Passive Solar Design
DESIGN WITH THE SUN IN MIND
Sunlight can provide ample heat, light, and shade and
induce summertime ventilation into the well-designed home. Passive solar design
can reduce heating and cooling energy bills, increase spatial vitality, and
improve comfort. Inherently flexible passive solar design principles typically
accrue energy benefits with low maintenance risks over the life of the
building.
Introduction
Passive solar design is the
process of creating a home that provides both shelter and comfort year-round
while responding to regional climate conditions and minimizing dependence on
energy-consuming mechanical systems. The goal is to build and occupy a home
that (a) utilizes solar heat gain in the winter to warm the interior of a home,
(b) controls solar heat gain in the summer, and (c) facilitates day lighting,
natural ventilation, and nighttime cooling to keep a home comfortably cool in
the summer. This fact sheet describes four steps in the first phase of
designing the basic form and layout of a home:
·
Shape
·
Orientation
·
Location
·
Aperture
Green Building Benefits
Passive solar design saves energy
by maximizing the home’s natural heating, cooling, ventilation, and lighting
options. Reduced energy consumption reduces utility bills for the owner or
occupant, reduces air pollution from power plants, and reduces the
environmental impacts of resource extraction associated with fossil fuels.
Moreover, homes featuring good passive solar design are typically healthier and
more comfortable.
Shape
S is for Shape: In a temperate climate, shape a
home to be roughly rectangular. An important factor in determining a home’s
thermal performance is its floor area-to-surface area (F/S) ratio. This is the
ratio of the finished floor area of a home to the sum of all its exterior
surfaces (the exterior walls and roof). The more living space enclosed per unit
of exterior surface area the less heat gain and loss will occur through the
building envelope. A home with a high F/S ratio is a more efficient design than
one with a low F/S ratio. The F/S ratio of a home should be as large as
possible but only up to a point. In order
to minimize the heat gains and losses through the enclosure of a home, a
compact shape is desirable. The most space-efficient orthogonal shape is a
cube. This configuration for a home, however, may place a large portion of the
floor area relatively far from the home’s perimeter. Consequently, passive
solar heating, day lighting and natural ventilation may be difficult to
implement. A house form that optimizes solar heat, daylight, and ventilation
will be elongated in the east-west direction (see orientation below) so that
more of the living area is closer to the perimeter and can take advantage of
these passive options. While this may appear to compromise the thermal
performance of the home, the heating and cooling load savings achieved by
well-designed solar heating, day lighting, and natural ventilation techniques
will more than compensate for the increased heat gains and losses through the
building envelope.
In summary, a home located in a
cold, cloudy climate with few opportunities for solar heating should have a
high F/S ratio and a more cubical shape. For a cubical shape, all four walls
are of equal length, and the ratio of length to width is 1.0. A home located in
a hot, humid climate should have a lower F/S ratio and be more narrow and
rectangular in shape to facilitate passive options such as day lighting and
natural ventilation year-round. For example, a narrow rectangle that is 48 feet
long and 16 feet wide has a length to width ratio of 3.0. A home located in a
temperate climate should have a shape somewhere in between a cube and a narrow
rectangle. To achieve an optimum F/S ratio in temperate climates, some
references indicate that a rectangular shape with a length to width ratio in
the range of 1.25-1.50 is best. For example, a home with a rectangular
footprint of 40’x32’ or 48’x32’ would fall within this ratio.
Orientation
O is for Orientation: Face the longer walls of a home to
the south and north. The orientation of a home on its site or lot is critical
to achieving energy efficiency and thermal comfort. The ability of a home to
properly utilize solar heat gain in the winter and mediate solar heat gain on
walls, roofs, and windows in the summer depends a great deal on where and how
the home is placed on the site and especially the direction the windows face.
The south side of the home must be oriented to within
30 degrees of due south. software that can improve the design and integration
of passive solar principles into modern residential structures.
Passive solar design includes the
proper positioning of a home on its lot or site as well as proper window
placement to make your house more comfortable year-round and save money on
annual heating and cooling costs. The first step is to find a true north-south
line on the home’s lot or site. When using a compass to find magnetic north, an
adjustment must be made for finding true (solar) north-south. In the Bay Area,
the magnetic declination (the direction the compass needle points to) is about
17° east of true north. This means that true north is 17° counter-clockwise of
a compass needle pointing to magnetic north. Another way to find true south is
to visit your lot or site on either the Vernal (Mar 21) or Autumnal (Sep 21)
Equinox. On those two days of the year, the sun rises true east, is located at
true south at solar noon (12:15PM PST in the Bay Area), and sets true west.
