[GSBN] The Reasoning For Hessien

Derek Stearns Roff derek at unm.edu
Sun Apr 20 15:02:22 CDT 2014

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.

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.”

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.

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.

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.

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.

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.

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,

On Apr 20, 2014, at 12:44 PM, Sarah Johnston <sol_design at yahoo.com<mailto: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!


Sarah & Sven Johnston
Sol Design, Ltd.
50A Connolly Street
Geraldine 7930  New Zealand
03 693 7369
sol_design at yahoo.com<mailto:sol_design at yahoo.com>
GSBN mailing list
GSBN at sustainablesources.com<mailto:GSBN at sustainablesources.com>

Derek Roff
derek at unm.edu<mailto:derek at unm.edu>

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