Hi all, Wondered what people thought about the idea of putting rice husk ash in stud walls as an insulator. I have access--thanks to an pioneering neighbor--to tons of the stuff and have heard, read about and watched videos of people mixing this with concrete and/or lime (or ramming it with earth)and turning into blocks for use in kilns and in other non-load bearing construction applications. An ash-heavy mix of 12:1 (ash:concrete) results in blocks that are light, fire and termite resistant and shapable with a hacksaw. My idea is to make a stud-gap sized form and pour the mixture in, let it set, and then insert between the wall studs. If I made a little "lip" to overlap each stud I might be able to decrease the studs' thermal bridging. I was also thinking I might just fill the floor spaces with the ash as is in bags. Apparently the insulation value of rice husk ash is four times that of styrofoam (see http://www.treehugger.com/files/2009/05/rice-husk-insulation-makes-fridges-more-efficient.php), so, allowing for the concrete in the mix, could I potentially achieve the effect of 14-16 inches of EPS with only 4 inches of ash?? Sounds a little too good to be true... Any thoughts greatly welcomed!

sailles;

there are many uses for RHA, but it is not a better insulator than EPS

http://webarchive.nationalarchives....e15138.pdf

Chris,

I just read the Executive Summary of the article you referenced.  I did not see any mention of the R or U value of RHA.  Does anyone know either of these values for RHA?

Alton;

Rice husk ash, an agricultural waste material, is available in large quantitaties in the rice paddy growing countries of the world at little or no cost. This ash is highly porous, mostly silica and possesses refractory and thermal insulation properties. It is therefore an attractive starting raw material for the manufacture of low to moderate cost thermal insulations for dryers, ovens, kilns and furnaces, including those employed in the ceramic industry.

Good properties for refractory bricks not wall insulation

Thanks for these replies. Lots of useful info, and great pdf link! Although I cannot find anything but positive comments in the file vis-a-vis RHA. It is the porous nature of RHA that means that it is not a good bet for wall insulation? I am building in a high humidity climate in Japan and one would worry about the RHA absorbing too much moisture out of the air. Is there not a way to manage this correctly to make it a beneficial feature of the material?? With automatic dehumidifiers around the house and breathable walls?? BTW, as many of you doubtless already know, rice hulls have an r-value of 3 per inch according to "the hybrid house", a book by Catherine Wanek, and are often used in walls and roofs. I was under the (misguided?) impression from various other sources that the ash could be of a higher r-value??? Although I can't find a specific R-value anywhere. thanks again,

What does it take to safely use rice hulls in an attic? Some type of fire and bug proofing?

According to this document, untreated rice hulls (not ash) test out as a Type I Class-A insulation material (which in  ASTM C 764 terms means that it's safe to use in open pour applications, no fire retardents or ignition barriers required.) 

They also have almost no nutrient value (to bugs, mold or fungus), so it may not need treatment on that end either. You can't  even compost the stuff, which is why they burn it to be rid of it, ending up with high silica ash to dispose of instead.

The biggest issue with attic applications is the ~9lbsft³ density at ~R3/inch  would yield a  9lbs/ft² attic floor loading to get R36 pour,  which is ~7x the weight of cellulose for that R value.  That shouldn't be as much of a problem in wall applications though.

I would think that for both wall and attic use, a little bit of glue would be a plus. In the attic case, it could add enough tensile strength to transfer load to the joists (vs the drywall). In the wall case, it could prevent settling.

I suspect it would take a LOT of glue for the stuff to be self-supporting in an attic application. The lignin already in rice hulls could be used as a glue, but would probably have to be heat-activated to get it to work.

In walls dense-packing it a bit should pretty much do it, bu tit may not be necessary. The paper indicated it's shippable at 25lbs density or 14.5lb density but is easiliy revived to it's 9lbs natural bulk density. It may well be that the silical/cellulose fiber structure isn't prone to the same kind of creepage & compression as celluose or low-density fiberglass. It probably requires more study.

I'd like to see a more technical discussion of the Technical University of Malaysia "Maerogel" aerogel-type insulator the Michigan Ross School folks developed a business plan around as their entry for the M.I.T. prize, as referred to in sailles' treehugger link in the original post:

ttp://www.treehugger.com/files/2009/05/rice-husk-insulation-makes-fridges-more-efficient.php

http://www.engineeringservicesoutsourcing.com/b/fe/2008/03/high-tech-insulator-maerogel-possibly.html

Clearly that's not raw rice-hull ash, and mixing that stuff in a cement slurry would surely degrade it's thermal performance by a large amount, but in batt or sheet or panel form it could be useful in any number of applications, possibly in building materials. In high-performance window/skylight/daylighting applications could be HUGE. With prior aerogel technology it's a bit pricey, but if this stuff can be produced cheaply, it's a real breakthrough.

I can't imagine that a material only 3x the density of air is going to have much mechanical strength on it's own, so in the refrigerator-insulation example it's probably protected by a mechanically rigid shell structure of some sort. It could be very reasonable sealed between rigid glass in a window app (but probably not so much if mixed into a lightweight cementitious material.)