Windows and Overhangs In Solar Home Design
How To Choose Appropriate Windows and Overhangs
This article will help you sort out what type of windows to choose and how to think about overhangs to optimize shading. Overhangs are discussed further down in the article, actually right here.
Windows In A Passive Solar Home
Window OrientationIn a passive solar home, south-facing windows should be maximized (if you live in the north hemisphere) and windows in all other directions should be minimized (or at least north-facing windows should be minimized. A general rule of thumb is that you south-facing windows should cover between 7 and 15% of your floor surface. More in a colder climate, less in a hotter and sunnier location.
East and west facing windows may also receive a fair share or total sun light during summer, and hence may contribute significant solar heat. As the sun path moves further south during the winter, solar radiation from the east and west decreases, limiting the potential for beneficial solar heat gain.
North facing windows rarely contribute any major solar heat in the Northern hemisphere. Instead they may result in significant heat loss, and hence should be minimized.
The size of your windows will affect the heat gain and loss they can provide. Bigger windows will inevitably lead to bigger heat losses and gains compared to walls. But windows are not only there for the purpose of energy efficiency, of course. Aesthetics, views, light are other very important consideration.
So how to decide?
Well, the good news is that with proper location, shading and type of windows, it is possible to optimize solar heat gain and minimize energy losses, making window size a less important issue.
Type of Window
The type of window you choose will also matter. Older windows are certainly able to admit solar light and heat, but often are not so efficient when it comes to maintaining the heat inside. Poor insulation and air leaks can be a problem.
Newer, more energy efficient windows are designed to let the heat in, but to keep the cold out, as they are air tight and well insulated.
Windows can be labeled (amongst others) according to their solar heat gain coefficient (SHGC) and their U-factor. The SHGC can be between 0 and 1 and expresses how much solar radiation is transmitted through a window. The U-factor on the other hand expresses how well insulated the window is, including its window assembly. A low U-factor means that the window is well insulated and hence the greater a window's resistance to heat flow.
For solar gain, south facing windows should have a relatively high solar heat gain coefficient (SHGC), of 0.5 or above, except in cooling dominated climates, where all windows likely have a SHGC of 0.35 or less.
Windows facing other directions may also benefit from gas fills and multiple glazing options to improve insulation.
Windows can have several different characteristics and I wouldn't make my choices without consulting an expert, who can give advice targeted for my particular situation. Since a variety of window technologies can improve window energy efficiency, including gas fills, low-E coatings, and high-performance frame options, it is not always intuitive what the overall performance of the window will be. How these technologies affect a window's energy performance depends on the sum of all parts. This is where whole window energy ratings help, accounting for the combined effect of glazing, spacers and frame. The only reliable way to determine whole-window energy properties are the ratings certified by the National Fenestration Rating Council (NFRC).
Without going into detail, the energy ratings included on the NFRC label are the U-factor, Solar Heat Gain Coefficient (SHGC), Visible Light Transmittance (VT) and, optionally, Air Leakage (AL) and Condensation Resistance (CR). To learn more, visit NFRC's website here.
Overhangs In A Passive Solar Home
Exterior roof overhangs are an important part of building a passive solar home. The purpose of overhangs is to shade the windows in different seasons and thereby prevent our home from overheating.
For summer months, overhangs should (ideally) completely shade windows facing the south. And during winter time, full sunlight must be allowed on windows.
Depending on where, geographically your house is situated as well as to what extent it is facing the true south, your overhangs should be designed in different ways and will be more or less efficient. If the building element bears more than about 30Â° off true south, the effectiveness of an overhang, as with any solar feature, begins to decrease significantly.
The below image provides a sample design for a house located at 40 degree North Latitude, facing the true south.
Every climate requires special design attention and few effective guidelines are available. Instead, as with window choice and design, make sure you seek professional advice for your particular situation.
Also remember that shading a window doesn't necessarily require a fixed roof overhang. Trees, awnings, and Bahama shutters (top-hinged louvered shutters typically propped open with wooden dowels) are examples of other options. All options have their pros and cons.
Your final choice will have to depend on both your budget, aesthetics and of course the aim to make your passive solar home as energy efficient as possible.
Other than windows and overhangs you should consider thermal mass and site location to get a good solar home design.
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