what's the best length for rice straw/ especially for slip clay walls. Conflicting advice!

I'm going  to be renovating an old house in Japan. I want to add extra insulation using strawclay infill  for the external walls, and probably internal too.  However, I don't know how long the straw should be.  I keep finding different sites have different advice, some say long, some short.

I assume short will be easier to work with, and coat with clay, especially if I use mechanical means, i.e. some kind of tumbler, rather than tossing with a pitchfork.

I would also assume that long would create a stronger wall as the fibers would interlock more.

I'm getting the rice straw free from an acquaintance. But, harvest time is very soon so I need to tell them what I want.Their mini combine harvester can cut the straw to various lengths.  Is there much difference between long and short. As I'll probably have a lot, would long or short be more useful for other construction purposes, that I haven't thought of? What would be the ideal length for rice straw? Thanks in advance everyone.

hey bob.... welcome to the forums.

from what i ve read, longer is better. it has only a thin coat of clay as a binder, so i think that you need long straws to lock into each other. this is why they often compress it with a very thin peace of wood. to kinda "weave" the straws together. and i assume that short straws will stick together. you ll probably end up with a pulp that hardens into a light-clay-brick. something more like cob than like straw-light-clay.
it may work. but it ll probably loose some insulation-value.

if you have enough time, make different batches with different lenght for testing purposes...
good luck

Thanks for the reply. Longer may result in stronger walls than short. But how much difference and what is "long", or short for that matter. I just can't seem to find any sites that have actually tested different lengths. What would be optimum length?
As for r values, I think you could get similar r values with long or short. Probably with short you would have to be careful about over doing the compression. Shorter  straw would probably compress easier than long.  But this is all conjecture.

I know that for plasters short is better so I want some short for that.

My main dilemma is that once I receive the straw, that's pretty much how I'll have to use it as most of the labour will be me alone so I can't afford to be spending time chopping it if I ask for the wrong length. And of course I can't uncut the straw.

On this site  
http://endeavourcentre.org/2016/04/light-clay-straw-insulation/  
they say "chopped straw, ideally 1-6 inches"

On this site http://mudstrawlove.com/natural-building-101/
They say "long".
Unfortunately I won't have time to do any tests until after delivery.
I'm finding similar advocates for both on other sites too. Hmm, what to do...

i m totally not experienced but i had the same questions.

i think, if you go with very short straw, you will loose r-value because it packs more tightly. it will go more into the direction of one big (more solid) brick. straw light clay is more like a slightly compressed strawbale bound together with a very small amount of clay. which means lots of airspaces.

i think, LONG will mean 12-16 inches? i ve seen several videos... the straw seems that long.

best would be to do a test batch. and maybe quick-dry it with a hait dryer and fan...

for plaster i think, it s VERY short. 1-2 inches for base layer and 1/4 to 1/2 inch for second layer?

but for your case... how long would that straw normally be when it comes out of the harvester? i m pretty sure that all "normal straw" will work. i think, you can t use the same lenght for SLC and plaster.
you can shred your straw. a leaf-sucker with shredder and bag would work best. or a garden shredder with knives like a kitchen blender. or you could try to run over the straw with a lawn mower. or you can make a bucket with a hole in the lid, then weld blades unto a rod. put that through the hole. fill bucket with straw. close lid. and use electric drill to turn the rod with the knives.
then screen it to desired lenght...

As far as I've been told they can have it come out of the combine at any length.  They want me to come and meet them and tell them what size I'd like  them to cut it.  They normally cut it short and leave it in the field as it's a "waste" product. I'll probably get a variety of lengths.
Because  I can have any length I like, naturally I'd rather know before hand and save myself extra work and expense in the future.
As for plaster, yes smaller is better I've heard, under an inch is better for rough.
For topcoat one good thing about Japan is that clay and lime plasterer are still quite common, so I can buy clay plasters, lime plasters, whatever I need relatively easily.
Hopefully someone with experience whill chime in sometime soon and educate me in the finer points..

Ideally you would have a "scratch coat" of long straws with a finish coat of short straws for smoothness.

The long straws will add better insulation value as well as a coat with "bite" to hold the finish coat better and tighter to the wall.

Hope that helps you.

Redhawk

another thought: will you get the straw baled? if not, long straw should be easier to pick up by hand or with a pitchfork