Shading windows in the summer to prevent excessive solar gain can greatly reduce overheating of your house and cut AC bills, or eliminate the need for AC in some climates. A hundred square feet of east facing window can gain over one hundred thousand BTU on a sunny summer day! This is like running your furnace for a couple hours, and is sure to result in overheating, high AC bills, and high greenhouse gas emissions. The pie chart from the Arizona Public Service Company shows that typically, windows account for about half of the total house heat gain.
South facing windows can be shaded with overhangs that will block the high summer sun, but allow the low winter sun to shine in and provide passive solar heating when its wanted. Overhangs do not work for east or west facing windows, since the sun is low in the sky when shining on the east and faces of the house. External shading devices work best for east and west exposures -- these include trees, trellis, external shades, -- anything that blocks the sun. Inside shading devices are not as effective as external shading, but can still be helpful. Internal shades that are reflective on the outside surface will be the most effective as they reflect some of the sun right back out the window. Details below.
For some areas that have hot days but somewhat cooler nights, a combination of reducing heat gain during the day, coupled with ventilation at night can reduce or even eliminate the need for AC. Shading can be a big part of reducing the daytime heat gains. Other techniques for reducing daytime heat gains are covered on the Cooling and Conservation pages. Techniques for providing effective night ventilation (e.g. whole house fans) are provided by the Cooling page. In addition, making the house outer surfaces reflective reduces heat gain.
The sections below give my 2 cents worth on window shading.
If you know of, or have experience with shading techniques that should be added please let me know. If you have have some pictures of shading devices you have installed that you think would be helpful to others, please send them in -- Gary.
A well designed overhang can shade south facing windows from the high summer sun, while still allowing the low winter sun to shine in and provide welcome solar heating. Overhangs are not effective on east or west facing windows because the sun is too low in the morning and afternoon for an overhang to provide any effective shade -- use the other techniques discussed below for east and west facing windows. The sketch shows how overhangs take advantage of the fact that the sun is high in the summer and low in the winter.
The overhang for south facing windows can be incorporated in the roof overhang, or can be a separate overhang or awning just over the window areas.
Look here for two easy to use tools that will allow you to design optimal window (or even house) shading structures: http://www.builditsolar.com/References/SunChartRS.htm#Overhangs
It should be noted that overhangs on south facing windows are only effective
in blocking direct sun.
In looking at the solar radiation numbers for a few locations, it appears that only
about half of the radiation arriving at a window is direct solar radiation. The rest of
the radiation is diffuse sky radiation and reflected radiation from the ground.
So, while overhangs are quite helpful, they will still let though about 50%
of the solar radiation. This means that a strategy that uses sun blocking solar screens
or trellis or landscaping (see below) may be more effective than an overhang.
Or, a combination of the two could be used.
Of course, for east and west facing windows, where overhangs are not effective, some form
of external screen, trellis, or landscaping will be the only effective solution.
You can get the average values of diffuse and total solar radiation on south, north, east, and west facing
windows in your location from the "Solar Radiation Data Manual for Buildings" at this NREL site:
My thanks to Richard Crume for pointing this out.
An exterior vertical trellis works well for shading East or West facing windows, and preventing excessive solar gain. These windows cannot be shaded by overhangs because the sun is low in the sky when shining on the East and West sides of the house.
Plants growing on the trellis can provide some additional shading in the summer and allow more sun during the winter when its desirable. A larger trellis can cool an entire wall plus windows and further reduce heat gain.
A wood trellis design that does not look so effective for low sun -- maybe with plants it would work better?
Or, angled vanes.
An aluminum trellis on the eco house that is populated with plants during the summer.
A moveable sun screen that
could be positioned for
summer sun protection outside
east or west windows.
Nice trellis and tree arrangement
The gone native
Looks nice and cool.
Horizontal trellis to shade deck and windows
Grapevine shading on a Trellis -- colonial Williamsburg.
Plans for a DIY Trellis:
|Elegant Shade Structure
for a very elegant shade structure. This could be used as a standalone
shade structure or adapted to shade a house window.
Very detailed plans.
|Alnet DIY Trellis
|Plans for a simple wood framed, shade cloth trellis fro Alnet|
|Windsor Shade shelter
California Redwood Association
plans for a free standing shading structure.
Could be used for yard/deck shade, or to shade a window.
Some links with external shading devices:
Note how the tilted glass shown in some of these examples is particularly difficult to shade. It also has much higher solar gains in the summer than vertical glass.
Some landscaping/shading advice from the eere site.
Some deck shading Trellis designs from the California Redwood Association
Trees work well to provide shading for windows on all exposures. In most climates, the leaves fall off in the winter to provide welcome sunshine and solar gain.