NORTH: The north side of a home is the
coolest side because it receives very little direct sun. A north wall will
receive sunlight only during the very early morning and very late afternoon
hours of summer. At these times of day, the sun’s vertical position in the sky
(the solar altitude) is at its lowest, so trees and adjacent homes quite often
shade north walls. North walls receive no direct sunlight during fall, winter,
and spring.
SOUTH: The south is the sunniest side
of a home since the sun’s position for most of the day is in the southern sky.
During the summer in the Bay Area (and other locations at 38°N latitude), the
sun is very high in the sky at solar noon (about 1:15PM PDT). Consequently,
overhangs or awnings can easily shade south walls and windows (see R is for
Roof below). Properly sized overhangs also have the advantage in winter, when
the sun is much lower in the sky, of allowing direct transmission of sunlight
through south windows for passive solar heating.
EAST: The east side of a home will be
exposed to solar heat gain in summer mornings. Careful placement of window area
on the east side of a home is recommended. Overhangs on east walls do not
perform well for shading because the sun is so low in the sky during the
morning (close to the horizon) that it travels beneath the overhang.
WEST:
The west side of a home will bear the brunt of the sun’s heat. Therefore,
minimize not only the window area, but also the total wall area facing west. In
those parts of the Bay Area where air conditioning is common, the overheated
period (the time when your air conditioner will run the most) occurs between
3PM and 5PM. Since the ambient heat of the day has built up (air temperature),
the sun’s added heat compounds the cooling problem. For this reason, the
western exposure of a house should have as few windows as possible. If windows
are required by the dictates of the site or design, protect them from solar
heat gain with porches, trees, trellises, sunshades, carports, or out
buildings.
To maximize the benefits of shape
and orientation, a home in the Bay Area should be elongated in the east-west
direction. This will increase the surface area of north and south walls and
reduce the surface area of east and west walls. With proper window shading,
solar heat gain potential in the winter will be maximized and unwanted solar
heat gain during summer mornings and afternoons will be minimized. If facing a
longer wall true south is not feasible, an orientation that is within 30° east
or west of true south will result in only a minor decrease in annual
passive solar heating and cooling
performance. In many Bay Area locations, the south, southeast, and southwest
sides of a home also receive the prevailing summer breezes. Consequently,
elongating the home in the east-west direction will also increase the wall area
and potential window area for facilitating natural ventilation. By placing
windows on opposite sides of the house, some on the windward side and some on
the leeward side, the home can be designed to take advantage of natural
ventilation in the spring, summer, and fall. Once the overall shape and
orientation of a home has been determined, proper room location should be
considered in order to take advantage of direct passive solar heating and
cooling.
Location
L is for Location: Locate major living spaces of a home on the south perimeter.
Allowing sunlight to enter directly into a living space is the most effective
way of implementing passive solar heating. By doing so, the space can be warmed
directly in the winter without having to rely on heat being transferred from
one space to another. Additionally, this allows enough daylight to enter the
room so that adequate illumination is provided for most daytime activities.
Spaces that need warmth in winter, cool breezes in summer, and light year-round
should be placed along the perimeter of the home near a south-facing wall.
Those that don’t require these conditions can be placed in the home’s interior.
Each of the rooms along the south-facing wall should have its own solar
aperture.
Aperture
A is for Aperture: Design south-facing windows for solar gain and ventilation. Winter
sun angles are very low making vertical and steeply pitched glazing optimum for
transmitting winter solar heat gain into a home. South-facing windows are the
least costly, simplest, and easiest way to accomplish this. They gain more heat
during a
clear or partly cloudy winter day by transmitting solar
radiation into the home than they lose at night. Thus, south-facing windows are
a net energy gain in the heating season. Windows that don’t face south
typically lose more energy than they gain each day during the heating season and
are therefore a net energy loss. South facing windows that are a net energy
gain are called the home’s solar aperture. Some references give a rule of thumb
that about half,
or even a majority, of a home’s windows should face south.
COST
It takes more thought to design with the sun; however,
passive solar features such as additional glazing, added thermal mass, larger
roof overhangs, or other shading features can pay for themselves. Since passive
solar designs require substantially less mechanical heating and cooling
capacity, savings can accrue from reduced unit size, installation, operation,
and maintenance costs. Passive solar design
Techniques may therefore have a higher first cost but
are often less expensive when the lower annual energy and maintenance costs are
factored in over the life of the building.
