The thermal resistance of raw earth is not good, similar to that of concrete. Thermal conductivity is around 10 W/m.K. To be compared to 0.1 W/m.K for a typical building insulating material.

But raw earth is porous and will store moisture, hence behaves like a two-phase material. Water in the earth may vaporize or condensate in the pores, which helps in regulating indoor moisture levels, and explains the high thermal storage capacity of the material. Since earth is cheap, it's also possible to build thick walls and get a high thermal storage capacity in the building envelope. Sun heat will be re-radiated a few hours later at night during winter, and some the cool of the evening nights will be available indoor during summer days. Because of this high thermal capacity, raw earth buildings are good at this so-called thermal phase shift. But raw earth is still a poor insulating material.