There is a consistent myth floating around the internet that common earthbag buildings are naturally super insulating. I don’t know how this idea got started but it has certainly spread widely. I see it written into basic descriptions of earthbag building on forums and websites frequently, often by folks who would appear to be experts on the topic, lending the myth a degree of false legitimacy.
Let me set the record straight: common earthbags filled with earth are not very good insulators. In fact the R-value of compacted earth is roughly R-1 per foot, so a standard earthbag wall might yield no better than R-2. That is roughly equivalent to a dual pane glass window.
Earthbags filled with earth are excellent providers of thermal mass, which serves an equally important function in a well designed house. Thermal mass has the capability to store heat (or coolness), so a large interior placement of this mass will help stabilize indoor temperatures. But it needs to be insulated from the outdoor environment or it will lose this heat or coolness to the atmosphere.
In some circumstances a solid earthen wall will seem like it is insulating very well, because the interior is comfortably warmer or cooler than outside. This can happen because of the time factor; it takes time for the mass to change its state. In some moderate climates this can help keep an interior space fairly comfortable, as happens with the “flywheel effect” of the mass giving off the heat gathered during the day at night, and then helping to cool the house during the day if the nighttime temperatures are cooler. But this only works well during certain seasons in moderate climates and cannot be counted on in most localities around the globe.
Fortunately earthbags offer the very flexible opportunity of being filled with other materials than earth. I built a very comfortable earthbag house by filling the bags with scoria, a lightweight volcanic stone. Other natural insulating fill materials are rice hulls, perlite or vermiculite. Or you can build a solid earth-filled earthbag wall that has an adjacent insulating layer. In some ways this would yield the best solution, since you still have all of that wonderful thermal mass on the inside.