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All of these
features are not anything new and revolutionary and have been
readily used in ancient civilizations where natural energy of
the sun has been carefully considered when building dwellings.
Let us now
explore in more details each of the listed elements.
House
Orientation
The building’s
southern exposure must be clear of large obstacles (e.g., tall
buildings or tall trees) that block the sunlight. Although a
true southern exposure is optimal to maximize solar
contribution, it is neither mandatory nor always possible.
Provided the building faces within 30º of due south,
south-facing glazing will receive about 90 percent of the
optimal winter solar heat gain.
Roof
Overhangs
The summer sun
rises higher overhead than the winter sun. Properly sized window
overhangs or awnings are an effective option to optimize
southerly solar heat gain and shading. They shade windows from
the summer sun and, in the winter—when the sun is lower in the
sky—permit sunlight to pass through the window to warm the
interior. Landscaping helps shade south-, east-, or west-facing
windows from summer heat gain. Mature deciduous trees permit
most winter sunlight (60 percent or more) to pass through while
providing dappled shade throughout summer.
Windows Selection & Placement
Heating with
solar energy is easy: Just let the sun shine in through the
windows. The natural properties of glass let sunlight through
but trap long-wave heat radiation, keeping the house warm (the
greenhouse effect). The challenge often is to properly size the
south-facing glass to balance heat gain and heat loss properties
without overheating.
Increasing the glass area can increase building energy loss. New
window technologies, including selective coatings, have lessened
such concerns by increasing window insulation properties to help
keep heat where it is needed.
In heating climates, reduce the window area on north-, east-,
and west-facing walls, while still allowing for adequate
daylight. Effective south-facing windows require a high solar
heat gain coefficient (SHGC)—usually 0.60 or higher—to maximize
heat gain, a low U-factor (0.35 or less) to reduce conductive
heat transfer, and a high visible transmittance (VT) for good
visible-light transfer. (SHGC refers to the portion of incident
sunlight admitted through a window, and U-factor indicates the
heat loss rate for the window assembly.)
In cooling climates, particularly effective strategies include
preferential use of north-facing windows along with generously
shaded south-facing windows. Shading from landscaping,
overhangs, shutters, and solar window screens helps lower heat
gain on windows that receive full sun.
Cost-effective windows for cooling climates have a U-factor
below 0.4 and an SHGC below 0.55 (a lower SHGC cuts cooling
costs). Wherever possible, climate-specific window-property
recommendations from the
Efficient Windows Collaborative should be followed.
In cold
climates, a strategy termed “suntempering” orients most of the
home’s glazing toward the south—a glazing area of up to 7
percent of the building floor area. Additional south-facing
glazing may be included if more thermal mass is built in. Such a
shift in window location is a great strategy for cold climates
and costs nothing beyond good planning. Many passive solar homes
are merely suntempered.
Thermal Mass
Thermal mass, or
materials used to store heat, is an integral part of most
passive solar design. Materials such as concrete, masonry,
wallboard, and even water absorb heat during sunlit days and
slowly release it as temperatures drop. This damps the effects
of outside air temperature changes and moderates indoor
temperatures. Although even overcast skies provide solar
heating, long periods of little sunshine often require a backup
heat source. Optimum mass-to-glass ratios, depending on climate,
may be used to prevent overheating and minimize energy
consumption. Avoid coverings such as carpet that inhibit thermal
mass absorption and transfer.
Natural
Ventilation
Apt use of
outdoor air often can cool a home without the need for
mechanical cooling, especially when effective shading,
insulation, window selection, and other means already reduce the
cooling load. In many climates, opening windows at night to
flush the house with cooler outdoor air and then closing windows
and shades by day can greatly reduce the need for supplemental
cooling. Cross-ventilation techniques capture cooling,
flow-through breezes. Exhausting naturally rising warmer air
through upper-level openings (e.g., clerestory windows, which
allow for what’s known as the stack effect) or fans (e.g., a
whole-house fan) encourages lower-level openings to admit
cooler, refreshing, replacement air.
Natural
Lighting
Sometimes called
daylighting, natural lighting refers to reliance on sunlight for
daytime interior lighting. Glazing characteristics include
high-VT glazing on the east, west, and north facades combined
with large south-facing window areas. A daylit room requires, as
a general rule, at least 5 percent of the room floor area in
glazing. Low-emissivity (low-E) coatings can help minimize glare
while offering appropriate improved climatic heat gain or loss
characteristics. Sloped or horizontal glass (e.g., skylights)
admit light but are often problematic because of unwanted
seasonal overheating, radiant heat loss, and assorted other
problems.
Passive Solar
Design Tools
One of the best
ways to design an energy-efficient house featuring passive solar
techniques is to use a computer simulation program.
Energy-10
is a PC-based design tool that helps identify the best
combination of energy-efficient strategies, including
daylighting, passive solar heating, and high-efficiency
mechanical systems. Another tool to optimize window area and aid
window selection is
RESFEN. Access these and other passive solar design tools
from the
DOE’s Building Technologies website.
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