[GSBN] [Cob] Lime Render - FAILURE

Graeme North graeme at ecodesign.co.nz
Sun Feb 26 17:38:56 CST 2017


HI Rikki - yes your first bold comment is right - I was was assessing risk as a professional designer.  Its being confident of the result that makes professional design or building trickier.

What people are prepared to put up with, or try out for themselves, is just fine by me.  

Over clay/lime mixes might also be tricky - I have done a small bit of work with trying to stabilise some clays with lime in the past and found it really tricky to get a good reliable result in many instances.  
Fragility of the material can indeed be one common result, when some or all of the clay is neutralised and there is not enough binder - either clay or lime - left in the mix. 
The less lime used the worse this can sometimes be - and the same comment applies to the use of the cement (there - I used the word) in earth.
 
Again - I think if you know your particular materials and it works well - fine - but as unqualified general advice it has its problems, and risks.

Graeme



> On 25/02/2017, at 4:31 AM, Rikki Nitzkin <rikkinitzkin at gmail.com> wrote:
> 
> Hi.
> 
> I am in agreement, generally, with Graeme. But I would modify his comments in this sense:
> 
> If you are professional builder, responding to a client, If you finish with lime, start with lime. The same with clay. It is too “risky” to apply lime over clay in a job you are paid to do.
> 
> However, I encourage self-builders to experiment (warning them first that there is a chance that it won’t work and they will have to fix it later). It is much more fun, and less expensive, to plaster with clay. It is also easier to use correctly (I know this is debatable…). I have had no problem with attaching lime-stabilized plaster (usually 5-10% lime added) to a clay sub-strate. And unless you are in an area with heavy hail, this is often enough to protect the walls. If with time it is not enough, it can be plastered over with lime or cladded.
> 
>  I haven’t heard of many failures of lime over clay-lime. Has anyone on this list had problems with this?
> 
> That aside, I am not totally convinced with stabilizing clay with lime. I find that when you trowel it, you bring the lime to the surface, and sometimes the surface becomes fragile - breaking when hit by hail or kids! I have had great experiences stabilizing clay (making it water resistant) with 4-5% linseed oil and/or lots of cow dung. For most walls this is enough. If the wall is exposed to driving rain, use cladding!
> 
> take care, Rikki
> 
>> El 23/2/2017, a las 23:37, Graeme North <graeme at ecodesign.co.nz <mailto:graeme at ecodesign.co.nz>> escribió:
>> 
>> Hi All
>> 
>> I have had time to look at the images and now leap in - sorry but I have been away and just seen the discussion.
>> 
>> Again the thorny issue of lime plasters over earthen substrates.  
>> As we know,  sometimes it works and sometimes it does not, and there are so many variables it is hard to be specific about predicting success or attributing any one reason for failure.
>> 
>> Really good keying is part of the issue, but the variability in expansion rates of different clays makes this a difficult area to work in unless you already know your materials and their behaviour really well and know what happens when the wall gets wet.  
>> 
>> I have heard of failure even from high humidities in otherwise sheltered walls too, so my cautionary side now leads me to believe that putting lime plasters over earthen plasters on strawbales it often too risky to be bothered with - I go for either lime or earth (or a cladding) all the way.
>> 
>> With an earthen substrate like cob, or mud brick, then there does need to be really savage keying, but also be aware that if you are relying on the waterproofing properties of the lime plaster to protect the earth materials then there is probably a high-risk design strategy already at work.  There is nearly always going to be a risk of moisture getting in behind the lime plasters and expanding the clay, leading to delamination, unless - once again - you already know your materials and their behaviour really well, and especially so if the clay should get wet.
>> 
>> As for a magic coating to fix this issue - there simply isn’t one that is reliable in my opinion. I have seen far too many failures of all sorts of "magic goo" coatings that have simply not worked or else trapped water behind them and caused real trouble. 
>> 
>> What does work is a design that builds in rain deflection either from roof overhangs or additional cladding.  As I have said before - in areas of risk of rain wetting of moisture sensitive materials, think of it this way - either take the roof out (more or less) horizontally to a degree required by the site exposure and local weather conditions, or run the “roof” i.e. cladding, down the face of the wall.
>> 
>> As the climate changes and the risk of severe storm events increases, this becomes even more important to take heed of.
>> 
>> Fascinating stuff.
>> 
>> Cheers
>> 
>> Graeme (I wish there was magic)
>> 
>> from wet windy and humid New Zealand
>> 
>> And I have just been to look at earth and strawbale buildings damaged in a recent 7.8M earthquake here. 
>> Interesting and scary stuff too - more to come, but generally modern earth and strawbale buildings did well.  
>> 
>> 
>> 
>> 
>>> On 19/02/2017, at 3:57 AM, Deborah Terreson <foodandart at comcast.net <mailto:foodandart at comcast.net>> wrote:
>>> 
>>>> On Feb 17, 2017, at 8:50 PM, Bill Wright wrote:
>>> 
>>>> ...  Any suggestions on a clear, water resistant coating I might apply OVER the exterior?
>>> 
>>> 
>>> Bill, 
>>> 
>>> I've had decent luck using LastiSeal on old crumbling brick homes on the Maine coast. The stuff isn't cheap, but seems to do the trick stopping both the brick and old lime mortar from worsening - for sure it holds back the efflorescence when applied after a cleaning.
>>> 
