[GSBN] The Reasoning For Hessian

[Sarah Johnston]

Thank you to EJ, Derek and Ian for your responses.  I must say the history lesson was not quite what I had in mind Derek but as per usual, very insightful!  Thank you for the Goat idea, I have some friends with goats so perhaps we can organize one or two to be offered to the ??  should we know which gods we are offering them to?  Goat blood would be great on the earthen floors I am sure, kill two birds with one stone...?  I will even try the US spelling (or perhaps the correct spelling) for hessian.  

To set the record straight, this lime over hessian (no clay slip) over wet plaster concept is not trying to replace tarps or misting, it is meant to be something which will help in the event the tarps blow down at the least convenient moment. Or are blown down in an event which thankfully we do not have any experience with in NZ (bombing).  This whole idea was founded upon an old article in the Last Straw Journal which is where I thought the history lesson may go but the lack of shared positive history here speaks pretty loudly as to how widely employed the technique has become.  

I share your concerns, Derek, with regard to trying to punch the holes and keep the hessian well keyed at the same time, Thus the test walls which my friend (the believer in the hessian technique) is constructing with the hole punching owner builder (wanting me to write the specification for the plaster system he will be installing on his own home).  I will be a physically (but not legally) safe four hour drive from all of the action.

To make things more challenging, the walls of this home are nearly all curved and the roof is also curved, creating a pretty dramatic compound curve to attach tarps to. The site is in a dry wind tunnel next to the Clutha river in Central Otago, South Island, NZ...  This is the third project we have been involved with in a 20km stretch of this river.

If anyone has suggestions of what we could do in these conditions to aid in a 'bomb proof' plaster system, I am 'all ears'. (suggestions do not need to include hessian :)


Cheers, 
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


>________________________________
> From: Derek Stearns Roff <derek at unm.edu>
>To: Sarah Johnston <sol_design at yahoo.com>; Global Straw Building Network <GSBN at sustainablesources.com> 
>Sent: Monday, 21 April 2014 8:02 AM
>Subject: Re: [GSBN] The Reasoning For Hessien
> 
>
>
>Hi, Sven,
>
>
>I’m concerned about the phrase:
>
>
>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. 
>
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>Some historians would tell us that “the reason that the Greeks sacrificed goats before the building of a temple, was to protect the structure from earthquakes, lightening, and other natural and godly risks.”  
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>I fear that both of the asserted “reasons" above have similarly large appeals to human psychology, quite separate from the amount of support from physics.  The “reason” (motivation) for our making a choice reflects our hopes, knowledge, experiences, and fantasies.  The “reason” (cause and effect) that a technique works or fails in a particular building reflects the physics and conditions involved.  Our motivation may or may not overlap the physics, but they are seldom identical.  
>
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>Returning to the “reason” given for using the hessien (hessian), the hope is that it will “stop the lime from cracking as the body coat dries”.  So let’s think about how congruent this hope might be with what we know about the materials.  The clay body-coat of plaster will still shrink as it dries, and the hessian can't stop the shrinkage.  Hence, the hessian must either move and shrink with the body coat, or move independently of it.  Each of these possibilities increases the chance of delamination, between some layers of the plaster system, and there will be some stresses between the layers in any case.  Similarly, the slip coat on top of the hessian will want to shrink as it dries, and will either move with the hessian, or not.  In the version described below, it sounds like we have eliminated the clay slip coat over the hessian described previously, and gone directly to the lime plaster.  
>
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>Next we apply the lime over the clay-soaked hessian, or else over a well-keyed, fairly wet clay slip layer, which shows our optimism.  Wet clay plaster is very soft.  The maximum possible strength of a physical key in wet clay is very small.  I have no idea about the bond likely between lime and clay-soaked hessian.  We now have a thick body coat of clay plaster, still drying, a layer of damp, clay-impregnated hessian, perhaps a layer of damp clay slip, and a layer of wet lime plaster.  All drying and moving at different rates.  But we plan for them all to cure strong and well-bonded to each other.  
>
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>Further, we believe that the water in the clay layers will make a substantial contribution to the moisture curing of the lime plaster exposed to the "100km/hr winds" of "a dry and windy climate."  During this cure, the moisture in the clay is entirely under the lime layer.  The wind and dry air are entirely outside the lime layer.  So we would expect a significant moisture gradient across the thickness of the lime layer.  The lime layer will likely be pretty stiff within a couple of days, while the clay layers will take a good bit longer to dry.  
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>The suggestion is made to punch holes in the hessian after it is applied to the clay body-coat, to “provide a bomb proof key.”  I assume that this reflects, felicitously, the very limited New Zealand experience with bombs.  I’m unconvinced that we can get a strong, reliable bond this way.  I’d like to learn how you would go about punching holes in the hessian over wet clay, without distressing the hessian/clay bond.  
>
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>I started the next to last paragraph with the words, “we believe”, but in fact, this system has too many unknowns and assumptions for me to believe in it, without very specific testing.  I am well aware that Sven also has his doubts about the proposals, and is trying to find a way to make things work under difficult conditions.  I have great respect for Sven, and his experience.  I fear that if he allows himself to get pushed into using this plan built on someone else’s fantasies, it could backfire very badly on him.  Putting your reputation on the line, using an untested system invented by someone else (conjectured would be a better word), is very risky.  Be careful, Sven.  
>
>
>Best wishes,
>Derek
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>
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>On Apr 20, 2014, at 12:44 PM, 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.
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>>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.  
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>>Does anyone have thoughts on the following......
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>>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 :)
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>>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! 
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>>Would love to hear any thoughts!
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>>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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>
>Derek Roff
>derek at unm.edu
>
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>
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