Hybrid Questions and Answers Involving Cordwood

Kelly Hart is your host at greenhomebuilding.com, and has built his own home using a hybrid earthbag/papercrete technique, which can be seen on the Earthbag page. He has adapted the concepts popularized by Nader Khalili and his "superadobe" building, by filling the bags primarily with crushed volcanic rock. This creates insulated walls that are similar to strawbale, except that they are completely impervious to damage from moisture, insects or rodents. The earthbags are plastered both inside and outside with papercrete. Kelly has produced a video, titled Building with Bags: How We Made Our Experimental Earthbag/Papercrete House, which chronicles the adventure of building this house, and shows other earthbag houses as well. Another video program that he produced is A Sampler of Alternative Homes: Approaching Sustainable Architecture, which explores a whole range of building concepts that are earth friendly. One of the homes shown in this video is a hybrid strawbale/wood framed home. Kelly spent many years as a professional remodeler, and is available to answer questions about what he has done, or consult about other hybrid projects.

Questions and Answers

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.

Q: I live in Eastern Tennessee. I have purchased several 40' steel shipping containers with the intent of converting them to a house. I am wondering about building cordwood walls around this strong exterior structure to provide thermal mass, insulation and improved aesthetics. My thought is to build a concrete block footer to get the logs away from the soil. The container would be on piers, providing an open crawl space underneath. Is there a method you could recommend to anchor the logs to the steel skin and frame that would not require a timber frame?

A: I think that what you propose is entirely feasible and practical. Such an external cordwood shell would definitely help insulate the metal container in very aesthetic manner. It would add some thermal mass, but not a whole lot. The cordwood walls would really just need to be self-supporting, since they are not necessarily part of the structural integrity of the building. It would not require much of an attachment to the container, only enough to connect the two together, and this could be accomplished with periodic metal bars (angle iron, etc.) that was welded perpendicular to the container wall and imbedded within the cordwood. You can actually stack cordwood at the corners in such a way that they interlace directions and are self-supporting.Whatever roof structure you devise would obviously need to extend over the cordwood walls.

Q: I live in Michigan in a manufactured home with a crawl space (rough guess - 1200 square feet). It currently has aluminum siding. We lose a lot of heat each winter and have wondered about 'wrapping' our home with some kind of alternative technique. We cannot afford to rebuild a more efficient home and are trying to make the best of what we have. We have concerns of both keeping 'green' and keeping costs low. Straw and cordwood are both resources that are available here, probably others (earth?). Which, if any, of these would work for this idea of an external 'wrap' and is this an option worth exploring?

A: Yes, wrapping your house with either cordwood or strawbales would make a huge difference in the comfort of your home. That metal siding is constantly wicking heat out of your home. Structurally, there are two main factors to consider: you might have to re-roof the home to make the eaves large enough to cover the wood or straw, and you will need some additional foundation to support the new walls and get them off the ground. The foundation could just be a simple rubble trench with earthbags of gravel above that. Detailing around doors and windows would need to be carefully figured out as well...but all of this is possible.

Strawbales likely provide a little better insulation and stack faster, but then they take a fair amount of time to plaster. Cordwood takes a little longer to do, but then once you reach the top, you are done...there is no need for further plastering or significant maintenance over time.