Building a Sustainable Home? Consider Using an Earth Render

Consider a strawbale house rendered with clay or earth. They may seem to be the domain of the alternative but strawbale houses are becoming more mainstream because of their longevity, insulation and the materials are readily available and sustainable.

For Greenies recently went to a clay render workshop in Conondale, Queensland Australia led by Nick Dunn from strawbalehouse.com.au

Watch the video to learn how easy, inexpensive and safe it is to render a strawbale house with clay/earth render.

Video transcript:

Nick Dunn: In my opinion strawbale is a great medium to build in, in terms of the speed that the walls can go up, the insulation value and the look of them.

We are going to apply a straw/clay plaster to the inside.

Straw is hydrophilic, it attracts water, cement is less hydrophilic than straw, which is why we put our strawbales on a bottom plate, they act like a big sponge. Cement through capillary action, because its full of tiny holes will actually suck water up and where strawbales are there is so much heat and if you imagine it, that will suck water up into the bales as well, because the straw is more hydrophilic. Clay and lime is actually more hydrophilic than straw. Clay and lime want water more than straw does. They’ll actually draw water out of a bale towards the render, and slowly evaporate it out that way so if you’ve got any moisture in your bale that will do that. If you cement render it, through that same capillary action it will go in towards the straw but because the straw is more hydrophilic than the cement it won’t actually let it get back out. #00:01:32-6#

Clay Slip #00:01:35-8#

So what we are going to do first up is apply a clay slip which is just a pure clay. #00:01:42-6#

Get it as fine as you possibly can, sieve it through a sieve to get a good clay slip otherwise you end up with lots of chunks which makes it harder to work. So basically the way to test a good clay slip is to dip your finger in it and if it hangs onto your finger like that then thats a good clay slip. #00:02:04-8#

What the clay slip’s doing is it’s grinding clay into the straw so you get a brush and you drag it up and down on the straw and it grinds that clay water into the straw which dries quite quickly. As clay wants to find clay it goes rock hard it wants to bind to itself. Each time you put another layer on you add more clay slip, it’s basically like a glue between layers and stops an ‘onioning’ effect. #00:02:29-8#

Can you see there’s a hole there I can stick my hand into? If there’s one there sure enough there will be one down there too because thats a corner of a bale. What I want to do is get a bunch of fine-ish kind of straw, dunk it into the clay slip, ball it up and shove that in. So fill that up until it’s about an inch away from the surface of the straw and then grab a couple of handfuls of this (render) and fill those holes up. Do that as you are applying your clay slip as that is when you will see them. #00:03:09-6#

Earth plaster is the same as any other rendering material- you’ve got a binding agent, same as cement if you are doing a cement render, and thats your clay, so clay is your binding agent. You’ve got an aggregate which is the same as any other cement- sand, and you’ve also got this fibre. Having this straw in there acts like a fibereglass and goes all sorts of ways and eliminates the cracking. If you work alot of renderers mesh into it, it creates a barrier that all that straw at that point stops and starts again on that side of the mesh so you can’t actually get that keying in of the straw. Thats why we don’t use that and it’s quite expensive. Whereas on the outside where we are liming we will use that because it doesn’t have the straw in it. #00:03:58-2#

The Base Coat- Straw Clay Plaster 20-25mm thick #00:03:58-2#

I generally put in around 3/4 bucket of water into this brew, so tip that in first. Then 1 1/2 buckets of clay, and 1 bucket of 3mm sand, turn the mixer on max, and you can see every now and then you’ll get a clumpy bit come to the top, if you just jump on top of it with the mixer it will come out. Just put a handful of straw into the bottom of the bucket and get the whipper snipper on it, just make sure you don’t get any baling twine into it that would be unfortunate. So 1 1/2 of straw into the mix and again you don’t have to be too exact with the straw it’s not going to hurt if there is a tiny bit more in there. #00:05:27-3#

The other way you can mix is 4 people stomping around in there until you can’t see any straw. #00:05:36-9#manually mixing clay render

Workshop Participant: The Zulu make the corrals in South Africa and Kenya out of cow dung and straw. I can categorically state there are no cow pats in this render. #00:05:54-5#

Nick Dunn: We grind it in with the heel of our hand first, and that makes sure that you get a nice bind into the straw. This is what we call a scratch coat or a discovery coat. Generally work from the bottom up so that each layer of render has something to sit on and then I can just force that up onto there and start to build it out. We build that out about 20mm. You can see I have a leading edge now, I’m ready to put plaster on there, so I force that upwards first bring it down to fill that up and then because that was my governing point I bring that across so its nice and straight and that’s my new governing point there so I apply to there and work my way backwards to the place I’m already happy with. #00:06:46-2#

Workshop Participant: Feels so nice to work with .#00:06:54-1#

Workshop Participant: Where else do I get to play with mud pies? #00:07:00-2#

Nick Dunn: The beauty about clay render, more so than lime and cement render, it’s so easy to fix, it’s so malleable and really easy to wet down and re-apply, that it’s a really great way to render your home. #00:07:17-6#

They’re just drying cracks and thats a good sign. It means you have a good quality clay because it’s doing that and that helps key in the next coat as well. See that crack and that crack and probably that crack? The reason its doing that because a hole hasn’t been filled enough. Just belt that in you can see it disappearing in there a little bit, thats filling up the hole now, that render will still be wet in behind it so tap it in until it won’t go in any further and fill that fella up, you’d never know it was there!

2 Responses

  • Chris Parker
    January 17, 2013 at 6:47 pm

    Hi Nick! And all the people that also took part in the above video. My name is Chris and I’m super pumped about the prospect of building my own straw bale home for my family in the not too distant future. I really love this video and have watched it many times. I’m living in Japan at the moment, but I’m moving back home to Aus soon and I’m keen to take part in any workshops or projects you need volunteer labour for in exchange for getting some experience in this technique. I really believe in this green way of living and I think it’s the way if the future for everyone to build that lives in the right climate.
    I was wondering if you could give me a ball park figure of how much a house similar to one in the video would coast to make from scratch?

    If you do have any projects coming up, then feel free to drop me a line and I’ll do my best to make myself available.

    Keep up the good work!

    Kind regards Chris Parker n family.

  • May 18, 2012 at 4:25 pm

    Hey there..!!I saw the video and I must say I found so many useful information related to this so thank you for sharing this with us.

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