[GSBN] Window Detailing Question

[Brian Hodge]

The cob is about 20mm thick so that it covers and seals the buck. The foam
is about 20 – 30 mm and extends the depth of the window, which is generally
about 140mm. I agree that the cob would generate a small thermal bridge
where it is used to cover the buck. Our buildings are all over Australia,
but mainly focused in the southern states including Tasmania. We are
located in central Victoria where frosts drop the temperature to  -4 C on a
regular basis in our winter, and on occasions as low as -10 c.



Brian



*From:* GSBN-bounces at sustainablesources.com [mailto:
GSBN-bounces at sustainablesources.com] *On Behalf Of *Derek Roff
*Sent:* Thursday, 22 November 2012 8:37 PM
*To:* Global Straw Building Network
*Subject:* Re: [GSBN] Window Detailing Question



If I understand correctly, the cob forms a thermal bridge between the
inside of the house and the outside, in the space around the edges of the
windows.  What percentage of the 100mm width is cob, and what percentage
foam?  What climate are you building in?



Derek



On Nov 21, 2012, at 11:58 PM, Brian Hodge wrote:



 Hi all,

Some time back we began enveloping the window buck in render/cob so that
the buck is  completely covered and sealed. The window is then fixed in
position with the use of expanding foam. The expanding foam is a mechanical
connection which also provides insulation and a water proof seal. By doing
this there is no entry point for water , which normally happens around the
junction of the timber to render. We complete the second coat of render
before fitting the windows, which also assists in the drying of the second
coat inside due to the increased air flow by not having the windows in
place.



The expanding foam needs a dust free surface to adhere to, so we treat the
surface of the render in alignment with the window with 10 parts water to 1
part bondcrete, which stabilizes the render. We have now been using this
method of window installation for about 6 years and even in high wind
applications we have never had cracking around windows or any water leaks.
Obviously in order to do this you must allow a significant amount of
clearance between the buck and the window to allow for the cob & render. We
allow 100mm in the width and 60 in the height.



The foam must be overfilled so that the excess foam can be cut of broken
off as this gives a surface that the render will adhere to, where-as the
natural finish is too smooth. Once the window is held in place with the
expanding foam, we use cob around the window to generate the final shape
required, which also provides a very strong structure around the window.



This method of construction is obviously of no value to people unwilling to
use expanding foam, but it has served us well, and completely solved the
problem of leaks around the windows.



I trust this helps someone.



Regards



Brian




Derek Roff

derek at unm.edu
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