The trap #
I’ve written before about the central tension in bio-aggregate blocks. The obvious ways to make them stronger are adding binder, or compacting them to a higher density. Both make them worse at insulating.
I spent a couple of months pushing on that anyway. I made a lot of blocks. They got denser, and they got a bit stronger, and their thermal performance got worse.
What I hadn’t questioned was why the block needed to be strong at all.
Following the thread #
The chain of reasoning went something like this, over a few days, mostly while taking my cat for a walk.
Start: my blocks are fragile at the surface. They shed material when handled. A render basecoat would fix that, and if I’m rendering anyway, I might as well reinforce it with mesh to give the block a nice tough skin.
Then: if the skin is carrying the load, the core doesn’t need to. Which means I could reduce the density of the core, which is the thing that’s been fighting my insulation value the whole time.
Then: if the core doesn’t need to be strong, how much binder does it actually need? Not enough to carry load. Just enough to stop it falling apart.
And then the uncomfortable one: if the skin is doing the structural work, why is the skin made of lime render, which is heavy, slow to cure, and has to be applied by hand?
Why not just put the husks in a box?
The reductio that didn’t self-destruct #
That was meant to be the absurd end of the argument — the place where you notice you’ve reasoned yourself somewhere silly and go back and question your assumptions.
Except it isn’t silly. Loose rice hulls have a thermal conductivity of roughly 0.05 W/m·K. That is better than any bound block I have made, and better than most of the published rice-husk composites. A plywood box full of dry husks outperforms the thing I’d been spending months trying to perfect.
It’s also not novel. Timber-framed prefabricated walls packed with straw or hemp are already available. Structural insulated panels are the same idea with foam. Same architecture: rigid perimeter, stressed skins, cheap low-density fill.
Does this replace blocks? #
The use case is nearly the same — an external insulating leaf, outside the structural frame — but the detailing forks. A block has a fibrous face that takes a render coat directly; a cassette has a sheet face and needs to sit behind ventilated cladding. Blocks are laid in mortar and tied back to the frame; cassettes shiplap together and screw to battens. And a block carries roughly three times the thermal mass.
So these aren’t really competing. If you want a rendered wall, or thermal mass, or a material you can actually see, blocks win. If you want something to put behind cladding, the cassette does. I’m still making both.
What the binder is for #
Blocks rely on their binder for objecthood — the ability of the material to hold a shape as it is demoulded, cured, stacked, shipped and laid.
Once a box provides that, the binder’s job list shrinks to three items:
- Cuttability. A builder needs to cut a panel to fit and have the cut edge stay where it is, at least until the cut edge is taped or something.
- Settlement. Loose fill in a vertical cavity slumps and leaves a thermal gap at the top. See measurements below.
- Mould. While plain rice husk is somewhat resistant, lime makes it better. In my test jars, untreated husk was well colonised at two weeks where husk-and-lime had only small patches — and the jar with borax added at 10% of fibre mass is still clean.
That’s the whole list. Notice that none of them is strength. So the binder dose is just the lowest dose that achieves settlement lock and a clean cut edge. That’s a much lower bar.
Where it’s up to #
I have four tray-scale prototypes curing: plywood mini-cassettes with kraft paper faces, cast with lime-misted husk at 12%, 25% and 50% binder-to-fibre by mass, plus a clay-slip one as a near-zero-carbon comparison. The point of that series is to find the lowest binder dose that still cuts cleanly and locks against settlement.
I don’t know yet whether the answer is 12% or 112%, or whether the paper facing holds up, or whether it dries out or grows mould. But I’ll know soon.
In fact the reductio can become even more absurd: why not just fill a cardboard box? It will be covered with weather resistant membrane anyway, and held in place by cladding battens. All that is needed is objecthood. I have four cardboard boxes curing in parallel with the plywood ones.
I’ve been calling them Bento.
Specs #
My mini-cassette testing module is 200 × 400 mm, but the full size cassette would be at least 300 × 600 mm. A reasonable thickness is 100 mm, giving R1.7–2.0 of insulation value, not yet measured. Comparable to wood fibre board: R2.0 at 80 mm thickness. The incumbent is $70/m² landed in Australia; my materials are a fraction of that, and the fabrication is the open question.
Working density target is around 150 kg/m³. I picked 150 because I thought geometric packing would make settlement self-limiting. It doesn’t — dry fill at 145 still densified 7% under repeated dropping, and at 121 it densified 13%. So the binder has to do the settlement locking after all, and the number it has to beat is a 20 mm gap.
The properties that actually matter for a cassette are:
- cuttability of the bound fill
- settlement of the fill under vibration over time (or after being dropped off a truck)
- edge crush and palletisation load — can you stack them on a pallet without the bottom row deforming
- clamp-point indentation and point abuse — what happens when someone leans a ladder on it
- and, ultimately, a declared R-value
Why I’m publishing this #
I’m publishing the design and the reasoning rather than keeping them quiet. I’m not seeking a patent on it. (I think the whole idea of patents misunderstands the social nature of thinking.) What publishing does is make it harder for anyone to patent the obvious version and block the rest of us from building it.
If you’re working on something similar, or you’d use this, I’d like to hear from you.