Glory Be; the root cellarGlory Be, the root cellar was conceived because I wanted a place to put homegrown organic vegetables for winter storage. The most intensive and frustrating part of the project was finding information and specifications for root cellar construction. I then resorted to the 'seat of the pants' method of design for this root cellar. I was also very interested to try out two alternative building methods; cordwood construction, and a green roof. Luckily, there is more information out there on these techniques.
I determined that the ideal place to build a root cellar was right into the hillside, so an excavator was enlisted to dig out a hole. After much deliberation, losing a track and almost falling into it, a hole about ten feet wide and deep was dug. Most of the soil was sandy, with a few large rocks at the bottom, so drainage isn't a problem here. 
A small footing was built with concrete, and a frame of 2x4” lumber was constructed. On the outside of this frame was screwed metal roofing which was carefully back-filled with the soil. The floor is just soil, with a layer of gravel over top. This ensures the humidity stays higher for vegetables such as carrots that keep longer when kept moist. 
A roof of two layers of oriented strand board topped with rigid blue foam insulation, and then a large piece of rubber roofing above that was constructed. The roof was back-filled with between eight and ten inches of soil, and eventually planted with Sedum. 
The front walls were built out of one foot (30cm) long larch cordwood, which was mortared with a mix of Portland cement, damp sawdust, lime and sand. The middle of the wall was insulated with dry woodchips mixed with lime to prevent insect infestation. So far, four years or so later, there appears to be no major cracking of the mortar, which was a concern. I stacked the cordwood in place to dry before mortaring them together.
I painted the outside to look like old wood, and surprisingly, after four years of full sun, winter cold and lots of abuse, it’s still looking as though I only just painted it. The paints were acrylic craft paint, coated with two layers of spray on urethane.
Once the plants get big enough to run into each other and fill in, it’s going to look great. 
Many other plants can be used on a green roof - here are Andropogon gerardii, the big bluestem, and Ratabida pinnata, the mexican hat which reseeds itself every year. Many other xeric plants are suitable too. Look for those that have a fine netting of roots rather than a tap root, as there isn't enough depth in the soil for tap rooted plants. The temperature inside on the hottest days of summer will get up to 17-20 degrees Celsius, and in the winter when outside temperatures can reach -25 degrees Celsius it’s kept about 4 degrees inside by the help of a 60 watt bulb.
All things considered, Glory Be is a hit on all fronts. Visitors are fascinated by it's hobbit house appearance. I also get frogs and crickets living in the cool damp inside, so I have to watch I don't crush them by accident. I'm very satisfied with the way it turned out, and I highly recommend it as a fun project to build in any xeric garden.
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