[Cob] Embedding Windows in Cob Wall

[Shannon Dealy]

Hi Bill,

Haven't seen the pictures, however, based on what you wrote ...

[snip]
> My window design was such that I  covered the fixed double-panes of
> glass with 2-3" of cob on all edges, and applied my lime stucco over the
> cob on the exterior, and my earthen plaster over the cob on the
> interior.
>
> It's the exterior lime to glass connection that I'm concerned about. My
> plan was to put a bead of silicone caulking at that joint to seal out
> the rain water. However, the glass delivery guys, and a friend who is a
> contractor (not experienced with cob, lime stucco, etc...) cast
> disparagement onto this idea saying it won't create a functional seal.

You don't need any additional seal, and they are correct, the caulking 
wouldn't provide one. The cob and lime stucco are porous, water will get 
in (and back out), no matter what you do at the edges. This really isn't 
an issue, it's not like there is some reservoir inside the wall for the 
water to get into and fill up. The cob will absorb what little moisture 
seeps in around the edges. I've done a number of embedded glass windows 
and seen many more. None of them ever had problems with interior moisture 
that I am aware of. I would suspect (though I have never investigated) 
that moisture around the edges causes the cob/stucco to swell and tightens 
the seal whenever it gets wet, effectively blocking any significant 
additional infiltration. Likely, there is a slow accumulation of moisture 
in the cob itself, but this will dry out fairly quickly once the rains 
stop. Of course, this assumes the embedding does not create a flat area 
for water to accumulate and stand against the window.

> The panes were installed, and in place and during the interior
> plastering process BOTH of them were shattered (pic of that included
> too). So I'm taking this OPPORTUNITY to cry A LOT. . . I mean, to pause
> and reassess, and email you.

If this breakage was a result of the plastering process and not due to an 
accident, then it is likely that the embedding resulted in too much cob 
load on the glass and the plaster just pushed it over the edge. I have 
only had this happen to one of my windows during plastering (a glass jar 
window which is fortunately trivially replaced), and it was in a location 
that I expected might have load problems as it was close to a support for 
the floor of a level above. On the plus side, unless there is a structural 
issue above it, when you replace a window in cob, the surrounding cob 
should now be much more stable since it has had more time to cure. During 
the replacement process, you can create a gap above 
the glass inside the wall if needed, or possibly install a lintel if there 
are local loading problems.

> I am thinking of a complete re-design if y'all think I should abandon my
> current design, OR I'm thinking of squirting a bunch of roofing tar
> along the edge of the glass and squishing it into the framing so it
> squirts out the edge on the exterior where the stucco meets the glass;
> thus creating a seal.

I can't speak to the re-design since I haven't seen the original, however, 
I don't see any way in which roofing tar would help. Is there something 
unusual about your design relative to how windows are normally embedded in 
cob?

One other thing that might be an issue is if you are using really coarse 
sand or have small rocks in your mix. This can create a point load on the 
glass where it sits on one of these pieces of rock. When installing 
windows or their replacements, it may be a good idea to use sand and clay
that have been run through a fine screen for the cob in direct contact 
with a pane of glass.

FWIW.

Shannon C. Dealy           |         DeaTech Research Inc.
dealy at deatech.com          |    - Custom Software Development -
Telephone: +1 541-929-4089 |    - Natural Building Instruction -
USA only:     800-467-5820 |            www.deatech.com