[GSBN] Window Detailing Question

[Derek Roff]

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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