In this article series we discuss the selection and installation of windows and doors, following best construction and design practices for building lighting and ventilation, with attention to the impact on building heating and cooling costs, indoor air quality, and comfort of occupants.
Shading of glass with overhangs, plantings, or shades will
reduce cooling loads and increase comfort in any climate
with significant cooling loads.
Shading of windows will also reduce glare
(Figure 3-19, below), fading of furnishings, and localized
overheating in rooms with south- or west-facing glass.
Window overhangs, plantings, or exterior shades that block the sun
before it strikes the glass are the most effective approach
since the heat never gets into the building. But light-colored
interior shades can also substantially reduce heat gains.
Spectrally Selective Window Glazing
From a shading standpoint,
using spectrally selective glass (SHGC below .40)
is like having shades or blinds on standard low-E glass.
However, adding good shading to spectrally selective glass
can reduce cooling costs by another 10 to 15%. This would
make sense in very hot climates or on houses with large
expanses of glass on the south or west side. In many cases,
the shading adds no cost or serves other design needs. For
example, a porch on the east or west side of a house provides
effective shading as well as outdoor living space.
Plantings for Window Shading
Deciduous trees can provide very effective
summer shade on the south side but, depending on the type
of tree, may block 20% or more of the solar radiation in
winter. Because trees follow the local seasons rather than
the calendar, the shading tends to occur when needed most.
For example, leaves appear earlier in the spring and last
longer in the fall in warmer climates, which need spring
and fall shading. Trees also cool the area around them by
their natural evaporative cooling—as water evaporates
from the leaves.
Other options for shading south-facing windows include
trellises with dense foliage or evergreen trees. Evergreens
should be tall enough to block the summer sun but
trimmed so their canopies allow the low winter sun to
reach the windows.
On the east and west sides of the house, trees or large
shrubs can provide very effective shading, since the problem
times are morning and afternoon when the sun is low
in the sky and easily blocked by a well-placed planting,
either deciduous or coniferous.
Fixed Overhangs. These are commonly used on the
south side of homes with clear glass or high-solar-gain
glass. To be effective, the overhangs must be sized correctly
to reject the high summer sun but allow in the low
winter sun. In most temperate climates, a 1 1/2 - to 2-footwide
overhang is adequate for average size windows.
However, to provide full shading from March to September
in hot climates may require a 3-foot or wider overhang.
One limitation of fixed overhangs is that the shading
will be the same on March 21 and September 21, although
the heating and cooling needs at these times may be very
different. The following guidelines for shading south facing
glass strike a balance between summer and winter
Cold climates: Above 6,000 heating-degree days
(HDD), locate the shadow line at mid window, based
on the June 21 noon–sun angle (see Figure 3-11 below). This
will shade the window 50% in mid-summer and provide
full sun penetration from late September to late
March. If more shading is required in summer, locate
the shadow line closer to the window sill.
Moderate climates: In climates with less than 6,000
HDD and less than 2,600 cooling-degree days (CDD),
locate the shadow line at the window sill based on the
June 21 sun angle at noon. This will allow full sun
exposure from late October to mid-February.
Hot climates: Above 2,600 CDD, locate the shadow
line at the window sill using the March 21 sun angle at
noon. This will provide full shading from late March
to late September and about one-third shading in
This dark plastic or fiberglass screening
is mounted on the exterior of the window in a frame or
retractable roller or, in some cases, applied directly to the
glass. Depending on the weave, it can reduce a window’s
solar heat gain by 40 to 70%. Solar screening reduces glare
and fading, but it also reduces daylight and obscures
Solar screens are best used on difficult to shade areas, such
as west-facing glass or skylights. Some are designed to
also serve as insect screens.
Interior Shades for Windows
Though not generally as effective as
exterior shading, light-colored drapes, shades, or blinds
can reduce solar gain by 10 to 50% (for typical curtains or
blinds) to as high as 70% for an insulated, reflective shade
that seals tightly around the window perimeter.
of an interior shade depends on the reflectivity
of its outer face, the density and R-value of the material,
and whether it seals around the window. Between-the-glass
pleated shades or mini blinds, available from Pella and some
smaller manufacturers, are more effective in blocking solar
gain than interior shades, because they block the heat before
it enters the building interior.
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