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Solar overhang calculator: Shade your windows!

6 Feb

An important component of passive solar design is calculating building openings (doors and windows) and roof overhangs. For instance, several features of a window will influence gain of solar heat and shade. The size of the window -width and height- and the window’s relationship to the roof -the space above the window- influences the direct and indirect gain of solar heat. For example, if the roof’s overhang does not shade the window during the hottest hours of the day, direct and reflected sun light will heat the interior. That might be desirable during winter months, but very uncomfortable during hot summer months.

A designer can draft the angles of the sun or refer to charts to determine heat gain and shading with laborious drawings. On the other hand, one can use interactive tools on the Internet to aid  decisions in placing windows to avoid heat gain in summer and maximize solar gain in winter. Additionally, this is an excellent tool to design a Trombe wall in a south wall for maximal heat gain and storage in an earthen wall!

An innovative and helpful solar tool is an Overhang Annual Analysis (the beta release of new version) on the website, Sustainable by Design. Developed and maintained by Christopher Gronbeck in Seattle, WA, two versions of the tool can be used. The beta release includes advanced features, such as accounting for diffuse and reflected light. What’s the difference, and does it matter? Wait and see.

In the examples below using the advanced beta version, I entered the latitude of Terlingua, TX, and basic data about one window, it’s location (e.g. south wall) and the overhang. Additional input includes sun mode (direct sun versus direct + diffuse + reflected) and overhang width. Since the eaves of our roof will overhang the entire wall, I did not change the default (‘wider than window’). For the first example, I chose direct + diffuse + reflected sun mode (in oval) and entered the dimensions in the appropriate field in the captured image below.

Percentage of shade for window with overhang; direct, diffused and reflected sun.

Now, compare the calculated data in the image above with that below after calculating for direct sun only (again, see field in oval).

Solar gain/shade calculated with direct sun only.

The first calculation is probably more representative of the true value of net solar gain, or total solar radiation on the window, depending on several factors. It includes direct sunlight, diffuse light from the sky, and ground-reflected radiation. If the window is close enough to the ground to catch reflected light on a cloudless sky, total radiation will increase that value (compare to same time/month in second example). This source of radiation should be factored into placing and sizing windows in a wall.

Also, if shutters are placed on the exterior of the window and opened and closed during hours of the day when solar gain is high, this will future reduce or increase total solar radiation. Another design feature to consider to windows in the overall structure.

Now I want to determine the solar gain for a 5′ tall Trombe in the south wall. A Trombe wall is a space incorporated into the wall with fixed glass exposed to the exterior. Some type of thermal mass behind the air space and glass captures and stores the heat from the solar gain, which diffuses into the interior during cooler hours of the day (diurnal flywheel). Our thermal mass will be adobe blocks or narrow earthbags. Also, similar to our windows, our Trombe walls will have shutters that will help reduce total solar radiation gain when needed.

Calculated for 5-foot south-facing Trombe wall.

Trombe wall.

The photo on the left shows an adobe Trombe wall in a building under construction by Joe Tibbets at the Southwest Solar Adobe School (Belen, New Mexico). The adobe blocks in the Trombe have a dark clay slip on them. It is positioned between twin sets of windows and as yet does not have the exterior glass installed. A dark surface inside the Trombe will aid in absorbing solar gain and heat.

As you can see, these online calculators can really help in design decisions!

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