The hope #
Having worked on housing energy efficiency, both on new designs and existing homes, I’m a fan of continuous insulation (that means a complete blanket without gaps where heat can sneak through). And sustainable building materials. I’ve built small structures using hemp-lime and wood fibre board: both are examples of continuous insulation made from natural materials.
The problem is cost. If you look at the cost breakdowns for those two projects, you will see that the insulation is a major cost. In the case of cast-in-place hempcrete, there is also a labour/time cost.
That sent me looking for alternatives, and I found rice husks. I used a simple rice husk + lime mix for roof insulation and underfloor insulation in a small renovation project, for a fraction of the cost of similar options.
Hundreds of millions of metric tons of rice husks are generated worldwide each year. It is usually considered a waste material, and does not compost or burn easily. Read more.
So it checks out on sustainability and cost. But to make rice husk accessible, it needs to be made easy to use. That means ready-to-lay blocks, or panels. As in the world of hemp construction, manufactured pre-dried blocks sidestep the labour and time issues with a cast-in-place material.
The challenge is to create a product with a good balance of
- cost
- insulation value
- strength
- usability (eg surface robustness)
- carbon emissions
Complications #
Rice husk does not contain the kind of cellulose or plant resins that allow wood fibre board or cork board to be bound by pressure treatment. So to make rice husk stick together, we need a mineral binder like lime. Hey, it works for hemp.
But rice husk is quite a different material to hemp hurd. Due to its geometric and material properties, hemp is more readily bound together and compacted. Rice husk tends to be springy and loose in comparison. However, these issues can be overcome. For example, the Italian company Ricehouse produces a rice husk block, and there are several academic papers on the subject.
Lime is the mainstay of the hemp block industry. The manufacturing of lime emits carbon, both from the chemical reaction of calcination, and the heating energy required for it. The chemically released portion is usually balanced by carbonation over many years, but it is still a material with a sizable carbon footprint.
The time required for the binder to cure, and its strength over time, are critical factors in scaling up the production of blocks.
The lime binder can be mixed with other binders to increase strength, or to give rapid early strength. Hydraulic binders react with water very quickly, while pozzolans react with lime and water and develop more slowly. Common hydraulic binders are natural hydraulic lime, magnesium oxide, or cement. Pozzolans include calcined clay (metakaolin), pumice, rice husk ash, slag, brick dust, etc. The main downside is cost. Hydraulic binders also have higher total carbon emissions than lime, while pozzolans are generally lower.
Compressive strength can also be straightforwardly improved by using more binder, or by compacting the aggregate material more. But both work directly against insulation value.
I am working on solving these problems. As I write this I have made 117 small test blocks. And I think I am finally close to asking the right questions.