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The Bottom Line on the New Energy Economy
Keeping Cool Part 1

Keeping Cool Part 1: Put Your Shades On

Eric Severance

Keeping The Sun Out In the First Place

June 5, 2009
by Craig Severance

As Congress considers new energy building codes to require a 75% decrease in energy consumption for new buildings by 2032, many wonder how this can be done without expensive new technologies.


Getting better buildings, however, begins with remembering simple things we may have forgotten.   These simple things don't cost much, but can make us more comfortable and achieve very large energy savings.  

40% of Cooling Load.  Roughly 40% of our cooling load comes from direct sunlight coming through windows. That's an average number.  However, many "stupid buildings" are built so thoughtlessly that major window areas face the summer sun, causing well over half of total summer heat gain. 

Keep the Sun Out.  The most important simple thing to keep us cool in hot weather is to keep powerful sunlight from streaming in through windows.  I don't mean closing the drapes -- because once the sun has already come in through the window, most of the heat is going to stay in the building (though a white surface on the outside of drapes will help reflect back some of the visible light).

Simple methods -- on the outside of windows --  can reduce summer sunlight through windows by up to 80%, cutting our total cooling needs over 30% (with more savings if you live in a "stupid" building with a lot of glass facing the summer sun). 

South, North, East & West.  The sun does not shine equally on every window.   The solutions to keep sun out therefore depend greatly on which direction the window faces.

South WIndows.  In colder climates, smart building design includes a large area of south-facing windows, to take advantage of the free heat from the winter sun, which tracks across the southern sky at a low angle.  

You might think this large area of south-facing glass would be a big problem in the summertime, however this is only true if the building is constructed in a thoughtless way.  If the south-facing windows are vertical, it is very easy to completely block direct summer sunlight with a very modest overhang. 


Overhangs that Protect South Windows from Summer Sun

As seen in the image on the left, even a typical (without thinking about it) roof overhang for a tract home can provide significant shading over south windows.  This is true because in the summertime, by the time the sun reaches the southern sky, it is very high in the sky.  More aggressive overhangs, such as in the picture at right, can also provide shading for late spring and early fall, which may be important for warmer climates.

Building Orientation is Critical.  The roof in the picture at left has an east-west axis, as does the house.  This is ideal for solar gain in the winter, and simple shading against solar gain in the summer.  Also, since the roof  faces south, it is well positioned for installation of solar hot water heating and solar electric panels. 

By contrast, a "stupid" building may be built thoughtlessly with a north-south axis.  Everything about a north-south axis orientation presents problems: low solar gain in winter because of fewer south-facing windows, no roof overhang on the south side to protect whatever south windows do exist, and large areas of east and west windows.  (As discussed below, east and west windows face the summer sun when it is low, so roof overhangs provide little protection from summer sun.)  Finally, with a roof oriented to face east and west, installation of solar water heating and solar electric panels will require costly and unsightly racks to point them toward the south. 

The direction a building is pointed -- its orientation -- therefore makes a huge difference.  The same building -- with identical costs of construction -- can be oriented (east-west axis) to save energy, or if pointed in the wrong direction (north-south axis) the summer sun will flood the building when not wanted, while sunlight for winter solar heating, and solar hot water and solar electric, will be unavailable or difficult to access.

Developers usually decide building orientation when a subdivision is laid out.  Many may think it impossible to orient all buildings in a subdivision with an east-west axis.  However, thoughtful design and layout can achieve this.  I  recently toured a very attractive new subdivision where this was achieved, though the houses had normal lot density and the subdivision did not appear "strange" in any way.   

East, West, and North WIndows.  In contrast with south-facing windows, which are easily shaded, windows that face east and west will experience the brunt of summer sunlight.  A great deal of this sunlight will flood into the building when the sun is lower in the sky, when modest overhangs will do no good. 

Even north-facing windows may experience significant summer heat gain, in the early morning and late afternoon/evening when the summer sun is low in the sky in the northeast and northwest portions of the sky.

Sun rising in the Northeast this morning.

This occurs because (unless you live on the Equator), there are only two days of the year when "the sun rises in the East, and sets in the West" -- the Spring and Fall Equinoxes.  In the Fall and Winter, the sun rises at some version of Southeast, and sets at some degree of Southwest.  In the Spring and Summer, it rises at some version of Northeast and sets at some degree of the Northwest. 

