Strawbale/Earthbag/Cordwood

Q: I plan on building a hybrid building (straw, earthbag and cordwood). This building will consist of an inner circular tower that will be 2 stories and capped by an earthen roof. This tower will be built using cordwood construction. The outer circle will serve as the first floor and be a post and beam framework with straw bale infill; this will also be capped with an earthen roof. My question is, is it safe to lay down a bed of sand (6-12 inches deep) and lay down earthbags as starter walls for the inner and outer circles? And what is the best way to fasten the posts to the earthbags?

A: You didn't say where you are building, what the climate is like, and what the natural soil is at the site. I built my earthbag house on a huge sand dune, with no foundation other than the earthbags themselves. Sand drains nicely and compacts instantly, both of which are good traits for building upon. It also can shift and blow and move around with currents of water, so it needs to be contained somehow. The big question is frost upheaval with foundations, since you don't want the structure moving up and down if the ground under it freezes. My house is built upon several yards of sand which drains well enough that frost upheaval has not been a problem. If, on the other hand, the soil you put the sand over holds water and the frost depth in the winter goes below your layer of sand, then upheaval could be a problem. One way to avoid this is to make a rubble trench foundation down to the frost depth, and then begin building with earthbags on this, filling the first course or so with gravel, so there is no way for water to wick up into the strawbales. The inner tower could be built right on the sand, since there would be no danger of freezing inside the outer perimeter.

Q: I am planning on building a cordwood and strawbale home using an earth bag foundation. I helped my neighbors down the road build their strawbale home on a earthbag foundation set on a pad of sand. My two big questions are how would I anchor the post to my building to the bags so that the house isn't lifted up by our strong winds?

A: I suggest that you not try to anchor the posts to the earthbags, but anchor them to conventional concrete piers instead, and then create the rest of the foundation as an infill to this. That way there is no question about settling or uplifting.

Q: How important is it to cover the bags with mortar? I realize the importance of covering the bags (polypro) to prevent ultraviolet deterioration, however in my plans the outside will be insulated with foam boards. Keep in mind that the foundation I'm attempting is simply a 16 by 18 foot rectangle three courses high. I will cover the insulation with a lime plaster. My big question is if by not covering the bags with plaster am I sacrificing strength?

A: The strength of the earthbags as a foundation under compression does not rely on the plaster, so as long as the bags are ultimately covered to protect them from the sunlight, this should be fine. It may be necessary to temporarily cover them with a tarp until they get covered, though.

Q: Could you use Kelly's double earthbag wall ideas to build a cordwood home for the foundation and for the earthsheltered parts of the structure - have the front (south facing) side of the home bermed two feet. My thinking is that the cordwood makes a great bond beam for the above ground sections of wall. You could even put a buried earthbag dome pantry behind the north end of the house ( like the one in Kelly's home). This would save on cement ( which contributes to greenhouse gases)- Insulation on the north side of the bermed wall would add greatly to the thermal mass.

A: There is no reason why this concept shouldn't work. Many strawbale homes have been built recently using earthbag foundations. They are a natural for situations with earth contact, whether as a foundation or as a bermed wall. There would probably be no advantage to the double wall concept, though, since an insulated single wall should work just fine. I would not rely on cordwood masonry to create a "bond beam"; a better approach would be to actually pour a concrete bond beam on top of the earthbags, and then start laying the courses of cordwood above this. If you fill the bags with crushed volcanic rock, as I did, you would not be adding much mass, but this is where the double-bag wall idea can excel, if the inside is soil and the outside is insulating.