SB: Re: Cob window Sills

["Bill Hunt" <billhunt@redrock.net>]

----- Original Message -----
From: "Michel Wingard" <carrrpenter@earthlink.net>
> I have a question then in regards to cob exterior window sills. The
> current wisdom seeems to be to use flush mounted windows when using
> earth plaster to eliminate water seepage.
> If I use thick cob say 6" or better to fill the sill area to the finish
> needed for the window reveal will it be a good water barrier? I'm
> talking of windows then inset into the wall say up to half the bale
> depth.
> Peace Michel

Hi Michel-  Back before I knew any better, I did some windowsills like this
on a little experimental building, with earthen plaster (more like 2" thick,
not 6" of cob).  Moisture probe testing last summer showed one of the
windowsill bales had the highest moisture content in the building. Around
15%, not terrible, but borderline.  I wish that building had bigger
overhangs all the way around, and membranes under the windowsills. Most
designers and builders will use a bituthene type membrane under any sort of
exterior windowsill detail, wether its cob, cast concrete, stone sills, etc.
How far to extend this membrane was a big debate on my last project. You
really want generous overhangs to keep driving rain off the windows and
sills, I think.

I think flush mounted windows on the exterior are more practical, if you
have bales under the windows.  It helps eliminate a weak point.  In severe
locations in snow and rain country, I still think it makes sense to have a
more water resistant material up to windowsill hieght, such as Rastra or
other foam block, Faswall or Durisol block, pumicecrete, cinderblock, etc.
Its hard to say if 6" of cob would be a good water barrier or not, but I
wouldn't bet my life on it.  The surface should be well sloped, what ever it
is.

Malcom wrote:
I will be making indoor sills as well, but my post refers to outdoor sills.
I want them to have a different color and texture than the stucco. I would
like the sills to look and feel like they were cut from stone. Thanks

Hi Malcom: A mason I worked with said he does this quite a bit.  Its cheaper
than stone, can be made to look like stone (somewhat), can be sloped,
sealed, etc. I think you'd form them. I don't know much more about it, other
than its possible.   One good detail for these, or stone sills, is to extend
them past the window opening slightly.  This provides more complete
protection for the bales below, and looks nice. I think stonecutters call
this a "pistol grip" type of cut.  They should extend out past the stucco
surface, too.
Cheers- Bill



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