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Topic:	Window Overhangs
Author:	Patricia Olson, PhD, LEED-AP
Company:	Ecological By Design
Phone:	(928) 778-9598
eMail:	ecodesign@earthlink.net

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Window Overhangs 101:  Passive Cooling Techniques

In our mild climate of Central Arizona, it's possible to achieve most, if not all, summertime cooling through passive (non-mechanical) cooling strategies.  Roof overhangs that provide shade for south windows are a key element of a naturally cooled home.  South windows allow solar radiation to heat a home in the winter, but they can become an overheating liability in the summer. 

When sunlight enters a window (short wave radiation) and strikes a solid object it's converted to heat (long wave radiation).  One of the ways to avoid summertime overheating is to keep direct sunlight, and the associated heat, from entering a house through the windows.  Therefore, shading is most effective on the outside of the windows.


Overhangs can be an extension of the roof or another horizontal element located over the south windows.  The key is to properly size the overhang depth (the horizontal dimension) to allow the low angle winter sun to be admitted for wintertime heating, but keep the higher angle summer sun out and therefore avoid the heat that comes with it.  See sketches for two overhang types (Watson, p.158).  

The way to calculate the proper depth of a south window overhang for the Prescott latitude (34.5∞) is to divide the height from the window sill to the lowest point on the overhang by 3.5 (the Shade Line Factor [SLF] for Prescott). The formula is:

W = H/SLF


W: The horizontal dimension of the overhang
H:  Height from the window sill to the lowest point of the shading element or overhang
SLF:  Shade Line Factor for location (see table in Climatic Building Design, p. 158)


This can be calculated in either inches or feet.  For example if the height of your window to the lowest point of the overhang is 5 feet, then 5 feet divided by 3.5 = 1.42 feet (or approximately 17").   Typical residential roof overhangs are between 12" and 18". 

If your overhang is not deep enough you can construct an extension to increase the depth or install a manual vertical shade.  This type of moveable shade will also assist in keeping the sun out in the fall when the sun angle is becoming lower in the sky, yet the daytime temperatures are still warm enough that overheating can occur.  For further information on overhangs and other shading techniques for homes, see the references listed below. 


References: 
Climatic Building Design by Donald Watson and Kenneth Labs.
The Solar Home:  Passive Heating and Cooling by Daniel D. Chiras

Patricia Olson, Ph.D., LEED-AP, is a registered architect with over 20 years of professional experience.  She is an educator and green design consultant.  She is also a Board member of the Central Arizona Renewable Energy Network (CAZREN) and co-teaches Solar & Renewable Energy at Yavapai College.