[GSBN] [Cob] Lime Render - FAILURE

Misha Rauchwerger misha.rauchwerger at gmail.com
Fri Feb 24 13:32:23 CST 2017


I have seen lime over cob fail on multiple occasions (especially cob ovens
and benches), and especially when exposed to the weather, or movement due
to settling (no continuous footings).  In most cases, the cob substrate was
not rough enough to create a good key-in.  Earthen plaster over cob, or
scratch/brown/finish all of lime do seem more effective in those
situations, and good mechanical connection is imperative.

Natural Hydraulic Lime as  brown and finish coats of ample thickness (1/2"+
1/8") have been very successful coupled with a heavily scratched base coat
of cob plaster on straw bales with adequate eaves to reduce wind driven
rain on the exterior render.  We have applied a lime wash to the scratch
coat in preparation for subsequent lime coats, but I don't know how
necessary or beneficial it really was.

Misha Rauchwerger

On Fri, Feb 24, 2017 at 10:12 AM, Ian Redfern <ian at adobesouth.co.nz> wrote:

> Good morning All,
>
> I concur with the comments below 100 %  and suggest  :
>
> 1   primary protection is essential – no excuses or magic bullets will
> overcome this
>
> 2  a full build up of lime plasters (30 mm + thick in total) over earthen
> substrates MAY work provided these are well keyed into the earthen
> substrate = really rough and a pocked surface – use a rough rubble/sharp
> gritty mix in the  earthen plaster (all coats)
>
> 3  the base coat of lime plaster is reinforced with fibre and a good
> dollop of fresh organic cow poo (faecal matter from other animals doesnot
> seem to have the same “glue” or roughage )  and that the animals are grass
> fed NOT fed from maise feedlot tucker. The scratch coat needs to be well
> hand massaged into the surface, left rough and cured slowly.
>  We now add a small quantity of paper pulp (not shredded newsprint) to
> even out the drying. This seems to be a partial substitute for the roughage
> in the cow poo
>
> 4  then apply 5 coats of traditional white wash (1 per day)  made from
> traditional mature lime putty
>
> 5  as said in previous comments  “keep it simple  and stick to one plaster
> type throughout” is best
>
> These are my humble comments after “experimenting” here at Adobe Cottage
> with a wide variety of exposure conditions,  and those of our
> owner-biuilder clients (about 100) in these  sub-tropical wet (2400 + ie 8’
> of rainfall) often as huricane force driving rain on a very exposed site in
> the northern part of New Zealand
>
>  Regards
>
> Ian
>
> Ps   I have not seen the photos so am unable to comment on the specifics,
>  but many coats of whitewash cures cracks in lime plaster
>
> Pps  the freeze/thaw phenomena should be investigated first  cos this
> action splits rocks !
>
> [image: Description: Macintosh HD:Users:ianredfern:Desktop:logo2.jpg]*www.adobesouth.co.nz
> <http://www.adobesouth.co.nz>*
>
> Ian Redfern
> *Adobe South*
> *A:*    5 Lancewood Rise, Onerahi, Whangarei
> *P:*     09 436 4040      *M:* 027 490 2324
> *E:*     ian at adobesouth.co.nz
>
> From: Gsbn <gsbn-bounces at sustainablesources.com> on behalf of Rikki
> Nitzkin <rikkinitzkin at gmail.com>
> Reply-To: Global Straw Building Network <gsbn at sustainablesources.com>
> Date: Saturday, February 25, 2017 at 4:31 AM
> To: Global Straw Building Network <gsbn at sustainablesources.com>
> Subject: Re: [GSBN] [Cob] Lime Render - FAILURE
>
> 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> 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>
> 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
>
> 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 <(541)%20929-4089> |      USA and the
> Netherlands
> Netherlands:   +31 85 208 5570 <+31%2085%20208%205570> |
> www.deatech.com
>
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