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Fiber options for Cob

 
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Hi,

We are wanting to build with Cob in hawaii... but straw is not something produced here.

I am wondering if coconut coir may be a suitable structural binder for the cob in place of straw? I've found articles using coir as a reinforcement for concrete but have found nothing for use in cob... When used in concrete it had to be soaked (sealed) with oil first. do you think this would be viable for a lasting construction component? Ideally we could use the cob structurally but we are open to doing a frame and using it for wall infill only if need be.

Any thoughts appreciated! Mahalo!

'Ilima
 
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Posts: 3875
Location: latitude 47 N.W. montana zone 6A
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Hi Ilima;  Welcome to Permies!

Let me start by saying I am no cob expert at all.
I have made a fair amount of it over the years.
My knowledge of cob revolves around rocket mass heaters.  
Straw is recommended (but not required ) to be used in them as a binder and by being hollow also as an insulator.
Horse poop , horse hair , plain hay . Quite a few things have been tossed in the mix, when straw was not available.

I think that coconut coir would work fine as a structural binder in a wall building situation.
 
pollinator
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Location: Bendigo , Australia
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I have lots of experience with cob.
I can say post and beam, even with steel, is a better way to build.
And use cob as infill panels.

You get a good roof to work under, collect water and store things.
I suggest you make a few trial blocks and see what happens.
Try some with unsoaked coconut and some with soaked fibre.
Then let them dry for a many weeks and test them.
Earth Blocks may be a better idea, they use less water during manufacture and do not need the straw etc to hold them together.
Look for Compressed earth blocks.
Ask more questions if you feel the need.
 
pollinator
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Location: Ashhurst New Zealand
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If you're doing a frame then you might want to have a play with light straw (light coir?) as infill. Basically, it's fiber impregnated with a runny earth mix (slip) and then pressed into forms in place. Strong, great insulation and sound deadening, decent thermal mass, and easy to coat with earth or lime plaster for a finish.
 
Ilima Smallwood
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Aloha John C. Daley 0 and thank you for sharing your thoughts. Steel frames become expensive here - mostly because they have to to be galvanized on the mainland and shipped over and also because of the size of the footings they require.
Is there any concern with using cob with an untreated wood frame? Typically in conventional construction I would use treated wood for the base plates only. I know that cob can absorb moisture and that's actually one of the things I think that would be great for our climate as conventional construction can have lots of mold issues here.
I guess I'm concerned that if the walls absorbed too much moisture it could over time rot out the posts that are embedded within the walls. We would use a natural plaster finish and have really wide overhangs (on average 6')so hopefully they wouldn't absorb too much water but with 98% humidity as a very normal everyday reality I want to plan for health and longevity.

Coconut coir does not tend to mold in nature at all, so hopefully it will prove to hold up the same in an earthen mix. I will try to do some samples of both cob and maybe a 'light coir wall.' Might anyone be able to point me in a good direction for the light straw wall methods and instructions? That's one I haven't done before.

Mahalo, 'Ilima
 
Phil Stevens
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For light straw infill, check out these for starters. On the top left there's a video on slip straw construction, and on the top right they have one on mixing the slip and adding the straw:

https://strawclaywood.com/videos/

Same guy (Michael Smith) in a Q&A about the method:

http://www.greenhomebuilding.com/QandA/cob/strawclay.htm

Another video with some context in the comments:

http://www.naturalbuildingblog.com/using-light-straw-clay-slip-straw-as-interior-wall-fill/

Some presentation graphics showing the process:

http://endeavourcentre.org/2016/04/light-clay-straw-insulation/
 
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Location: Ladakh, Indian Himalayas at 10,500 feet, zone 5
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I second the person above who suggested that you make some trial blocks, dry them, and then kick, drop, rub and soak them to see how they do. That's how we started building with rammed earth at our school, and it worked great. We didn't even realise that the stuff we were calling clay was technically a fine silt. It worked great for construction.

In our rammed earth we didn't use any straw or other biomass, because none was available at all in our barren desert region, and for rammed earth it was not a problem. We are still using the same technique 25 years later. As others mentioned above, if you have a frame of any kind, maybe wood, and if the earthen material will only be infill, then you can be more experimental and not worry much about strength. But make some blocks with different proportions of materials you might have access to, and test them.
 
John C Daley
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Normally the steel does not need galvanising if it is not exposed to the elements.
I am surprised you say there needs to be large footing? What size?
The wall foundation would consist of a concrete strip footing perhaps 15 inches wide and 12 inches deep on original ground and not filled ground.
There would be no timber used at all except for window and door frames, which could be steel if you wanted.
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