The design decisions that you make on paper for your green home will determine the impact it has on the environment. The ideal green home is one that will last for over 100 years, yet, when the time comes, will dissolve harmlessly back into the earth. Straw bale, adobe (mud bricks) and wood all meet this criteria. They came directly from the earth with minimal processing. Nature recycles everything, and the concept of waste does not exist. When a structure's life comes to an end, many of the materials will merge back into the soil with no harmful effect. Materials, such as plastics (used in pipes, wiring, etc.), paints, some flooring materials, etc., are a potential environmental problem. Others, such as portland cement (which is called concrete when sand and gravel are added) can create pollution during the manufacturing process. This article will discuss the use of concrete in foundations.Concrete is one of the most common building materials used today. The advantages of concrete are its exceptional strength and the ability to be poured into a form of almost any shape. Through a chemical reaction, it will turn into a stone-like material. When using concrete in a green home, there are several points to be aware of. First, the production of portland cement accounts for approximately 7% of all greenhouse gasses in the world. This is a surprise to most people. Because of this, it is best to carefully consider how and where the material is used. It is a miraculous product that is essential in house construction. One of the major uses of concrete (and therefore portland cement) is the foundation itself. Straw bale walls will be at least 18" in width, which means that the width of the foundation may be the same. When a
foundation trench is poured with solid concrete beneath a straw bale wall, the resulting mass is overkill for the typical home.
One solution is to use a rubble trench foundation where only the top eight or so inches are poured with concrete. The remainder of the foundation trench is filled with "rubble" that consists of 1.5 to 2-inch crushed gravel. The depth of the trench is determined by building code and must go below the frost line in your area (the depth of potential freezing of the earth). This system was used frequently by the architect Frank Lloyd Wright. In my home, a rubble trench foundation reduced the amount of concrete by over 50%. The system is structurally sound and will meet code if your construction plans are stamped by a structural engineer.
CFC-free foam insulation is placed on the outside of the foundation to prevent heat loss from the floor. Sometimes it is placed on the inside as well. Foundation insulation is required by code in most areas. Concrete can be made greener by including fly ash in the mix. Fly ash is a waste product from coal-powered electrical plants. When added to concrete, it not only reduces the amount of portland cement in the mix (thus reducing greenhouse gasses), but it also makes the concrete stronger. The end result is that a waste product is put to use, less green-house gasses are generated due to the reduced amount of portland cement, and the concrete is structurally stronger.
Next week I will continue to discuss the use of natural materials when building a sustainable home.
A trench is dug down to the frost line. In this case, it is around 18" deep (see first picture on top of this post). Gravel is then placed in the trench. Straw bales were used as forms for some of the foundation, in addition to the foam insulation.
The top 8" is then filled with concrete. This concrete beam sits uponthe gravel in the rubble trench. The foam insulation is required by code. The rebar will be used to anchor the first row of straw bales to the foundation.
Update: Part 6 is here.
[This has been a guest post by Ted Owens, a green designer and filmmaker. More details on green building design and construction can be found on his website and in the Building with Awareness DVD and Guidebook. -Ed.]