Large trees can also shade full walls, which can reduce the outside wall temperature from in the 140F range in full sunshine down to something more like the air temperature -- the heat transmission through the wall, which depends on the difference between the inside and outside wall temperatures is correspondingly reduced. Reflective walls can also help.
Evergreen trees can also provide wind protection from cold winter winds.
Here is an excerpt from the North Carolina Solar Center: Passive Cooling for you NC Home
Landscaping is an effective and pleasant means of providing shading for your house. An effectively planned landscape will block out the hot summer sun, encourage warming sun to enter the house in winter, deflect the cold winter winds, and channel breezes for cooling in summer.
In general, an ideal landscape plan for North Carolina would include trees to the east and west of the house to provide summer shading, with the area to the south of the house left relatively clear in order to allow solar heating in winter. Trees will be most effective if they shade east and west windows, where the most heat can enter, but shading east and west walls and the roof is also important. Even trees which do not directly shade the house, such as those planted to its north, are valuable because they reduce the
temperature of the air surrounding the house. Figure 1 shows an "ideal" site plan for most of North Carolina.
These two guides provide some additional information on using landscaping for shading and cooling in southern climate:
Passive Cooling for Your NC Home -- NC Solar Center
Landscaping for Energy Efficiency -- South Carolina Energy Office Energy Brief
External rollup shades made from a shade cloth material (as is used in greenhouses) can block a high percentage of the sunlight while still allowing some useful daylight to enter the room. Such shade cloth can be ordered in various grades that block different amounts of sunlight. Greenhouse supply places and some home centers carry a wide variety of shade cloth. Awnings (either fixed or moveable) are also effective for blocking unwanted sun. Awnings to block the sun on east or west facing windows are more difficult to design, since the sun is low when it shines on east or west walls -- moveable awnings may be preferred for windows on east and west walls.
Awning with slats.
Plans for simple shades you can make for rollup shade cloth exterior blinds:
Rollup Shades made from shade cloth:
|Alnet DIY Rollup Shade Cloth Shades
|Plans for a simple rollup shade from shade cloth.|
|Alnet DIY Awning Plans
for a simple wood framed, shade cloth awning to provide window shade and
reduce heat gain.
An article on this product:
The screen material from Phifer:
And another -- there are lots of
Exterior window shades -- search for shades:
seems like a pretty good idea for reducing summer heat gain on windows.
The shades block up to 90% of sunlight, and mount in vinyl frames (like bug
screens). It looks like they would work on a wide variety of windows,
including ones that would be difficult to handle with other shading
techniques. They do the shading on the outside, where its most effect.
It seems like a DIY version could be worked out. The Phifer SunTex material is said to be available at some home centers. Frames could probably be made from the same material that home centers sell for making insect screens.
|Rollup shade plans
From Charlie's Green House
The Plans (pdf)
|Plans for a simple rollup shade from Charlie's Greenhouse in Seattle.|
Local awning makers and shops that do canvas work can also be quite competitive in some areas.
Some links to commercial suppliers:
There are many, many commercial suppliers of exterior shades and awnings
Rollup exterior shade cloth shades.
Inside shades are the least effective way to block unwanted solar gain through windows, since the sun has already penetrated the window before it gets to the shade, but they can still be effective. To be most effective the face of the blind that faces out should be as reflective as possible to reflect as much sunlight as possible back out the window. Blinds that fit fairly tightly and minimize air circulation behind blind are also more effective.
One supplier of inside shades (one of many, many)
Reflective window films can be applied to inside of the inside pane of glass. These films reflect and/or absorb some of the solar energy, resulting in less heat gain to the house.
Here is one supplier of DIY window film:
Gila Window Film:
It appears from the "performance" data that some of the films may absorb/reflect up about half of the solar heat.
A negative effect of window films is that they reduce solar heating gain in the winter.
Windows can be designed to absorb a high fraction of solar energy, and this helps to reduce solar heat gain. The SHGC (Solar Heat Gain Coefficient) is the fraction of solar heat absorbed by the window. For new windows, the Solar Heat Gain Coefficient (SHGC) will be marked on the window. For climates where heating bills are low and cooling bills are high, windows with high SHGC's are preferred to cut down on summer solar heat gain. The high SHGC does have the negative effect of reducing winter time passive solar heating. If you are in the market for for new windows, the website www.EfficientWindows.org has excellent material on selecting the best windows for various parts of the country and different window situations. It provides detailed information on zillions of different windows by jillions manufacturers.
Greenhouses require shading, and many products and techniques have evolved to provide shading. It may be worth a taking a look at some of these to see if they can be adapted to your house. The greenhouse solutions tend to be simpler and cheaper (e.g. shading "paint" applied directly to the glass -- it washes off for the winter).
Gary 4/23/06, May 18, 2008