Foundations for Natural Buildings

Dan Chiras has been involved in natural and alternative building since 1994 and lives in an off-grid passive solar/solar electric home in the foothills of the Rockies. His house is built from straw bales, rammed earth tires, and numerous green building materials and is powered entirely by wind and solar energy. Dan is author of The Natural House: A Complete Guide to Healthy, Energy-Efficient, Environmental Homes, published by Chelsea Green; and numerous articles on natural building and sustainable design, which have appeared in Mother Earth News, Natural Home, and The Last Straw. Dan embraces a comprehensive systems approach to building that offers a wide range of benefits to people, the planet, and our economy. He will field general questions on natural building and offers consultation on project design and construction, as well as lectures and workshops on various aspects of natural and sustainable design and construction.

Questions and Answers

Q: I live in south Louisiana which is not well known for sustainable architecture although there are plenty of trees (cypress) for building. I am drawn to some of the other natural building materials such as strawbale, earthbags, etc.. mainly because of the aesthetics and the relative ease of building (unskilled). I have read several of the books on the subject. However, none refer to building in a wet climate like Louisiana. I am thinking of adding a porch and wall using a combination of strawbale and earthbags but I don't know what to do for either the foundation or a water barrier between the porch and the already existing home. Our ground holds a lot of water so I don't know if a gravel foundation will work without wicking water up into the strawbale. I could use cement as a foundation (even though it is not a natural material, per se). But I still don't know what to use as a water barrier between the cement and first layer of strawbale

A: I'd strongly suggest a rubble trench foundation (be sure to install a French drain in the base of the excavation) with a grade beam, either made from concrete or earth bags. If you use earthbags, be sure to stabilize the earth with about 5% cement. This foundation works well in non-seismic (earthquake) zones. The rubble trench foundation will keep the stem wall draw. Be sure to install a French Drain to remove water from the rubble trench and be sure to grade your soil so it slopes away from the foundation.

You'll need to waterproof the top of the grade beam with a nontoxic sealant such as AFM's Dynode. It's expensive, much more expensive than standard foundation sealers, but you'll only need one or two buckets and the stuff is safe to work with! I highly recommend it.

You may want to provide further waterproofing by building a bottom plate using 2 x 4s.. You may want to use pressure-treated 2 x 4s as well or seal them with a nontoxic sealant. Place pink foam seal beneath the 2 x 4 bottom plate and fill the space with crushed rock. There's a picture of this in my new book on natural plasters.

Q: Any thoughts about a foundation that would be "green"? If it is the Portland cement that makes greenhouse gas emissions, would concrete be okay if something like lime was used instead, or is it still not the best choice (and then what might be the best choice for foundation?)

A: (Kelly) The question of using Portland cement in a foundation is a good one; yes, the manufacture of cement does produce greenhouse gas, so minimizing its use is important. I'm not sure that using lime instead for a foundation would be structurally sound enough. Other alternatives are using fly ash to replace some of the cement in the concrete, or making a rock foundation that uses less cement in the mortar. Another possibility is a rubble trench foundation in conjunction with earthbags filled with gravel. These sorts of foundations are described in Paulina's book "Building with Earth", available at our store.

Q: I am from Victoria B.C. and I have almost finished the planning stages of a passive solar, ecological, energy efficient house; but I can't seem to find how to waterproof cement that I plan to submerge up to 20 feet underground that won't affect the quality of water that will travel through my house. The exterior walls will be accessible in the early building stages but the medium is what I am unsure of?

A: To waterproof concrete safely, I recommend AFM's Dynoseal. It's a marvelous product for this application and very environmentally friendly. You will pay about 10 times more for it than standard foundation sealant products (petroleum-based materials). So plan on spending about $55 per five-gallon bucket. It's well worth the higher investment. It's much more pleasant to work with than standard foundation sealant and cleans up with water!!! I love working with this stuff.

Q: What are our alternatives for a friendly safe building system for the foundation for a 3 story Enertia log home? We like the pumicecrete but accessibility may be be a problem in Charlottesville, VA. We also considered Fawswell blocks but they are 8 weeks backed up before delivery is possible.

A: Why don't you try insulating concrete forms? There are about 50 manufacturers of this type of product, so you shouldn't have any trouble getting them. If you can get Rastra blocks, that would be nice as they're made with recycled polystyrene.

Q: I currently looking at building a eco lodge in Sri Lanka. The land is very challenging with a slop of 30- 40degrees. I'm looking to build 8 huts that are made from local materials, but I'm having problems finding an alternative to cement for the foundations. Could you make any recommendations?

A: Have you considered a rubble trench/earthbag foundation? A rubble trench is a trench filled with crushed rock to grade. Then you build a grade beam from earthbags. If this is a wet climate, you would want to fill the earthbags with crushed rock, too. You can read more about both earthbags and rubble trench foundations in my book, The Natural House.

Q: For foundation for posts, what would you recommend?

A: (Kelly) For an appropriate foundation for posts, I would suggest elevating them on piers of concrete or stone, perhaps pinned with steel to keep them from slipping. This will keep them up away from the soil and away from hermits or dampness that might cause rotting.

Q: Is a certain type of foundation necessary for natural building techniques like cob and strawbale building?

A: (Kelly) The requirements for a good foundation for both cob and straw is actually similar to wood-framed buildings; they must be solidly built, be able to withstand frost upheaval, not be adversely affected by moisture, and high enough to keep the material they are supporting away from surface water. Concrete, stones, tires, earthbags, and rubble trenches have all been used effectively.