Michael G. Smith has been the expert on cob and light straw/clay use at greenhomebuilding.com for nearly two decades, and has answered innumerable questions on these topics, most of which are posted on the website. In response to a question about using light straw/clay for a particular application, he wrote:
Another possible solution, that I have become quite excited about lately, is rice hulls and lime. Rice hulls, hydrated lime, and water are mixed together and installed as an insulating system similar to straw-clay. Advantages: better insulation than straw-clay, extremely mold-resistant, can be installed in wet weather, very easy to mix and install. Disadvantages: higher embodied energy for the lime production; requires permanent lath to hold the hulls in place since they are much more crumbly than straw-clay; only appropriate where rice is grown. Rice hulls are light and easy to transport (especially if you can buy them bagged); the weight would be much less than an equivalent volume of straw.
Here are the ratios I use for rice hulls and lime (“ricecrete”):
1 part dry hydrated lime
equal part by volume of water
equal part by weight of rice hulls
For example, here in California I can purchase both lime and rice hulls in 50-lb bags. I put one bag of lime into the mixer with an equal measure by volume of water – about 10 gallons. I let that mix for a few minutes then add one bag of rice hulls. The resulting mix fills about 6 to 7 cubic feet of space, so that will give you a sense of how much material you need. You don’t need to tamp or compress while installing the mix. Just pour it into the cavity and move it around a little to make sure there are no big voids.
The ricecrete mixture is far too light and crumbly to accept plaster directly. Rather than install ricecrete in moveable shuttering as I would for straw-clay, I hold it in place with permanent lath, installed prior to the infill. For lath I have used split bamboo, thin wooden slats, and also reed mat which is readily availble in hardware stores here. I don’t know whether mesh would be stiff enough on its own to do the job – it probably depends on the distance between your framing members. With the lath in place, there is no need to compress the ricecrete as it goes into the wall. This is advantageous because less compaction means more air in the wall, which means better insulation – and also less materials cost, less weight, less mixing, etc. I would not add any kind of fiber to the ricecrete because it would make it harder to mix, harder to install, and less likely to fill narrow cavities completely. Do be careful as you are filling to move the ricecrete by hand underneath electrical boxes or any other horizontal obstacle inside the wall cavity. You can lath one side of your wall completely and then on the side you plan to fill from. Don’t do more than 50cm vertically at a time so that you can reach the bottom and make sure that the cavity is completely filled.
I’m considering doing light clay or rice crete (with a plaster finish) in the 4” walls of my tiny house on wheels I’m building this summer in Northern California. The roof and floor will be well insulated and it’s a mild climate so I think that the walls not being super insulated will be okay. I don’t plan to move the house often or far, and understand there may be cracks in the plaster but that I can repair that. Any feedback, concerns, major things I’m overlooking and need to consider? Thank you so much! I am very inspired.
I was excited about light straw but when I heard about rice Crete and that it’s lighter weight, more insulative and less mold prone, I was drawn in. Of course lime is extracted and that’s a draw back.
I’m still deciding if I should do modified post and beam or conventional 2×4 framing.
I think that ricecrete could work for you, especially if you follow Michael’s directions. Lime is extracted, but it does reabsorb co2 from the atmosphere as it cures.
I would appreciate a video if possible.
This is a great idea, thank you for posting! I am about to buid a hybrid earthship adapted for my climate here in northern Thailand. I want to use tires to hold back a berm, but then I want to use bags of rice husks to build the walls of the rooms, which help to dry the air in our humid climate. But I did not know what to put in between. So I will think about ricecrete… Seems like a great idea!