[GSBN] The Reasoning For Hessien

Graeme North graeme at ecodesign.co.nz
Mon Apr 21 16:54:41 CDT 2014


In the last storm we just had here an experimental strawbale wall that only cops strong storm wind-driven rain, and that I have had up for nearly 10 years, finally came to grief. It had lime plaster over an earth plaster and the lime plaster was shed off the earth plaster, and the earth plaster was shed off the strawbales because it turns out that the straw behind had completely rotted out   =  hooray  -  total failure at last!   But it took quite a surpassingly long while to really come about.

So Sarah -I would have a few concerns - 

1) I suggest that you make sure that you have fantastic weather protection to those walls, esp as you were talking two story building were you not? This is a piece of the puzzle that you have not mentioned so none of us has any idea what is being proposed here.
If the building is really exposed to 100 km per hour wind the question I would ask is if plastered strawbale is the right material at all to be using in this situation?  
If any rain comes with that wind the walls will get periodically hammered so a full rainscreen cladding sounds really really sensible to me.

 2) Can you be sure that the hessian will have good adhesion to the earth plasters below? As already commented on, a clay slip over the hessian is not the right thing to do at all.  From my own experience with earth plastered ceilings over draped hessian, it is really hard to get good penetration of clay through hessian but with a ceiling you can work from both sides so that is fine, but there were significant areas where very little plaster or slip came through the hessian, even when pushed from above, esp with a tighter weave material.  

3) You are right to try to get lime to adhere to the earth with physical keying and this is very necessary.  But the keying you are suggesting has, I would suggest, way too small a diameter hole to push lime plaster into to give you a reliable or strong connection.


Graeme North Architects
49 Matthew Road
Warkworth 0981
New Zealand
tel/fax +64 (0)9 4259305


On 21/04/2014, at 4:44 AM, Sarah Johnston <sol_design at yahoo.com> wrote:

>  Thank you to those who replied to my earlier post.  Here is some background on this project/concept.
> The reason to install the hessien is to stop  the lime from cracking as the body coat dries and the reason to apply the lime while the body coat is still wet is to keep the lime damp in a very dry and windy climate.  It is also very nice to be able to get all the plastering completed at one time rather than having to come back in 6 or 12 months.  While tarps are always used to protect the lime walls, it is very difficult to keep the tarps in place during 100km/hr winds.  Summer is too hot and winter too cold so the window of opportunity is very small for lime application.
> While I have not personally used this technique, a friend who does use it is a true believer.  My concern is delamination in the long run and his experience can not debunk that concern.  
> Does anyone have thoughts on the following......
> The hessien is worked into the face of the body coat, wire brushed to lift the fibers then punch holes through the hessien to create a solid key into the body coat.  Lime would then be applied directly to the hessien and would key into the holes as well as the fine fiber of the hessien.  The hope is the hessien will prevent the cracking and the holes will provide a bomb proof key.  This technique could  help in situations that Derek has pointed out, when you have a lime that does not like to bond to the clay, the hessien and the holes create that key.   Test walls are being prepared.  We will try 10mm dia, 20mm deep @ 75 centers and 10mm dia, 15mm deep @ 50mm centers.  sorry if you do not speak mm, I have mostly used US spelling :)
> This project is being covered by the TV show Grand Designs so the techniques that are used could be very well publicized, thus a double reason to make sure everything works very well! 
> Would love to hear any thoughts!
> Thanks,
> Sven
> Sarah & Sven Johnston 
> Sol Design, Ltd. 
> 50A Connolly Street 
> Geraldine 7930  New Zealand 
> 03 693 7369 
> sol_design at yahoo.com 
> www.soldesign.co.nz
> _______________________________________________
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> GSBN at sustainablesources.com
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