Building Green: Energy Efficiency and Aesthetics From The Same Materials (Part 7)
In this article I will discuss the advantages of straw bale for the exterior walls. Unless you live in a climate where heating and cooling are not necessary, some sort of wall insulation will be required. Code will specify the minimal amount of wall insulation for your area. Although the bales will not be stacked until the post-and-beam framing is finished, the material will be discussed here as this will dictate how the house is constructed.
Straw bale walls have many advantages over conventional insulative materials. First, straw is a waste product. Straw bales come from food crops such as wheat, barley, and oats. During harvesting, the upper nutritional part is removed and used for food. The lower part is usually discarded and becomes another waste problem. Many farmers are now selling this material for use in straw bale homes. When used for structures, the bales are tightly compressed and are held together with polypropylene string. Depending on the size of the bale, they will be either 2-string or 3-string. The 2-string bales, which were used for this project, weighed around 35 pounds each. Straw bales are not always available year-round, so you will need to check for availability well in advance. Depending on the thickness of the bale, an insulation value of R35-R45 can be achieved. The R-value increases with the application of the interior and exterior plaster finish. The straw bale itself also forms the surface of the wall. This eliminates the need for wallboard.
Straw bale walls are quite resistant to fire. Tests show that they have a better fire rating than most wood-frame walls. Loose straw, such as that which collects on the ground while working with bales, is very flammable. It is important to rake up any loose straw and place it in a sealed container. This loose straw can be saved and mixed with the earth plaster finish.
Excessive moisture must be kept out of the straw bales. This is achieved by installing a good roof and a breathable wall plaster that permits excessive moisture to evaporate. The roof for this house will be a pitched metal roof with overhangs. This will insure that the walls will stay dry from above.
Of course, the insulation that is achieved from this natural material is only part of the reason for using straw. The insulation is an invisible asset that will lower your energy bills in both the summer and winter. The aesthetic of thick walls is something that will be appreciated every day and for the life of the structure.
Next week I will discuss options for a thermal mass floor.
[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.]