From time to time we answer reader’s questions.
Jeff: We are looking at building a home on acreage in central north Michigan. It is extremely cold in the wintertime where we are looking. Would an earthbag home insulated with rice hulls (using 10″ tube sandbags on exterior of earthbag walls) be sufficient for that kind of climate? How do you protect the lower courses of rice hulls from moisture from the ground to prevent rot?
Owen: Adding 10″ of insulation on the outside of walls should be plenty. That’s more insulation than 2×4 and 2×6 walls. At R-3 per inch, that’s about R-30 walls! (It’s probably less than that because of the shape of the tube, so let’s say R-25.) This will give you a superinsulated wall with a thick layer of thermal mass covered by a thick layer of exterior insulation. That’s ideal.
To protect the rice hulls, build a footer and fill lower tubes (or bags) with scoria or something that doesn’t rot. Use rice hulls once you’re above the level where moisture can cause problems. I have several articles on the Internet about building in cold climates such as this one: Insulated Earthbag Houses
I’m building a small earthbag cabin in Chiang Dao, northern Thailand, using rice hulls as the fill material for the walls. I’ve done as you suggested above in order to protect the rice hulls from ground moisture. I’m wondering, though, if there are special concerns regarding what plaster to use over rice hull-filled bags. I’ll have wide eaves to protect the walls, but as you know those monsoon rains can come in horizontally some times, and my main concern with these walls is the potential for rot.
Thanks for your help.
Make sure the areas where blowing rain can hit will resist water. Most people use cement plaster for this. Detail the windows and doors carefully. This is where most moisture problems occur. For instance, you can add sloping window sills with a drip edge cut on the bottom. Check the seal between plaster and wood frequently, and caulk as often as needed. Put the door on the downwind side away from storms and maybe add a buttress to block the wind and rain. You could also extend the roof over the door area for added protection. Also, you can add plants to help block the blowing rain.