperatures. What about the real world in which temperature differences are dynamic, not steady?
In much of the United States, the outside air temperature changes greatly over a 24 hour period, a situation referred to as diurnal swing. In the desert environments and Mediterranean climates that dominate the West and Southwest, the temperature outdoors changes greatly during the day, rising above a comfortable indoor temperature in the afternoon then falling far below a comfortable indoor temperature at night. Over a course of 8-12 hours, the temperatures on two sides of an exterior wall can change dramatically. So the steady state R-value determined in a lab is no longer so valuable in determining how the indoor temperature can be best maintained when applied to a dynamically changing outdoor temperature.
From this more dynamic and changing environment, the concept of “effective R-value” / “mass enhanced R-value” has emerged and relates to the idea of high mass exterior walls functioning as a thermal flywheel. Warm daytime temperatures and direct sun slowly warm exterior walls but this heat does not reach interior spaces until after the sun has set. The latent daytime heat in the exterior walls warms interior spaces during the evening, at which point the process reverses and the walls slowly begin to expel the daytime heat through the thick exterior walls into the now cooler nighttime air. By morning, the thick exterior walls have cooled again, and the process continues, offsetting the daytime heat to the evening and the evening cool to the daytime.