[Strawbale] Bottom of wall detailing

[Rob Tom]

--- In SB-r-us at yahoogroups.com, Jeff Ruppert <jeff at ...> wrote:

> First, this detail came out of California and the detail masters out 
> there.  In my opinion, it is not what I would refer to as a standard 
> plaster detail that one would see on any other type of home.  Most ofthe  
> time, plaster crews use J-strip or weep-screed to finish the bottomof  
> their plaster.Second, this detail, in my opinion, is suited for extreme  
> load-bearingconditions where you would want the plaster bearing directly  
> on thefoundation.

I don't think that "what one would see on any other type of home" WRT  
plaster details necessarily applies to SBC.

For instance, on wood framed buildings, where there is just a weep screed  
to finish the bottom, the plaster is keyed into metal lath and there is a  
nailing schedule for the nails from which the lath is hung, calculated  
support the weight of the plaster. ie The lath nails are carrying the  
weight of the plaster in a similar manner that drywall screws carry the  
weight of gypsum board on walls, except that the plasters used on SB walls  
are 2 to 3 times as dense as gypsum board and three or more times thicker  
than either plaster or gypsum board so we're talking about weights that  
are 3x to 6x heavier than with other types of homes.

Furthermore, as the Colorado tests clearly demonstrated, the plasters on  
SB walls aren't always well bonded to the straw substrate or even to base  
coats. So if the plaster isn't bonded to the straw very well and there is  
nothing underneath the skins to carry the weight of the plaster, just what  
is keeping the plaster from slipping down over time ?

Given that the plaster skins weigh more than the straw bales and that  
people seem to go to great lengths to provide foundations upon which the  
straw bales can sit, IMO it makes sense to support the plaster skins on  
the foundation too even if the bales (and skins) aren't loadbearing or in  
seismic areas.

As for the concerns about the plaster bonding to the foundation etc. , I  
think that's a non-starter.

There should be a sill flashing at the base of the plaster with an upstand  
that extends up behind the plaster and with a horizontal leg/drip that  
sheds water out and away from the foundation.

There should also be a moisture barrier/capillary break that separates the  
foundation from the moisture-susceptible materials above and it will  
likely be the same piece of material that is underneath the drainage layer  
under the first course of bales. This membrane should lap over the outside  
of the upstand on the sill flashing mentioned above.

This _necessary_  damp-proof coursing would effectively isolate the  
plaster from the foundation thereby eliminating any chance of a bond being  
formed.

But again, I've not yet seen the sketch to which Ned was referring so once  
again, I may be talking about something completely different than what the  
original question was actually about.

-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
<A r c h i L o g i c   at  c h a f f y a h o o   dot   c a >
winnow the chaff from my edress in your reply