>>> You could always get in touch with the manufacturer and inquire if they think it will work on your render. 
>>> 
>>> http://www.radonseal.com/concrete-sealers/lastiseal.htm <http://www.radonseal.com/concrete-sealers/lastiseal.htm>
>>> 
>>> Deb.
>>> 
>>> 
>>> On Feb 17, 2017, at 8:50 PM, Bill Wright wrote:
>>> 
>>>> 
>>>> 
>>>>> On Feb 17, 2017, at 3:03 PM, Shannon Dealy <dealy at deatech.com> wrote:
>>>>> 
>>>>> 
>>>>> [snip]
>>>>>> The images show the cracking of a lime render applied less than (1) year ago over an earthen brown coat. The Brown coat was 3 parts sand to 1 part - 90% indigenous site clay and 10% lime (NHL 3.5). The final lime render was 2.5 parts sand to 1 part - 90% lime (NHL 2.0) and 10% indigenous site clay. A pigment from Trans-Mineral USA was added for color to the recommended acceptable amounts according to the manufacturer of the pigment. (I think TM USA was the source, I know we got our lime from them - St. Austier NHL).
>>>>> 
>>>>> Hi Bill,
>>>>> 
>>>>> Based on what I am able to see, it appears to me that pictures 1, 3 and 4 are settling cracks. Number one could be due to foundation issues in that area, or issues with how the wall was constructed near that area. It doesn't look particularly bad given the amount of time that has passed, so it may be that everything has stabilized. Pictures 3 and 4 appear to be due to shifting/settling/shrinking around embedded wood (door/window lintels and embedded log - I'm assuming something is embedded in the cob above the door) note that the cracks originate and end at these points.
>>>>> This is quite common if the wall and wood haven't had a long time to cure around these areas, since wood and cob shrink very differently as they dry.
>>>>> 
>>>>> I do not believe that any of these cracks are due to rainfall issues as the moisture from rainfall heavily favors the base of the wall and these cracks are not preferentially occuring at the base of the wall. Why is water more of an issue at the base?
>>>>> 
>>>>> - The upper wall is better protected by roof overhangs
>>>>> - The upper wall receives greater airflow so it dries much faster
>>>>> - rain splash from water hitting the ground and rebounding onto the
>>>>> building is limited to the first couple of feet
>>>>> - rain splash from plants next to the building can be higher up,
>>>>> but the plants themselves are generally mostly at the base
>>>>> - the humidity is higher closer to the ground (where the water from
>>>>> recent rainfall is), slowing the drying at the base
>>>>> 
>>>>> Additionally, the finish render mix you describe should be highly resistant to water. If you have water issues behind this mix, I think it would be more likely to come from moisture within the house working its way through the walls (breathing, cooking, showering, ... give off a lot of water, I've discussed this issue previously), though I think it is unlikely given where you live.
>>>>> 
>>>>> I've seen and worked with similar exterior plasters on a number of buildings in Oregon with 40+ inches of rain each year. Some had problems, but it was not due to the rainfall in any case I am aware of.
>>>>> 
>>>>> The only picture that causes me any concern is number two and my question would be what (if anything) is embedded in the wall in that area as well as what is on the other side of the wall? Embedded wood, plumbing, shower on the other side, etc. Even this area is not a major concern to me unless the render starts flaking off (at which point you may be better able to assess what is going on behind it).
>>>>> 
>>>>> 
>>>>>> A. I'll bet my bottom dollar the County will not pass my building w/ cracks in the render - big ouch!
>>>>> 
>>>>> For this issue, I would simply make a small batch of plaster mix, matching as exactly as possible the original mix you used. Wet the area around the cracks, then fill them in with an excess of the new mix and refloat the surface of the wall around the crack to blend it in. I would start with the crack in the least visible area and get your technique down before doing the others. It is important to note here, that the color will not match for quite some time, even if your mix is exactly correct as the area needs to reach the same level of moisture as the surrounding wall, and the lime needs to react with CO2 in the air for a time to catch up with the state of the rest of the wall. I don't recall how long it took for my last patch job (jar window broke) to match the rest of the wall, but I think it was measured in months.
>>>>> 
>>>>> 
>>>>>> B. The process of water expanding the clay underneath is progressing to the point that the lime is delaminating and falling off in certain areas. While this is not really a huge structural issue, it's a bummer, and it's unsightly to say the least. Over time, it could start to wear down the cob/earthen layer beneath it, which will create more work to fix it.
>>>>>> 
>>>>>> So, my question is this. . . What would you do next to remedy this situation and create a functioning wall given my current situation?
>>>>> 
>>>>> None of your pictures show an area where the lime is actually falling off, so it is hard to say for that case. One of the problems that cracks can cause once they are present (due to settling or other issues) is they can allow water a pathway into the wall, and in some cases, even channel the water in. At this point, the greatest danger is usually from freezing as this causes the trapped moisture to expand and forcefully separate the plaster from the wall. If you have freezing temperatures in your area, I think this is more likely than water expanding the clay in the walls.
>>>>> 
>>>>> FWIW.
>>>>> 
>>>>> Shannon C. Dealy               |       DeaTech Research Inc.
>>>>> dealy at deatech.com              | Biotechnology Development Services
>>>>> Telephone USA: +1 541-929-4089 |      USA and the Netherlands
>>>>> Netherlands:   +31 85 208 5570 |          www.deatech.com
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