Building Straw Bale Homes: Rubble Trench Foundations vs. Frost Protected Shallow Foundations
by Tony Sirna
In trying to decide on whether to use a rubble trench foundation or a frost protected shallow foundation for the straw bale home that I've been designing I've come to realize that despite the fact that everyone and their sister has taken a workshop on straw bale construction, the technology is still in its infancy. There is still plenty of innovation and research to be done as well as the task of adapting the systems to different climates and bioregions. And even if we were using the most tried and true construction techniques there would still be a big learning curve for someone like me who never even built a tree house as a kid.
Construction Plans For Our Load Bearing And Timber Frame Straw Bale Homes In 1998
So here's the plans as we currently have them for one of our two homes being built in 1998. This will be a two-bedroom 15-ft by 30 ft straw bale home with no plumbing and a simple solar electric system. It will be built with load bearing straw, meaning that the straw bales will actually bear the weight of the roof and there will be no structural timbers in the walls (our other home is a timber framed structure with straw bale infill). The goals of these structures are to create new housing for members by this winter, to allow us to experiment with new building materials and techniques, and to serve as an example of ecological building techniques that could be used at DR and around the world. Now remember that we haven't actually done all this yet so we can't tell you if all these ideas are good or not but we have certainly learned a lot during the design process so we thought we'd share our new knowledge. In this issue we will talk about foundations and in the next we should be ready to talk walls and roofs.
We Wanted Alternatives To Traditional Cement Foundations
Like any building project you need to start from the ground up. Foundations need to provide a couple key functions. One is to support the weight of the building so that it does not sink and sag unevenly. A second is to get the walls off the ground and protected from moisture so that things like wood and straw don't rot. The third and sometimes trickiest is to provide a level, stable surface that won't shift and move when the ground freezes and thaws.
In mainstream construction people generally use concrete in one way or another to create this firm stable foundation for their buildings. Concrete is a really handy material - instant rock in any shape you want, just add water. It's made from mixing gravel, sand and cement - all substances readily available in most places. Unfortunately, cement is made from heating limestone in a kiln to very high temperatures. This requires a lot of energy and creates a lot of CO2 emissions - by the year 2000 10% of global CO2 emissions will come from making portland cement for construction.
Given this, one of our goals was to minimize (or ideally eliminate) the amount of concrete we used in our foundation. We started by planning to have an earthen floor instead of a cement slab. Thus we can use local clay and sand atop a bed of gravel (for drainage) for the main floor area of the structure (more on earthen floors in a future March Hare). This leaves us with finding a method for supporting the walls.
Option 1: A Rubble Trench Foundation
Our first design was for what is called a "rubble trench". In a rubble trench foundation you dig a trench down to the frost line (4 feet in our area) under the wall and fill that area with gravel. In the bottom you place drainage tile so that this area is free of water. On top of this you create a "grade beam" of reinforced concrete which gives you a firm, level surface and lifts your bales above "grade" (ground level). So this could allow you to use only 6-8 inches of concrete instead of 4 feet - quite a savings.
Unfortunately, our soils here have very poor drainage and during wet parts of the year the water table can be very near the surface. So proper drainage for a rubble trench would be critical to its success - if water stays in this trench it can freeze and as it expands it will "heave" upwards and crack your grade beam and mess up your walls. For proper drainage we would need to run a sloping pipe out to daylight. Unfortunately, the chosen site is fairly flat so we would have to run a pipe for hundreds of yards which just didn't seem appropriate, ecological, or energy saving.
Option 2: A Frost Protected Shallow Foundation
On to design number two. After a little research we discovered a paper from some government research on a technique called Frost Protected Shallow Foundations. These have been in use in northern Europe for 40 years but are relatively new for this continent. The idea is that by insulating the outside of the foundation the heat of the building will warm the foundation and the area under it, thus preventing it from freezing. This only works for buildings on grade (i.e. on concrete slabs or earthen floors) since if there is a crawl space then the building heat will not heat the ground. So with this system we are able to have a foundation depth of 20 inches (based on our climate zone, and the fact that our building will only be partially heated - average monthly temperature not below 41F). The government designs then call for insulating with 2 inches of polystyrene and pouring concrete which sort of pushes our ecological buttons.
Enter Rastra. Rastra is a material like a concrete block made out of recycled polystyrene and cement which can be used as a "wallform". The Rastra wallform is not itself structural but it creates a supportive wall when you pour concrete into the holes within it thus forming a matrix of horizontal and vertical concrete. With Rastra we can use much less concrete and it is insulative (up to R35) giving us the frost protection we need.
Unfortunately Rastra is not a widely available material and is therefore somewhat expensive and requires shipping from quite a distance. But we figured it's worth the expense for this experiment and to support this fledgling ecological industry (remember, if ordering small quantities, order early, it took us quite awhile to get our Rastra, since we had to find someone else to finish off the truckload).
So after a fair amount of research and some innovation the net result is using 4.5 cubic yards of concrete, only about 20% of what we would have used in a normal foundation (which is good but not great). In the future we hope to create structures that use almost no cement or petroleum products. For example, on sites with better drainage we hope to use a rubble trench and gravel filled bags to bring the walls above grade (like sandbags used for flood control).
But it certainly taught me that there is still a long way to go before there are inexpensive, simple, and ecological foundation techniques for all climates and soil types. There is much experimentation and creative work to do in the field of alternative building and there are good shoulders to stand on in both recent innovations and age-old techniques.
Intentional Community Bookstore