Insulation
Table of Contents
Cottage Deep Retrofit - This article is part of a series.
Ricecrete #
Know what hempcrete is? Same thing. But with rice husks.
No, it does not involve concrete.
I had recently built a small hut using hempcrete (hemp + lime). It’s a compelling material: a high-mass insulator. That means it does a lot better at buffering extremes of temperature — “decrement delay” — compared to standard insulation like glasswool or foam even with the same insulation R value. This will be true wherever there is a significant day-night temperature variation. Also, hempcrete enables full insulation coverage; is vapour permeable; naturally mould resistant; and low-carbon.
The downside is cost. Here in Australia where hempcrete (and other kinds of natural building) is generally not taken seriously, the market is small. And when building such a small structure, there is more overhead in shipping cost. After spending nearly $6000 on hempcrete materials on that project — more than half the total construction cost — I went looking for alternatives.
Wood fibre board has almost all the advantages of hempcrete, but is similarly expensive. Straw bale is good and cheap if super thick walls are an option, but can’t really be used in floors or roofs.
Eventually I stumbled upon Michael G Smith’s introduction to rice hulls and ricecrete. It was a revelation. Here is a deep dive. However, I have only heard of one house in Australia using rice hulls: this 2019 Castlemaine house… and I was not able to track down the owners to find out more.
Insulation value #
Rice hulls have an insulation value of at least “R3 (imperial) per inch” which equates to R2 (metric) per 100mm. That is essentially the same (for the same thickness) as loose-fill hempcrete or standard glasswool batts. As I said above though, the effective performance is better than glasswool because of its higher mass and decrement delay.
Cost #
Rice hulls are cheap. I was quoted less than $50 per half-cubic-metre bag. That does not include shipping though.
I am now in a position to compare the costs across two small projects.
| Hempcrete project | Ricecrete project | |
|---|---|---|
| insulation area | 21.5 m2 | 24.7 m2 |
| thickness | 250mm | 290mm (avg) |
| hemp hurd / rice hulls | $1540 | $277 |
| + shipping | $1430 | $383 |
| binder (lime) | $2318 | $369 |
| + shipping | $551 | $0 |
| total | $5839 | $1029 |
| total / m2 | $272 | $42 |
Note these are not completely equivalent:
- This ricecrete project included roof and floor but not walls. Unlike hempcrete, ricecrete is too loose to form a wall surface on its own; it would require a lath or strong mesh to support a plaster finish. For the same reason, exposed hempcrete feature walls (popular in Australia) would probably not work with ricecrete.
- My hempcrete project used a proprietary lime-based binder, whereas my ricecrete project simply used hydrated lime off the shelf from Bunnings. I could also have made my own binder for the hempcrete project which would have reduced its cost.
- Some work has been done to smooth the way for building approvals using hempcrete; building approval is likely to be harder with rice hulls.
Mix #
Michael G Smith’s super simple recipe for ricecrete is
- 1 part dry hydrated lime
- equal part by volume of water
- equal part by weight of rice hulls
Using my pan mixer, each mix was roughly:
- 50L rice husks
- 5kg hydrated lime
- 7.5L water
The mix is light weight and pours easily.
Roof insulation #
The roof is insulated with ricecrete.
I was initially unsure about using a wet-cast material in a flat roof. It needs to be able to dry in place, and I wasn’t sure that a flat roof would allow enough air circulation through the cavity above. But this detail from The Hempcrete Book (Stanwix & Sparrow, 2023) reassured me:
Obviously I’m not doing everything in the diagram. Just ricecrete in the roof (sitting on the ceiling), covered with a vapour-permeable membrane, and a ventilation cavity above.
At the last minute I decided to lay fibreglass mesh above the ceiling, attached to the ceiling joists. Because if I ever need to take down the ceiling panels, I don’t want several cubic metres of rice husks falling on my head.
A few precautions are necessary when using materials with hydrated lime. Because lime corrodes even galvanised steel, exposed nails / screws / etc should be painted with bitumen. I realised just in time that my aluminium exhaust fan duct could also react with lime.
I replaced it with a PVC pipe — which is also stronger and has less air resistance.
The process of mixing and placing ricecrete is quite easy. It is not compacted at all, just poured in place and evened out a bit.
Filled to the top of the rafters, the depth of insulation is 290mm on the low edge and 410mm on the high edge. That means the insulation value is R6 - R7 on the low edge and R8 - R10 on the high edge. The performance is likely to be better than the same value of glasswool however because of higher mass and complete coverage (no bridging or gaps).
Floor insulation #
I also filled the floor with ricecrete.
This means closing off the bottom of the floor space so it can be filled from above. Whatever is used to close off the space should be permeable so the ricecrete can continue to dry. Options include fibre cement board or a fine mesh. But I had some spare bits of vapour permeable membrane; I got them for free as they have a few holes.
I stapled them to the joists and then fastened some random battens for extra support. This turned out to be difficult at the top edge. The space is so tight I could not even turn my head upright. But I eventually managed to do some awkward diagonal nailing.
I closed off the space with timber blocking along the diagonal corners, and around the bathroom (the bathroom floor was insulated later; see below).
Bathroom floor #
The bathroom floor was insulated later, from below, after the plumbing was all done. I fastened more membrane to the joists, and stuffed the cavity full of alpaca wool.
Natural wool is a good insulator. It almost certainly has a better thermal resistance than glasswool but is more resilient to wetting, and has more thermal mass. It is also lovely to work with. I have developed an intolerance for glasswool. I insulated the whole bathroom to a depth of 200mm (at least R5) with a bag I bought for $60.
Wall insulation #
The wall framing is only 70mm thick. Not really enough to bother with anything but batts. I decided to use polyester batts rather than glasswool. I hate working with glasswool: lots of coughing and itching. Polyester is much nicer. It also survives getting wet, unlike glasswool.
The walls all have a reflective foil wrap behind the cladding. That is not ideal because it can trap moisture: it is not vapour permeable. But I figured it won’t be a problem since my roof and floor are vapour permeable. The bathroom however is more at risk due to high humidity. So I removed the cladding and foil from the bathroom walls…
…and replaced it with a vapour-permeable membrane.
Between each wall section there were sizable gaps (average 10mm) that had been covered roughly with trim. I filled them with expanding foam.
