Design Variations-Shading
Shading in the Northern Zone (mostly heating)
In order to receive solar radiation in winter,
the house must be located so that it is not in the shadow of other buildings or
landscape elements. If possible, locating the building on the north end of the
site (in the northern hemisphere) provides a greater assurance of future solar
access. Of course, deciduous trees can be located within these limits.
Moving from no shading to more shaded conditions in a cold climate increases
heating costs. These increases in heating costs, however, are largely offset by
decreased cooling costs if the house is air-conditioned. The benefits of
high-performance windows can offset undesirable shading conditions that reduce
passive solar gain. Although this figure suggests that shading makes little net
difference when heating and cooling costs rare combined in Boston, this should
not necessarily be generalized to other cold climate cities. Different climates,
fuel costs, window orientations and glazing areas may produce different results.
Window A |
Window B |
Window C |
Window D |
Window E |
Window F |
|
 |
 |
 |
|
|
 |
|
Glazing |
||||||
Frame |
||||||
U-Factor |
1.16 |
.63 |
.49 |
.37 |
.34 |
.18 |
SHGC |
.76 |
.62 |
.56 |
.53 |
.30 |
.40 |
VT |
.74 |
.62 |
.58 |
.53 |
.50 |
.49 |
Shading in the Central/North and Central/South Zones (heating and cooling)
In climates where there is both a significant
heating and cooling season, there is a desire for solar gain in winter while
preventing gain in summer. This can be accomplished with overhangs on the south
and other shading devices elsewhere. As shown in the figure, for Sacramento, the
cooling season benefits of shading are notable.
When the house has clear single glazing (Window A), interior shades, overhangs
and typical combinations of shading devices significantly reduce energy costs.
Naturally, a completely shaded house has the best performance in a hot climate.
Reliance on any form of shading is not as important when windows
with a low solar heat gain coefficient are used. Using a low-solar-gain low-E
coating (Window E), results in great energy cost reductions for all conditions
even with no shading. This is because the glazing itself provides the necessary control of
solar radiation, so these additional measures become less important in terms of
energy use. The triple-glazed low-solar-gain low-E window performs best (Window F). Note that high-solar-gain low-E (Window D) performs more poorly than
the low-solar-gain low-E (Windows E and F). With more west-facing window orientation or greater glazing area, the savings from high-performance
windows would be even greater.
Window A |
Window B |
Window C |
Window D |
Window E |
Window F |
|
|
 |
|
|
|
|
|
Glazing |
||||||
Frame |
||||||
U-Factor |
1.16 |
.76 |
.47 |
.49 |
.37 |
.34 |
SHGC |
.76 |
.68 |
.33 |
.56 |
.53 |
.30 |
VT |
.74 |
.67 |
.53 |
.58 |
.53 |
.50 |
Shading in the Southern Zone (mostly cooling)
The best place to shade a window is on the outside, before the sun strikes the
window. Exterior shading devices have long been considered the most effective way
to reduce solar heat gain into a home. To most effectively reduce solar heat gain
on the interior, the shade or drapery used to block the sunlight should have high
reflectance and low transmittance. If your goal is to minimize cooling energy
use, or you live in a house without air-conditioning in a hot climate, then the
combination of good shade management with low SHGC windows will be the best
strategy. This can be accomplished with overhangs on the south and other shading devices elsewhere.
In addition to shading the building from direct sun, trees have been found to
reduce the temperature of air immediately around them by as much at 10¡F (5¡C)
below the temperature of the surrounding air due to evaporation of moisture.
Trees and bushes can provide strategic shade from low east or west sun angles
that are extremely difficult to shade architecturally.
When the house has clear single glazing (Window A), interior shades, overhangs
and typical combinations of shading devices significantly reduce energy costs.
Naturally, a completely shaded house has the best performance in a hot climate.
Reliance on any form of shading is not nearly as important when windows
with a low solar heat gain coefficient are used. Using a low-solar-gain low-E
coating (Window F), results in great energy cost reductions for all conditions
even with no shading. This is because the glazing itself provides the necessary control of
solar radiation, so these additional measures become less important in terms of
energy use. Note that high-solar-gain low-E (Window E) performs more poorly than
the low-solar-gain low-E (Window F). With more west-facing window orientation or greater glazing area, the savings from high-performance
windows would be even greater.
Window A |
Window B |
Window C |
Window D |
Window E |
Window F |
|
|
 |
|
|
|
|
|
Glazing |
||||||
Frame |
||||||
U-Factor |
1.16 |
1.16 |
.76 |
.59 |
.37 |
.34 |
SHGC |
.76 |
.65 |
.68 |
.37 |
.53 |
.30 |
VT |
.74 |
.56 |
.67 |
.57 |
.53 |
.50 |
Note: The thermal performance properties of specific glazings and frames can vary depending on product design and materials. The results presented here are averages. Consult specific manufacturers for NFRC rated U-factors and SHGCs for products of interest. The annual energy performance figures shown here were generated using RESFEN for a typical new 2000 sq. ft. house with 300 sq ft of window area (15% of floor area). The windows are equally distributed on all four sides of the house. U-factor, SHGC, and VT are for the total window including frame. Energy use and savings between different window options will typically be higher for homes which are not as well insulated as typical new homes. The costs shown here are annual costs for space heating and space cooling only and thus will not correlate to utility bills. Costs for lights, appliances, hot water, cooking, and other uses are not included in these figures. The mechanical system uses a gas furnace for heating and air conditioning for cooling. These figures are based on typical energy costs for this region. Natural gas prices and electric prices are provided by the Energy Information Administration (www.eia.doe.gov).