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GBlist: Re: Stucco

On 9/5/97 12:00 AM Joyce Coppinger wrote:

>What polymers are used or can be used in stucco products used for
>strawbale construction?  Concern is for breathable wall which won't
>crack over time because of climatic conditions (freeze and thaw cycles,
>moisture, hot to cold climatic changes). What types of stucco products
>are available using "natural" ingredients which would have the same or
>similar properties as stucco or coatings with polymers?

I've spent a bit of time researching stucco, and I still know amazingly 
little about this important material.  I would like to hear other's 

First some terms, all of which apply to the Southwest only, and are 
totally casual and unofficial.   "Stucco" tends to mean whatever is 
applied to the exterior, and "plaster" means its on the interior.  Whats 
called "traditional" stucco is cementitious;   its usually applied in 3 
coats ("scratch", "brown", and "color").  Of course, this isn't 
traditional at all;  that would be mud.  The word is used to 
differentiate from the acrylics, such as Dry-Vit, etc, which are 
essentially heavy duty paints.  I have no idea what you might mean by a 
"polymer", unless it was mistaken for acrylic, or is one of the roof 
coating products more common in commercial and industrial construction.  
So there are three general residential classes of stucco:  1. mud;  2. 
cement;  3. synthetic (acrylic).  

These are often mixed together.  For example, it is rare for someone 
today to use a pure mud;  it is usually "stabilized" with a percentage of 
cement.  Thus, there is a huge range of possibilities, plus all the 
additives to the above (cactus juice, horse hair, sheep blood, etc).  A 
fun treatment is to use a cementitious stucco, but utilizing sand from 
your site for the mix;  thus you get a truly harmonious color scheme.

In practice however, I think the choices narrow to something much more 
manageable.  Straw bale requires 3 coats, due to the gaps to fill, which 
mandates either a mud/cement or cement base.  Then the color coat can be 
either acrylic or cementitous.  Frankly, I think this is simply personal 
choice;  they both work fine.

Which brings us back to your question regarding "a breathable wall which 
crack over time".

I find this "breathable" question somewhat annoying, in part because I 
don't know enough myself, in part because I think its a false alarm, and 
in part because I suspect it doesn't matter.

1. If its cementitious or mud, it will crack, and often not over very 
much time.  Guaranteed.  If a stucco doesn't crack, then its not a 
"natural" product.  Sorry about that.  The mud stucco's were usually 
refinished on a regular basis.  The hairline cracks in the cementitous 
products are considered to be a cosmetic issue only.  The acrylics 
stretch and cover the hairline cracks in the underlying basecoats.

2. Personally, I wonder how much the two differ in breathability anyway.  
Imagine being inside a sealed room with no openings, with one encased in 
3/4" of concrete, and the other encased in the same with an 1/8" of paint 
on top.  I think you would suffocate in either one.  Maybe not though;  I 
would love to hear the air transmission ratings for these different 
stucco products from a more technically proficient list subscriber.

3. At any rate, "breathability" in walls seems easily achieved by opening 
a window.   
The house is what needs to breath.  (How much air diffusion is optimal 
for a wall is a very complicated and controversial separate issue.  Its 
much easier to install an HRV than it is to waste valuable brain time by 
studying the problem). 

Probably the most common treatment in the SW, is to use a cementitious 
basecoat (2 for strawbale or adobe, 1 coat for anything else), that has 
fiberglass mixed in for cracking resistance (required by code if doing a 
single base coat).  Then an acrylic is used for the color coat.  

Good luck on this one.

Buzz Burrell           "Time flies like an arrow,
Boulder, CO             Fruit flies like a banana".

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