Hybrid Questions and Answers Involving Geodesics

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

C: I was thinking about building a meditation hut using the sandbag method. I'd be interested to know if anybody has tried the following: My idea was to put a small geodesic dome at ground level -maybe 3-4 high and then dig out some of the interior to finally make it 7-8 high in the center. Take the dirt from inside (we have heavy sand here in Bakersfield, CA) and then put those around and on the dome. I imagine, I would have to line the "pit" with concrete blocks reinforced with rebar. I have metal plates that form the centers of the dome sections called "starplates". The reason I wanted to go with sandbags was for silence and also insulation.

R: Well, I think your idea is basically sound...I don't see why it wouldn't work. I helped design a small (14 feet interior diameter) sandbag dome home for a women once, where the top was spherical and used a geodesic framework made of 2X4 lumber connected with metal plates. Bags of crushed volcanic rock were stacked over the framework, and then the whole thing was stuccoed. This worked well. For your project, the whole structure, except for the entrance could be underground. We made our underground pantry by just stacking bags over a conical pole framework, and then covering that with several layers of polyethylene plastic, and backfilling with dirt. If you have available crushed volcanic rock to fill the bags with for the roof section, it would be lighter and much better insulation, but dirt would also work if the geodesic framework is strong enough. As for using cinder blocks and rebar to shore up the underground walls, I would suggest just using the bags again here. That is basically what we did with our pantry, and it has worked marvelously. You could actually start by digging a pit to the level that you want, and somewhat larger than you need. Then stack the bags in a circular pattern and come up either vertically, or leaning in slightly up to the ground level. At this point you can set the geodesic frame on the inside edge of the bags, and continue stacking up and over it. The final thing would be to drape the plastic over the whole structure, all the way to the bottom of the trench, and then toss dirt all over the whole thing. The entrance would need some special consideration, but I'm sure you could figure something out. To keep moisture from coming up through the floor you might put down more plastic there and covering it with carpet or whatever...there are lots of possibilities.

Q: I want to build a 30' geodesic dome made by www.pacificdomes.com, the outside must get an earthbag shell. The inside dome consists of a Fuller construction made of 1'' steel tubes. Does this inside construction withstand the burden of the outside earthbags or what should I do?

A: According to their website, Pacific domes "are engineered with steel frames that withstand heavy snow and hurricane winds". With this in mind, I would say that they could likely withstand the minimal stress of supporting an earthbag shell during the construction process. Once the earthbag shell is completed as a dome, and has been protected with some plaster (hopefully reinforced with stucco netting or such), the whole unit would become more or less monolithic.  I once designed a similar arrangement, using wooden struts for a partial geodesic top to an earthbag dome, and it has worked out quite nicely.

Q: We are researching various methods of building underground housing. I was wondering if one could build a hybrid earthbag/geodesic dome structure. My thoughts were to use the bags to a 9' level, place a geodesic dome on this at the loft level then continue with the bags to the top of the dome. Would a structure as this be strong enough to withstand the pressure of berming with earth?

Also, what about radon in an underground earthbag structure? Are the bags enough of a barrier between the earth and the inside - with of course some type of plaster - to alleviate the need for more than the normal ventilation that one has to put in an underground home?

A: I designed something similar to what you describe for a small earthbag dome, and it worked out well...but it was not earth-sheltered. Geodesics, and domes in general, are very strong structures and can withstand quite a bit of force from above. I would suggest that you use a light-weight insulating material in the bags (such as scoria) if possible, and that you consult an engineer regarding the exact details of the geodesic framework, how it is attached to the bags, and how much soil can be placed over this.

As for the radon issue, you are going to have to cover the bags with a moisture barrier (such as polyethylene sheeting) where ever the structure will be bermed, so this will also serve as a barrier to potential radon. You will want to do the same on the floor area.

Q: I am interested in building a geodesic dome made of bags that are filled with paper-cuts. Do you think this will work and could you give me some additional advise?

A: I am not sure what you mean by paper-cuts, but if they are loose bits of paper, or shredded paper, then they should provide fairly decent insulation, but would not likely be able to bear much weight. This might not be a problem if the bags of paper were used as infill in a geodesic structure that was made with stiff struts of wood or metal. All you would need to do is somehow get the bags to remain stuffed into the triangular spaces until a rigid plaster was placed on both sides...or the bags might be stacked over the geodesic frame, with it supporting the weight of the bags. In either case you will need a completely impervious plaster or membrane on the outside to keep all moisture from entering the walls/roof. Sometimes borax is added to cellulose insulation as a fire retardant, and you might consider this as well.

Q: My spouse and I are planning on building a sustainable home out of straw bale, cob and lime render around a geodesic dome structure in a northern climate (Alberta, Canada) where there is 6-7 months of winter, sometimes reaching below -40C. We have researched into various building methods, but have still found no examples of another dome being created in such a way, even though geodesic domes have long been known as one of the world's strongest and most efficient structures. We just wanted to know if you have heard of this type of building before and what you think the pro's and con's would be.

A: I have not heard of a geodesic dome covered with either strawbales or cob...and this might be for good reason: neither of these materials do well in situations where they are subject to possible moisture exposure and retention. Even a good lime plaster covering them will not guarantee that they will be free from moisture, especially given your lengthy winters and snow exposure. Additionally, cob is not an insulating material, so even if it were used where it is safe from the wetness, it will not be very comfortable in the cold. I suggest either changing your concept to a different shape that will protect those materials, or cover the geodesic in a different way. One possibility might be to cover it with earthbags filled with lightweight volcanic stone or perlite, which are not adversely affected by possible moisture. Of course there are more conventional ways to sheath a geodesic dome.

Q: In your opinion could one build a wood framed geodesic dome, attach metal lath to both interior & exterior surfaces and plaster with an appropriate mix? Was thinking of stuffing the wall cavity with a natural insulation such as sheep's wool.

A: I suspect that this idea would work fine. Often when you mix a variety of materials in a wall system you tempt problems with differences of expansion and contraction, leading to cracks. In this case, the fact that the entire shell would be encased in metal lath initially may keep this from happening. You might consider using traditional ferrocement techniques, especially for the exterior, with a rich and dry enough mix to render the cement virtually waterproof. I would suggest that you try a small project first to see how it works out.


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