Building Green: Energy Efficiency and Aesthetics From The Same Materials (Part 9)
Depending on the area that you live in and the local building codes, the straw bale walls can either support the roof directly (load bearing) or the roof can be supported by a post-and-beam structure. There are many ways to construct a straw bale or green home. I chose a post-and-beam structure because it was required by code in this area. The advantage of this approach is that the roof can be completed before the straw bales are stacked. This will help to protect the bales from rain during construction. The advantage of a load-bearing straw bale wall is that the vertical posts are eliminated from the construction process, thus saving some wood.
This particular post-and-beam system uses 4x4" vertical wooden posts that are capped with a 4x8" beam. This beam was made from two 2x8s nailed together. A box beam, which is a hollow beam constructed on site, can also be used, in addition to engineered structural lumber (laminated lumber or wood I-beams). The roof framing sits upon the beam and the posts transfer the load to the foundation. Metal brackets anchor the posts to the foundation. The straw bales are used as infill and do not carry the load of the roof.
With a 4x8 beam, our structural engineer calculated that the posts could be placed up to 8 feet apart. This span is often much less due to the need for vertical posts around doors and larger windows. The posts are positioned to the outside of the wall and the bales are notched around each post. Posts can also be positioned inside the bales themselves, thus avoiding the need to notch the bales. This approach will require a bit more engineering for the roof due to the overhang over the bales and beyond the wall.
Steel can also be used for framing. The first straw bale post office constructed in the U.S. is located a couple of miles down the street from this home. It uses steel-stud wall construction with the bales placed on the outside for insulation. Wall board was then used to finish the interior walls. Again, there are multiple ways to approach the structural framing. Always check with the local experts as to what system is best for your project.
Pre-engineered roof trusses were used to frame the steeply-pitched roof. To create a pre-engineered truss, the specifications of the roof (the pitch and the depth required for insulation) are given to a truss manufacturer, who will then engineer, manufacturer, and deliver the trusses to you. The advantage of this system is that the truss can be made with smaller dimensional lumber, thus saving trees. In this home, the pre-engineered truss were made entirely from 2x4s. Each truss was light enough to be carried by two people, which made the assembly very easy.
In the next post, more on the construction process of this straw bale green home.