Our straw bale house plans called for a slab foundation with a poured footer. The dimensions allow for a 25' x 50' interior dimension home (1250sq ft). However, due to the thickness of the straw bale walls, the exterior dimensions are 28' x 53' (1484sq ft). Talking with your local feed store or strawbale supplier can get you the specific dimensions of the straw bale you'll be using. Determining the desired thickness is a requisite early in the design process as the architect will need to draw the house plans accordingly. However, as we quickly learned, no matter who supplies the bales, all of those bales will not be exactly the same size. There will be slight variations and this is one element that makes a straw bale house have the slight imperfections that makes it unique.
Since the foundation is what supports the entire house, once the foundation was completed, all other elements were pretty much locked in. Location of doors, upright beams supporting the roof, plumbing lines, etc, were all locked into place. We chose to use exterior pinning of the bales, so we did not need the traditional rebar pins sticking up from the foundation to hold the first straw bales in place. However, we do have bolts which stick up to hold the footer in place. This footer keeps the bales a few inches off the floor should there be interior flooding. Getting the foundation right was critical.
The footer consists of two rows of double stacked 2x4's the width of the straw bales. We could have used 4x4's, but they are more expensive. The 2x4's were filled with pea gravel, then covered with rigid foam insulation. This provides the base for the straw bales to sit upon. Again, the straw bales should be raised so that if there is ever any water on the interior floor, it does not soak into them, but rather flows under the bales.
As for the roof, there were many reasons we chose metal. One, we knew we were installing a rain catch system for landscaping. Two, the durability and life expectancy is exceptional. Three, by going with a galvanized aluminum color, we got the simple look we wanted to achieve. Four, the reflective nature of this color helps keep out the summer time heat. And, five, our cost of home insurance would likely be less expensive.
Just under the metal roof is a decking with heat shield. And, just under that is blown cellulose insulation equal to R-38. All these elements work together to keep the outside temperature out and the inside temperature in. Our first winter, the internal temperature remained quite consistent. In just the first month of being here, our outside temps ranged from 20 degrees F to over 70 degrees. But, the inside temperature remained pretty constant with a range from 67-72 degrees.
As things heated up in June, we quickly learned that by getting up at 530am, opening the windows and adding a box fan to the large south facing windows, we were able to pull in a large amount of 60-70 degree air. We then closed the windows and shades by 7am and the house generally stayed in the 70's throughout the day. We also had to add solar screens to the East facing windows to prevent heat penetration.
Both the foundation and roof were coordinated by the general contractor.
