exterior wall protection

[Shannon C. Dealy]

On Sat, 2 Nov 1996, christopher j. meyer wrote:

[SNIP]
> concrete, which I wrote about;  and shrinkage/expansion.  Clay which has
> been thoroughly dried will expand during its lifetime due to something as
> slight as humidity.  Concrete will shrink, fact.  Definition: differential
> movement.  At this point in time I'm led to believe that p.c. should not be
> used in plaster over anything but concrete block.  Although, I understand
> some people put a little p.c. in their plaster to quicken the set time.
> Maybe clay plaster with wood fibers should be used over cob.  Sounds like a
> pleasing combination for the ultimate in material compatability.  You may
> have to replaster every so often ( I wonder about material lifetimes).

Earthen plasters using a variation of the 'standard' cob formula (as if
there could really be such a thing) are often used.  One of the most
important differences is that the sand and clay are run through screens to
provide finer materials for a better finish, and the straw is finely
chopped or even replaced with dung (which provides very fine fibers).
One advantage of using a cob plaster over a cob wall is that the materials
will expand/contract at similar rates which will virtually eliminate the
chance of cracking/peeling due to dissimilar expansion rates.  To provide
for longer life of the plaster, it may be desireable to use a natural
paint such as a lime wash to protect the plaster from weathering, since
it is easier to periodically paint the wall than it is to periodically
patch/replaster.

> 
> Hydrated Lime:  ASTM C207
> 
>                         Type N:  (normal)  no limits on oxide content, type
> NA is air entrained
> 
>                          Type S:  (superior)  limit on oxide content, type
> SA is air-entrained
> 
>                         note:  oxides are what causes lime to swell when wet
> 
> I thought that might be of some interest.

Thanks for the info, I'd been wondering what the difference was between
type S  and type N lime because of all the discussion regarding where to
find it (here and on the strawbale list) and the different types
available.  It sounds like type S would be preferable for plastering since
it would expand/contract less when it gets wet/dries, which would make it
less likely to crack or peel.

Shannon Dealy
dealy at deatech.com