Rice hull concrete for cool room insulation

UC Davis designed a CoolBot cool room (excellent project for homesteaders!) using rice hull concrete for insulation and a CoolBot controller to keep farm produce fresher. The cool room paid for itself the first year and even helped make a good profit for local farmers. The rice hull concrete was poured into forms to make insulating wall boards. This blog post will briefly summarize the outcome of their project. The full report is available online for free and can be found by searching for the title of the report: Cool room insulation – UC Davis D-Lab

Design considerations for the cool room:
1. Insulation material has to have a high R-value, better than EPS foam.
2. In order to increase the R-value, the insulation material has to be able to trap air in as part of its structure.
3. Insulation material has to be water resistant.
4. Insulation material has to be able to avoid air moisture condensation.
5. Insulation material has to be non-flammable.
6. Insulation material has to be wind resistance.
7. Insulation material has to be locally available.
8. Insulation material, if not commercially available, has to be made out of local resources.
9. Insulation material, if not commercially available, has to be made by local inhabitants according to their technical skills training.
10. Insulation material has to be low cost as possible.
11. Insulation material has to be malleable as possible to adjust to the different building options.

Summary of the mixing process: soak the whole rice hulls in equal volume of water for 30 minutes, mix cement separately, drain the rice hulls and slowly add them to the cement and then mix until uniform in consistency. A cement mixer speeds the process. Ratio of rice hulls to cement is between 3:1 to 4:1, so only minimal cement is needed.

Rice hulls are typically very low cost and sometimes even available for free. Over 100 million tonnes of rice hulls are generated each year throughout the world. Rice hulls are naturally fire resistant due to the silica coating on the hulls and therefore meet fire code requirements untreated. The insulation value of whole rice hulls is over 3.0/inch. Using this abundant, low cost, highly insulating agro waste to build affordable homes and other structures has lots of potential. There are numerous rice hull homes around the world that you can read about on our blog.

20 thoughts on “Rice hull concrete for cool room insulation”

  1. I haven’t seen anything saying if you can build a dome using RH tubes. I’ve seen a lot of things saying that RH is not good for load bearing and they either suggest rebar, saplings, or post and beam with a connector beam on top. I’m planning a 20′ diameter main portion with a domed roof. Which way would you recommend?

    • I would not rely on rice hull fill alone when constructing a dome. It might be that using rice hull concrete would be structurally sound, but this would take some testing to be sure. When I designed an insulated domed roof for one project, I specified a geodesic supporting structure beneath the bags.

      • Thank you for the quick response. What do you recommend as a material for the geodesic structure? Also, if I were to have a 2′ deep gravel trench and drove rebar 2′ deeper into that. Could I have 4′ exposed on both sides of the RH bags to stablize then lash 4′ or more to those to get an 8’+ wall instead of post and beam?

        • The geodesic struts can be made from either wood or metal or even bamboo. What you suggest sounds feasible to me.

          • It might take some testing to find the best ratio of Portland cement to RH, but I would suggest starting with about 1:8 by volume. You could also try using hydrated lime for this, which would be more ecologically benign.

          • Not that I know of. BTW, the blog post suggests a ratio of rice hulls to cement is between 3:1 to 4:1

  2. We are wondering about the weight of a cubic foot of rice hull + lime concrete. Would it weight more or less than rice hulls alone?

  3. Keep the idea of rice hull concrete fresh in your mind. Very soon I will publish my latest ideas on low cost housing made with this material.

      • India, like other big countries, loves Portland cement because it’s a big profitable industry that gives kickbacks (payoffs) to investors/insiders/politicians, etc. One reason that Portland cement is affordable is because the industry is subsidized with tax payer money. This boosts profits for the industry and makes it harder for competing products. So it may be difficult to find other types of cement.

      • Any type of geopolymer such as magnesium cement will work. The challenge is finding affordable products nearby.

      • Hempcrete is basically made with a thick slurry of hemp hurds, water, and Type S hydrated lime. I paid $9 yesterday for a 50 pound bag of Type S hydrated lime off the shelf at a big box store. I’m now trying to source ‘construction grade’ hurds…more of a problem. Until I can get them it is worthwhile to practice with rice hulls. Here’s a youtube vid of a high end project using hempcrete: https://www.youtube.com/watch?v=cm23l_VLyp4


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