[GSBN] Lime plaster and expansion joints

Tue Feb 10 16:56:19 UTC 2009

Interesting discussion to say the least.  I've got a couple of things  
to add as well as a few questions.
First of all I've never seen a great difference between the two when  
it comes cracking with either one.
I think it the conditions for cracking are present it's going to  
happen to either one.  On paper lime may be more flexible than a lime/ 
cement mix, but again, in the field I've never seen much difference.

I do have a question as to why the preference for lime by itself on  
the exterior versus lime/cement.  The embodied energy isn't that  
different between the two, I suppose you could make a case for  
hydraulic lime, but then it's gotta be brought across the ocean and  
it's not exactly what you'd call cheap.  Lime became popular in recent  
times with many in the conservation field because it did well with  
masonry buildings and mass walls.  I think it's important to note that  
straw bale walls are a different creature and the water absorption  
that the mass walls can tolerate is not a good thing for straw bale  
walls.  Therefore the need to reduce water absorption in both  
plasters.  And if one decides to go with something like siloxane, just  
watch the guy behind the sales counter roll his eyes when he looks at  
the price.  Don't get me wrong, I love lime and think there are many  
places to use it on a building, I'm just not quite so sure that it is  
all that advantageous on the exterior of the building over lime/cement.

That being said I've always thought the use of expansion joints a good  
thing and like some of you, I've never seen anyone propose a good way  
of executing them with straw bale walls.  I know Rob Tom is a fan so I  
propose he and some other willing individuals come up with a way to do  
it that the rest of us mortals haven't thought of yet.

I think that detailing the interface between the structure of the  
building and the bales/plaster is something that we could greatly  
improve.  Unquestionably, isolating the frame from the plaster makes a  
lot of sense when the structural elements are to the exterior of the  
wall. I have also found that additional layers of reinforcement or  
mesh embedded in the subsequent coats also to be of help.  I faced a  
similar problem last year when our crew from Obregon, Mexico did a  
good sized building on a ranch along the border.  It had steel posts  
to the exterior of the wall.  As we often do, mostly with good  
results, we left much of the plastering strategy to the Mexicans.   
Exterior plaster was lime/cement, the cavities and spaces around the  
posts were filled with straw/clay and then their choice of wire  
reinforcement was remesh.  The short of it is that there is not even a  
hairline crack in the finish.  I have no point to make here, mostly  
I'm offering it as additional data for whatever it's worth.

As for straw bale buildings in general I've always been amazed how  
prevalent cracking can be with cementitious coats.  Sometime back we  
did some experiments because I got to thinking that one of the major  
causes was the highly uneven thickness that can happen with the  
scratch coat. I don't think it would be an exaggeration to say that in  
some cases, depending upon the joints between the bales, the way the  
bales have been stacked, etc. etc. the thickness of the scratch coat  
can range anywhere from 1 to 4 inches making it highly susceptible to  
cracking.  I mean you would never do that with any other type of wall,  
at least I wouldn't.  Therefore, we adopted the strategy of filling  
out all cavities and low spots in the wall so that the base or scratch  
coat was of similar thickness.  That has always worked well for us and  
was what our Mexican friends did on the building I mentioned above.

Although not related to lime or cement finishes I'll throw in an  
additional note about how we've eliminated the cracking problem with  
earth plasters.  I've not seen any type of crack in our earth finishes  
for as long as we have been adding very high amounts of straw to the  
plasters.  Since the mix has to be very high clay other detailing has  
to happen for the shrinkage that results, but posts and the like pose  
no problem whatsoever in terms of needing additional detailing.

I think that's enough for now.

Bill
On Feb 10, 2009, at 8:56 AM, Laura Bartels wrote:

> Thanks for everyone who has chimed in on this expansion joint with  
> lime question. The points made about isolating framing (particularly  
> steel) from the plaster is something that has always made sense as  
> of course keeping any steel, if used, to the interior. Even placed  
> on the interior, I still would rather isolate movement of one vs.  
> the other.
>
> Who else remembers hearing Andy DeGruchy talk at the ISBBC in  
> Ontario about the flexibility of lime vs. cement? I'll have to dig  
> up my notes (or refer to that very comprehensive CD that was  
> compiled) to see what the details were. Going by memory (often a  
> dangerous thing for me), it leaves me thinking that by eliminating  
> issues such as differential expansion rates as John points out, and  
> by proper technique in finishing the lime as RT points out, that not  
> using expansion joints could be the safer thing to do given that  
> expansion joints can be a moisture risk and lime, besides its  
> flexibility, is easier to repair than cement stucco, where this  
> whole expansion joint thing got started.
>
> Andre, I have not before come across information on mixing gypsum  
> and lime historically. Thanks for raising this. Of course, I too,  
> have no idea what kind of gypsum in the states would be comparable.  
> Anyone know any more about this?
>
> And on Andre's last note, below, I am surprised. I have always  
> thought of rough, sanded textures as being more water resistant. You  
> have brought the most durable part of the mix to the surface to  
> collect water and allow it todrip off, severely lessening the  
> sheeting action that occurs on smoother surfaces such as glass and  
> smooth troweled plasters. Hence harled lime on the lower part of  
> exterior walls in England. This is something that Barbara reinforced  
> when we were at the ISBBC. On the other hand, it does make sense to  
> me that scratched plaster would be less likely to crack. Thoughts on  
> this?
>
> And let me add my huge congratulations to Barbara and all those  
> awesome women over there. I am especially excited as a woman to see  
> this kind of recognition being bestowed on you, Barbara. It's  
> wonderful. (It also brings hilarious memories of our women straw  
> bale olympics team!)
>
> Laura
>
>
>> I also heared a couple of days ago (at the St Astier hydraulic lime  
>> plant) that scratched lime plaster is much less likely to crack  
>> than smooth (troweled) plaster but that the latter is more water  
>> repelling. They also claimed that in the old days plaster was  
>> almost always allways troweled/smooth.
>>
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Athena & Bill Steen
The Canelo Project
HC1 Box 324
Canelo/Elgin, AZ 85611
absteen at dakotacom.net
www.caneloproject.com