Solutions to Block East, West, and North Summer Sun.  Since modest overhangs will not block the lower and very powerful summer sunlight coming from these directions, other solutions must be adopted.  The best solution is to avoid having a lot of windows facing these directions in the first place, through proper building orientation.  If the building is already built or cannot avoid this, however, there are low cost solutions.  These solutions include awnings (either permanent or retractable), sun-blocking exterior sunscreens, reflective glass, and shade trees.


Examples of Awnings to Block West, East, or North Summer Sun

Awnings.  Though housing developers routinely build houses with windows facing directions staring directly into summer sunlight, seldom do you see awnings installed on a new house.  In residential applications, therefore, awnings are usually installed after the fact by the homeowner to solve a problem the builder left.  In commercial applications, however, it is quite common to see very attractive awnings installed as part of the original plan.  There are a wide variety of awnings available in different styles -- every building owner can usually find a style that looks good with their buidling. 


Examples of Sunscreens that block 70 -90% of Sunlight  source:wswcgj.com

Sunscreens.  A quite attractive and almost invisible method to block 70 - 90% of sunlight from entering a window is a "sunscreen" -- a special screen of tightly woven synthetic material which installs instead of the normal bug screen, providing good ventilation.  As seen in the photo on the left, they still provide excellent outward visibility, especially important for windows with a cherished view.  They are pet resistant, thus suitable for patio doors.  They are often manufactured locally to each window's exact dimensions, including custom windows such as arches. Sunscreens have been quite popular in desert cities such as Phoenix and Tucson for decades, and are just now being discovered by much of the rest of the country.  The screens typically come in a variety of earth tone colors to complement the outward appearance of the home.  This product should also gain acceptance in new housing construction as a cost effective means to cut cooling loads, especially as new energy building codes take effect.


Commercial building with high-refectivity glass

Reflective, Tinted, or Low-Transmissivity Glass.  Sometimes a large expanse of windows face the wrong direction (e.g. directly into the west, baking occupants with the late afternoon summer sun).  This typically occurs in commercial buildings, and often the solutions noted above simply don't work.  It is therefore quite common to see commercial buildings with large expanses of reflective or tinted glass, especially for upper stories.

Another option (offering a clear window solution) is the use of special "Low-E" coatings. Most Low-E windows allow solar heat gain (e.g for south-facing windows in cold climates), however special versions of Low-E are specifically designed to block a majority of infrared rays from transmission through the window. 

All of these coatings or tints are quite effective and can make an otherwise unbearable heat gain load much more manageable. Some of these coatings are also available to apply to existing windows, however the quality of retrofit coatings varies widely, so it is very important to find a high quality product with a good reputation.

Don't Coat South Windows in Cold Climates.  Because you want the southern sun to enter the building in the wintertime, and it is so easy to prevent this in the summertime, the more aggressive window treatments described above for windows facing other directions are not appropriate for south-facing windows.  For cold climates,  in general south-facing windows should not receive reflective coatings or sunscreens.  For south windows, these are usually unnecessary in the summer, and would block valuable sunlight coming through those south windows in the winter.  (Sunscreens are removable, however, so if a building owner is ready to take down & put them up every 6 months, these might be acceptable if it is truly a problem building.)


Shade tree giving whole house & yard cooling

Shade Trees.  You can feel the difference when you go into an older neighborhood with mature shade trees -- it is noticeably cooler.  A large shade tree can shade not only a window but an entire house, and perhaps most of the yard.  The tree cools not only by blocking the sun, but by the transpiration process, where its leaves give off enormous amounts of water vapor that is evaporated into the warm air, thus cooling the air itself.  (Think of it as giant "misting" cooling device.)  It is typically at least 10 degrees cooler around a large shade tree than the surrounding area. 

I suggest shade trees as great for cooling if the tree is to the north, west, or east.  However, it is generally a bad idea to plant a shade tree on the south side of your house. Where will you get sun for solar hot water and solar electric if you block the southern sun? Even in the winter when it drops its leaves, a tree trunk or limb casting a shadow on solar panels can drastically cut ouput.  (Many solar electric panels shut down the entire panel if a thin but distinct shadow falls on the panel.) 

30% Savings Already.  The new Energy Building Codes will require a beginning goal of 30% reduction in energy use from buildings in the first round of codes, likely to go into effect in 2012.
 
For cooling needs, we can get to this goal simply by doing a good job blocking the summer sun from entering windows. 

No Buck Rogers inventions needed, just common sense low cost solutions. 

Can someone hand me that iced tea now?



This article was first posted on June 5, 2009.

 

 

 

 

 

 

 

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