Thank you Athena, Bill, and Robert for all you've learned and taught about earth floors! Books on patio floors might give you some other clever floor ideas. Remember, you can use more than one type of floor in different sections of your home.
The temperature in the ground below frost line is the average yearly air temperature. (The temperature in the ground is the same all year round.) Find out what that temperature is and if it's a comfortable temperature you'll want to connect your floor to the ground. If your average yearly temperature is not pleasant, you may want to move to a more reasonable climate or consider insulating your floor from the ground. To get an idea, call the local concrete folks and ask them what the insulation requirements are in your area.
If some insulation is added under the floor, the heat absorbed by the floor will reflect back to you quicker, but you then lose some of your connection to that average yearly temperature. There may be data on optimum thickness for an insulated thermal mass floor in your climate.
When you build, you will be creating a protected zone in the ground under the structure. It will be shaded from the heat and protected from the freezing by the house. The indoor heating and cooling and the sun shining through the windows will also affect the temperature in this area. If you live in a climate with extreme temperatures, you may want to insulate around the perimeter of the foundation down to the frost line to keep the heat from escaping through the frozen and/or wet ground surface. This is usually done by burying a special water resistant styrofoam (sometimes called Formula) between the foundation and the ground all the way around the house. (2 inches is rated at R10, 3 inches at R15.) The styrofoam stuff needs to be protected from the ultraviolet sunlight. Check with local builders or the building department about how deep, and how much to insulate in your climate's conditions.
The floor is one of the best areas to store the sun's heat, because it's where most of the sun rays hit after coming through the windows. This is why it is valuable to make the floor out of something that has plenty of thermal mass. That means make the floor out of something heavy and dense like earth, brick, or stone.
When I lived in an old house with an uninsulated concrete slab in a temperate climate, I was amazed at how comfortable it was. In the hot summers, that house was cooler than all my friends' houses. And in the winter, the floor was cold to the touch but that house needed a lot less heating than I expected. I put down rugs in the areas where my feet touched the floor and was cozy enough. In the rooms where the sun shone onto the floors, I left the floors bare so they could soak up the heat from the sun.
Cold air is like water. It will flow to the lowest point on your floor. If you design your floor with a low spot, the coldest air will flow off the floor where your feet hang out and into the lower cold air reservoir. Put the cold, sink spot on the cold side of the
house. Because this will be the coldest place in your home, it is an ideal place for cold food storage if you don't have electricity, or if you'd rather not have a refrigerator. You might be delightfully surprised, with a little adjustment to your habits, how easy it is to live without an electric fridge. If you live where it's very hot, you may want to make the cold sink area a living space, a cool bedroom, or summertime living room.
Stepping the floors to different levels is an easy way to deal with a sloped site. The differing floor levels define and separate indoor spaces and enhance the interest in a room. Steps from one floor level to another must be at least 4 inches high and at most 9 inches to prevent stubbed toes. The step(s) where the floor level changes can be flowing organic shapes or straight edges. Take this into consideration when planning your foundation. During construction, make temporary ramps where the steps will be. This will make life easier while you are dragging the tarps full of cob around the floor area during cobbing. Make the actual step later when you've finished the walls.
The door's threshold limits your choices about the height of the floor
at the entrance. When designing the foundation and threshold, it is helpful
to estimate floor height(s). Ideally, the threshold will be at the same height
as the floor surface. Your floor can be built directly on the ground.
Read on to help you decide how thick your floor will be.