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Solar overhang in Mexico (C) Daniel Friedman Passive Solar Design Roof Overhang Explanation & Design Details

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Use of roof overhangs & shading to control heat gain in buildings:

This article discusses the use of roof overhangs as a component of passive solar design; we include links to additional passive solar design as well as solar design evaluation questions and answers. Our page top photo illustrates extensive use of solar shading on a government office building in Queretaro, Mexico.



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Guide to Passive Solar Home Design: Using Roof Overhangs

Solar overhang in Mexico (C) Daniel FriedmanPassive Solar Home Design for Summer Comfort - using shade

It makes little sense to save money on winter heating just to spend it on summer cooling. So in most climates, a passive solar home design must provide summer comfort as well. The solar heat in the summer must be blocked by an overhang or other devices, such as awnings, shutters, and trellises.

Roof Overhangs Control Building Heat Gain & Sunlight

The physical dimensions of an overhang are an important element because overheating will occur unless the overhang provides enough shade.

[The solar overhang shown in our photo (above-left) is located on a home in San Miguel de Allende, Mexico and is discussed in more detail at PASSIVE SOLAR HEAT PERFORMANCE. The addition of these small overhangs above windows on the East and West facing sides of this studio made the difference between having a comfortable space and having a space that was just too warm to occupy late in the afternoons. For a contrasting example, another home in the same area that has no roof overhangs to control solar gain is discussed at PASSIVE SOLAR HOME, LOW COST. -- DF]

Many variables—including latitude, climate, solar radiation transmittance, il luminance levels, and window size and type—need to be considered for properly sizing an overhang in a specific locale. Therefore, it's best to have an experienced solar designer or builder calculate the proper overhang dimensions. For more information, see roof overhangs [below] for shading building elements.

In passive solar home design, exterior roof overhangs provide a practical method for shading building elements such as windows, doors, and walls.

How Roof Overhangs or Other Window Overhangs Work to Control Heat and Solar Gain

Overhangs are most effective for south facing elements (in the northern hemisphere) and at midday. If the building element bears more than about 30° off true south, the effectiveness of an overhang, as with any solar feature, begins to decrease significantly. Overhangs usually only affect the amount of direct solar radiation that strikes a surface. Diffuse sky and reflected radiation gains are not often directly affected by overhangs.

The higher overhead the sun is, the shorter the shadow a person will cast on the ground. However, the short brim of a baseball cap can create a long shadow across the body of a standing person. The same concept applies in designing overhangs for buildings.

The higher, or more vertical, the arc of the sun, the longer the shadow that the building overhang generates along the face of the wall. Summer shadows extend down walls the furthest, winter shadows the least. Sites closer to the equatorial path of the sun have deeper-extending wall shadows than ones farther from the equator, assuming the same overhang length.

Types of Roof or Window Overhangs for Sunlight & Solar Gain Control

Passive solar overhang, Vassar College Campus (C) Daniel Friedman

Overhangs may be solid, louvered, vegetation-supporting, or a combination of all of these aspects. Some shutters, eaves, trellises, light shelves, and awnings serve the same purpose as an overhang.

Our photo (left) demonstrates a passive solar overhang in use on the East-facing wall of this building on the Vassar College campus, Poughkeepsie, NY.

Overhangs may also be fixed, operable, and/or removable. Examples include roof eaves, awnings, and Bahama shutters (top-hinged louvered shutters typically propped open with wooden dowels) respectively.

Fixed overhangs offer perceived longevity and low maintenance at the expense of flexibility or the ability to adjust to site-specific factors. Although adjustable devices allow the user to fine tune the amount of shade or direct sunlight, they require more maintenance. Removable fixtures generally provide flexibility and longevity plus some personal involvement with installation and removal.

Roof or window overhang US DOEOpenings, such as windows, do not always require fixed overhangs. A fixed overhang designed for optimal shading on the autumnal equinox (September 21) casts the same shadow on the vernal equinox (March 21).

While northern-hemisphere shading may be welcome in September because of the heat, shading in March may be undesirable. Vegetation, on the other hand, can follow the climatic seasons. Vines that shed their leaves for winter usually leaf out about the time shading is needed. Movable shading devices, while adjustable, often become maintenance problems.

Sizing Roof or Window Overhangs

Unfortunately, there is as yet no universally simple formula for sizing overhangs. While one overhang methodology works well for some locations, it can be completely inappropriate for others.

For example, there are a limited number of overhang-sizing guidelines acceptable for buildings located in southern states, particularly hot-humid climates. Guidelines acceptable for the high plains of Montana are unlikely to work for a site in Florida.

Due to the varying microclimate conditions encountered across the United States, the methods presented here are general in scope. Anyone seeking a more specialized analysis should seek professional advice from an architect trained in passive solar design.

Every climate requires special design attention. The following general guidelines may be useful in determining a suitable overhang design. The guidelines are listed by climate type, for solar noon, when the sun reaches its maximum altitude for a given day. Solar noon is very rarely the same as noon in local standard time.

*(HDD and CDD data is available from local weather services.)

Roof or Window Solar Gain Control Overhangs and Zoning Ordinances

Overhangs may be inappropriate for sites with restrictive regulatory guidelines. For example, your calculations indicate your house needs a three foot (~1 meter[m]) overhang on the front. The local zoning ordinance restricts eave extension to two feet (0.61 m) beyond the front yard setback.

If your house will be or is located precisely on the setback, you must do one of the following:

  1. relocate your house at least one foot (0.3 mm) back from the front building setback [OPINION: Surely this is absurd advice, not to mention the decimal point error-DF];
  2. redesign your building fenestration (windows, doors, grilles, vents, and other openings) [OPINION: surely this is cost prohibitive in retrofit applications -DF];
  3. redesign your overhang; or
  4. apply for a variance (an exception to the ordinance) [OPINION: common -DF.].

List of Passive Solar Design Key Reference Books including Online Texts

The first three passive solar design handbook links below are to free, online documents - in PDF format.

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

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