[lhpz]

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.

[Bender]

Thankyou for that really detailed explanation. That reaffirms for me that I will most likely stick with aircrete and concrete/shotcrete when it comes time to build a few hobby structures I had in mind.

Out of curiosity, could you suggest what material would be best for storing heat from the sun? I am going to build a greenhouse that will face the south and have a wall on the north side to absorb heat. I have seen some people use clay with a black metal wall and some use black barrels of water. I want to be able to extract some heat from it using pipes. Any thoughts on what might be even better?

[lhpz]

If you want to store solar heat, you cannot beat water @4000J/kg.K. Black barrels of water is probably the best solution, yet a very simple solution. I've checked the thermal capacity tables, only ammonia would do a slightly better job than water !

I also toy with the idea of building a greenhouse myself, that's why I have done some research. If you want to harness the thermal capacity of earth for a greenhouse, I know of two tricks:

- Excavate one or two meters of soil to build a “pit” greenhouse. You will get earth walls connected to an immense thermal storage capacity. This is the so called Walipini greenhouse concept https://en.wikipedia.org/wiki/Walipini

- The diurnal and seasonal temperature oscillations dampen quickly with depth in the ground. Below some meters of soil, the temperature will be stable at the average annual temperature at your location (Ta). If you can dig a trench to lay underground pipes to create a ground/air heat exchange connected to your greenhouse, you will get a free source of air heated at (Ta) in winter or cooled down to the same (Ta) in summer. Fans to force air circulation will help.