Digging In for Comfort
What the animals know is that the earth can shelter them from the extremes of temperature, from the wind and sun and snow. If you dig several feet into the ground, you will discover that the temperature does not vary much there, year round. Water pipes are usually buried because they won't freeze when below frost level. In fact, the temperature five feet under the ground stays about the same year round. I'm sure you have experienced the delicious coolness of a basement room on a scorching summer day. Perhaps you've gone into that same basement in mid-winter and been surprised how warm it felt. This is the moderating effect of the earth at work.
Of course, most of us would not be comfortable in a house kept at 50 degrees, so we would need to bring the temperature up maybe 20 degrees to relax at home. Compare that 20 degree increase with say the 70 degree increase necessary to be comfortable in a conventional home when it's zero outside. It would take over three times as much energy to stay warm if you have not taken advantage of the earth for shelter. And on a hot day it works in the other direction, requiring tremendous air conditioning energy to stay cool enough, if you have not dug into the earth.
Many people think that an earth-sheltered house must be dark, dank, dirty and doubtful as a pleasant abode. They are wrong. There are many ways to introduce light, views, and an airy feeling into a bermed house. I'm not suggesting that we live in a hole in the ground, although even that can be pleasant if the hole is big enough to provide an atrium/central courtyard. More commonly, an earth-sheltered house is dug into a hillside, especially if that hillside faces more or less south. Then the windows for solar heating are naturally at ground level, and much of the rest of the house can be surrounded with earth. Of course, even on flat land, soil can be pushed up around the sides of the house to provide the berm.
In building an earth-sheltered house, it is important to pay attention to certain details. As with the rest of the house, the walls that are in contact with the earth need to be well insulated, or else the soil would continually suck the warmth out of the house. Also, these walls need to be strong enough to withstand the pressure of that earth and waterproof to keep out the moisture. Traditionally, reinforced concrete has been used to build subsurface walls, and this works well, but it is not the most environmentally friendly way to do it.
Mike Oeler, in his books on building underground, suggests using heavy timbers to frame the structure, with boards to form the walls, and then using plastic sheeting to waterproof it. He has lived in such a structure in Idaho for quite a few years and is still happily advocating this approach.
I have been experimenting with earthbag construction, much of which is underground. The polypropylene bags were filled with either sand or volcanic scoria. The lower bags that are filled with sand are insulated from the outside with scoria. I covered all of this with a double layer of 6 mil polyethylene before backfilling. Between each course of bags are two strands of four-point barbed wire. The wall itself has a convex curve set against the earth to withstand the pressure of the soil. The beauty of this is in the simplicity of construction and the fact that it uses very little industrial material.
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