Sherry Crow

Thanks for the feedback. The earthbags will be protected from outside precipitation with a covering of epdm and/or heavy plastic vapor barrier. The EB root cellar that we built at my neighbors has more issue with interior moisture. Improvements are being made with that ventilation system that should help.

One other issue that I hope to address is the potential for radon exposure. I understand that I can test for it and that there are ways to mitigate should it prove to be unsafe levels. The structure will be used as a storm shelter, as well as for food. I doubt that the time it's used for personal shelter would provide sufficient exposure to be an issue, but I don't know if it would affect the food that is stored. I would appreciate hearing from folks that are knowledgable about this.

I think it's a matter of degree. The catenary curve is stronger than a vertical curved wall. Buttresses are another tried and true way to increase strength. Given our frost depth, the amount of fill needed for insulation is a significant load. If I integrate another type of insulation (i.e. foam board), I can reduce the amount of soil covering. Because this structure will be its own independent hill, I'm not as concerned about the hydraulic pressure as I would be if it was part of a larger drainage area. It would be great to see examples that are comparable and how they perform. Given the variables with siting, materials, skill, and regional climate, it's challenging to make comparisons. When I sent questions directly to some of the pioneers in EB construction, they weren't ready to make recommendations for underground construction parameters. It's great that we have forums like this one to share our efforts and hopefully accumulate pertinent knowledge.

A flat roof would certainly be easier. I think my preference for a pitched roof is from the inherent ability to channel water away...and experience with failure of flat roofs on above ground structures. I'm not ruling a flat roof out yet.

Thanks, Christopher. I hadn't thought of using interior posts for both roof support and shelving. I have been thinking of yurt type roof construction with a compression ring at the peak and a cable tension element towards the heel of the rafter. I would also tie them to the bond beam. Besides insulating the roof and covering with earth, a used grain bin roof could simplify and reinforce it as well. I have used epdm roofing for a vapor barrier. There are occasionally silo roofs on Craigslist.

Making design choices on a 10' diameter earth bag root cellar project. We have very poor soil percolation and have already prepped a compacted raised pad to build on and will build a hill over the structure. We completed an earthbag root cellar at my neighbors property and I hope to improve this one based on the experience. The existing structure has an interior diameter of 8' and we used the catenary curve for wall structure. The inherent stability of that style is a proven technology, yet has some drawbacks that I would like to avoid. It yields a sloping wall with excessive height in the center. The compound-curve wall makes storage more difficult and the added height requires more fill for insulation and an overall larger profile to the resulting mound. I would like to do a circular structure, but with vertical walls. If we use earthbags to the level of the door lintel and cap the bags with a concrete bond beam and build a robust insulated roof attached to the bond beam, I'm hoping to reduce the overall height of the mound while retaining sufficient insulation and earth cover. The structure will be used as a storm shelter as well, and the interior space should be more accommodating with the vertical walls. A local expert on root cellars suggested that I tent the entire structure with a layer of foam board sandwiched in the soil covering. I am considering that option along with internal earthbag buttressing to resist the thrust on the walls. The interior buttresses would take up interior space, but I could use them for shelf supports or as dividers for different zones. I am in an area where 5' frost depth is not unusual.