Many people are interested in retrofitting their existing home with straw bales. This is possible; however, a specific set of concerns exist with this type of construction. The most important thing with straw bales, no matter if it is retrofitting or new construction, is keeping them dry. This includes rain water and moisture. With that in mind, it is important to retrofit the building with large overhangs to protect the new walls from rain. Keep in mind that even if the building had reasonable overhangs to begin with, the thickness of the bales will have cut down on the overhang by at least 14 inches.
The application of new overhangs may require a change in the roof design. For example, if the slope on a roof is 6/12 (six inches in vertical gain for every 12 inches in horizontal movement) the overhangs will drop the fascia and gutters very low on the wall if you extend them an additional 2 to 3 feet. You may even end up blocking views from your windows and interfering with door swings. You may need to change the roof design by lowering the slope to 2/12 and then extending it out 2 to 3 feet. You would, in this case, be creating something a kin to a deck roof over the new walls. In fact, this may be the best solution for some of the walls: a patio roof or veranda.
Be sure to keep solar gain in mind if you choose this solution. This is not to say that you have to change the slope of the entire roof, just the new section you are adding. Another situation to be aware of is deep wells created by leaving the windows and doors in place. Having window seats and deep wells on the outside of a building is asking for water damage. I suggest moving the windows to the outside of the bale wall by building new window and door bucks. Extra attention needs to paid to the details around the windows and doors, as always, and also to the space from which the windows and doors were moved.
With the walls protected from direct rain, we move to consider moisture. If the house or mobile home you are wanting to wrap with bales already has siding, as I assume it does, there will be a chance of trapping moisture behind the bales. Many homes use building paper (Tyvek®, Typar®, or the like) behind the siding to reduce moisture problems in conventional homes. We have learned over the years that such paper is a major problem in bale homes as it traps moisture in the walls and contributes to bale rot. The siding itself has the potential to cause similar problems. Therefore, some kind of separation needs to be maintained between the bales and the siding.
One option is to remove the siding entirely and notch the bales in and around the framing. This is time consuming and potentially dangerous as the structural sheathing in the walls will need to be removed as well. This will weaken the structure and may, in fact, undermine the original engineering of the building. Another option is to add a breathable netting and air space between the bales and the existing siding (www.MortarNet.com). These systems are designed for masonry applications to stop the mortar from filling the air space and creating mold and moisture damage behind brick and stone veneers. By using the same design and materials, moisture can be directed away from the bales and out through weep holes in the bottom of the assembly. This is the easiest way to ensure a moisture break and air circulation behind the bales.
Finally, the foundation would have to be big enough to handle the bales and high enough that you can get the bales up off the ground. Use the standard design for raising the toe ups, and creating the foundation unless the inspector requires the mobile or existing home be on the same footing (very possible). In this scenario you will need to make a wider footing or stem wall that either spans the bales and the mobile home, or can be attached by way of epoxy bolts to the existing foundation. You may want to incorporate a rubble trench foundation with a bond beam/stem wall to minimize the use of concrete. Much of this may be at the mercy of the building department.
I like the idea of retrofitting because it minimizes the waste that we as a people produce. Imagine how many trips to the dump you will save by adding bales to an existing structure rather than tearing it down. As with any project, there are specific places of concern that need to be addressed within the retrofitting process. Pay attention to the details and get into the mind of moisture. If you were water or moisture, where would you create problems? Find those areas and plan ahead. It is crazy to think, in this project or any other, that we as builders can stop water from “getting in” forever. Plan a way for the water to get back out without damaging the bales or the structure and you will have a house that lasts longer than you do!
ABOUT THE AUTHOR
Andrew Morrison has a passion for straw bale construction that is matched only by his desire to teach his knowledge to others. He has a wealth of experience in designing and building both conventional and straw bale homes. After years of building, he has moved his practice entirely to consulting and teaching. He shares his knowledge with thousands of people via his DVD series and this website and teaches roughly six-eight hands on workshops each year. For more on his workshops, please visit www.strawbale.com/store/category/workshops. Andrew received a BA degree from Hampshire College in 1995 for Glacial Geology. He also has a degree in construction technology.
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