[Greenbuilding] windows - flush vs recessed

[John Straube]

I have heard the PH wants the window in the middle, but I have not seen much benefit when I model non-conductive framed windows in wood frame walls using FRAME.  This may be a "rule" that comes from European walls in the 90's, and does not apply as well to North American walls today.  In fact, with a plywood buck, and exterior foam insulation, locating the window nearer (not flush) the outside tends to get a few percent better than installing near the middle (because the exterior rigid insulation has a higher R per inch and has no thermal bridges).  The center of wall maybe the best answer in a double stud wall however.
I have taken IR photos of triple-glazed fibreglass windows installed near the exterior in a plywood buck in an R45 wall and I could not see too much effect, and it mostly validated the THERM runs.

It is at least as important to pull the windows in  a bit (even an inch makes a big difference) to protect them from rain.
In practise, we find the design is often driven by how easy it is to integrate subsill drainage to the drainage plane of the wall.  Some walls allow easier drainage at the middle, others easier at the exterior.
One design to avoid at all costs is the one which covers the window frame with exterior insulation, therbey trapping rain leaks in the wall (but reducing building heat loss by as much as 1%).

John

On 12-01-16 9:49 AM, Alan Abrams wrote:
> thoughtful article. PH teaches that the optimum location for windows
> is near the center of the section, based on modeling in THERM. an
> important consideration is preventing a metal flange from acting like
> a cooling fin--. this, however, is based only on conductance, and
> does not consider wind lashing or exposure to rain, which is
> discussed in the article.
>
> the conductance approach also does not consider shading/solar gain,
> another important factor--but this is carefully analyzed in other
> portions of PH design.
>
> lastly, all this should be integrated with the location of the vapor
> barrier, air barrier, and rain screen and flashing...
>
> -aa
>
> *Alan Abrams** Abrams Design Build LLC* /A sustainable approach to
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>
> On Sun, Jan 15, 2012 at 8:06 PM, Kat <molasses at q.com
> <mailto:molasses at q.com>> wrote:
>
> Hi all,
>
> I came across an article from 2009 that recommends installing
> recessed windows in super-cold climates
> (http://www.insulfoam.com/__images/stories/docs/JLC-__finalfeature.pdf
> <http://www.insulfoam.com/images/stories/docs/JLC-finalfeature.pdf>).
> I think it's an interesting idea and could see how it might be
> beneficial in some ways but wonder if the potential problems outweigh
> the benefits. I'm not having much luck finding details on the web.
> Everything I've found on BSC shows flush windows.
>
> Any thoughts on or experience with recessed windows in a mild *rainy*
> climate (Portland, OR)? Has this, by chance, been discussed before?
>
> -Kat
>
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--
>
Prof. John Straube, Ph.D., P.Eng.
Dept of Civil Engineering / School of Architecture
www.buildingscience.com