Design Your Home Using Passive-Solar Heating

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PHOTO: CATHERINE WANEK
You can save money when you design your home to use passive-solar heating.

Learn how designing your home using passive-solar heating will save you money.

Design Your Home Using Passive-Solar Heating

If you’re planning to build your dream home someday, this
article could save you thousands of dollars. Including
simple, passive-solar features in any style home can cost
next to nothing up front and save you unbelievable amounts
over the long-term in reduced energy bills.

Millions of homes easily could be designed to capture free
heat directly from the sun. But instead we are
burning — wasting — huge amounts of oil and natural
gas every winter. The missed opportunities to tap into
solar energy are so fantastic they boggle the mind, and
nowhere is our blindness to the potential of solar more
troublesome than in the home-heating arena.

You can incorporate passive-solar heating in any style
home, as the photos that accompany this story show. Or you
can add solar features when remodeling an existing home, as
long as the south side of the house receives full sun most
of the day. When correctly designed, solar homes provide
unrivaled comfort in winter and summer They offer large,
south-facing windows, generous views, sunny interiors and
open floor plans.

Architect Debbie Rucker Coleman, who has been designing
solar homes since 1985, says her clients are impressed with
how spacious the sunlight makes the home feel. “In addition
to low heating bills, passive-solar homes are cool in
summer. They are delightful places to live,” she says.

Coleman’s drawings show how two conventional house styles
could easily become passive-solar homes.

How Passive-Solar Works

Heating homes with sunlight, known as passive-solar
heating, is based on the simple idea of using south-facing
windows to admit low-angled winter sun. Sunlight streaming
into the home warms the interior space. Thermal mass, such
as tile floors and interior masonry walls, stores the sun’s
heat and releases it when room temperatures fall at night
or during cloudy weather. Choose a house design that
accommodates the right amounts of south-facing glass and
thermal mass. Add careful caulking and ample insulation
(usually slightly higher than building codes currently
require), and you’ll have a solar heated home that requires
little or no heat from any nonrenewable fuel source. In the
summer, a solar home’s thermal mass and insulation,
together with properly sized overhangs to shade the
windows, keep the home comfortable and reduce cooling
requirements.

Simply orienting a conventional house to the south will cut
annual energy bills by at least 10 percent, saving
thousands of dollars over a home’s lifetime. Add a long
south-facing wall of windows and some thermal mass and you
easily can tap sunshine’s free energy to meet 50 percent to
70 percent of a home’s heating requirements. Do your
homework or hire a solar architect to create a rigorous
passive-solar design and you can reduce your energy bills
by 80 percent to 100 percent. Given the probability energy
costs will increase steadily in the coming years, the
long-term savings from a passive solar home could become
very substantial, as we’ll show in detail below.

Up-Front Costs for Passive-Solar Heating

According to Ron Judkoff, director of the Buildings and
Thermal Systems Center at the National Renewable Energy Lab
in Golden, Colorado, passive-solar features increase the
cost of building a new home by anywhere from nothing to
about 3 percent. (On a $200,000 home, for example, the
maximum additional cost of incorporating passive solar
heating might be only another $6,000.) Since many building
codes now require much more energy-efficient windows,
walls, ceilings and foundations than in the past, and
you’ll need a much smaller furnace or other backup heat
source, passive solar frequently adds very little or
nothing to the cost of a new home.

Judkoff bases his cost estimates in part on a series of
case studies sponsored by NREL and the American Solar
Energy Society, headquartered in Boulder, Colorado. (See
“Comparing Passive Solar Savings”.) Data on passive-solar
homes were collected from a variety of locations across the
United States. The study found the addition at cost of
building a passive-solar home ranged from nothing to 3
percent, while the annual savings from passive-solar
heating in the homes ranged from $220 in New Mexico to
$2,255 in New Hampshire. Based on recent energy prices, a
passive-solar home in a northern location could save you as
much as $67,000 on heating and cooling costs over a 30-year
period.

We all know energy prices are only going to rise. Some
regions already have experienced sudden 100-percent spikes
in natural gas prices. Major increases in natural gas and
oil prices seem inevitable. (See “Running Out of Gas.”)
Without an inexpensive, reliable fuel source, millions of
homeowners who rely on natural gas and oil may suffer
enormous economic hardship. So let’s take another look at
those estimated cost savings from the NREL and ASES case
studies. If we assume a 5 percent annual increase in energy
costs, the potential 30-year savings from a passive solar
home jump to $141,400, quite a return on a maximum
investment of just 3 percent of your construction costs. If
energy prices increase 10 percent per year, the estimated
30-year savings in cold climates could be more than
$400,000.

No one knows for sure how fast fuel prices will increase.
But whatever happens, passive solar clearly makes fantastic
sense from a financial standpoint. And the bonus is the
environmental benefit of reduced air pollution. Coleman
estimates that compared to the average home, her
2,100-square-foot solar home — with its average heating
or cooling costs of $12 per month will save 574,410 pounds
of carbon dioxide emissions over 30 years.

To harvest free energy from the sun, just orient the long
side of the house to the south, add more windows to the
south side and pay attention to overhangs, as we’ve shown
for this conventional ranch house (above) and
colonial-style home (at right)

Passive-Solar Design Basics

Here are the basic principles to follow in designing a new
or remodeled passive solar home:

1. Choose a site that receives south sun during winter.
Obstructions to the south of the site, such as tall
evergreen trees, buildings or hillsides, need to be kept at
least 1.7 times their height away from the home. When in
doubt, visit the site around December 21, when the sun is
the lowest in the sky. The site should receive full sun
from 9 a.m. to 3 p.m.

If you’re choosing to build on a small lot, select one that
is deep from north to south, to ensure good solar access.
Locating the house’s septic drainage field within the solar
access zone is another strategy for maintaining good solar
access, since that area will need to be kept clear of trees
and shrubs, which would otherwise block the southern sun.

2. Choose a home design with few projections below the roof
line and no porches on the south. Projections shade
adjacent windows; porches on the south prevent the sun from
entering. Porches on the east and west can be beneficial by
shading windows from the hot summer sun.

3. Orient the longest wall of the house so it faces true
south. Rectangular floor plans minimize the exposure of
east and west walls to summer sun, which is especially
helpful in hot climates. The front, back, or side of the
house can be the south wall. (True south is not the same as
the magnetic south shown by compasses. Check with a local
surveyor’s office to find out how many degrees to adjust
from magnetic south.)

Can you deviate from a due south orientation? Sure, but
you’ll pay a price in dollars and thermal comfort. Straying
from a solar-south design reduces wintertime heat gain and
may increase summertime solar gain, leading to overheating.

The more rooms that have some south windows, the better.
This helps eliminate the need for fans or ducts to move
warm air from one area to another. Place rooms that require
less heat, such as workshops, bedrooms and kitchens, on the
north side of the house.

4. Maximize windows on the south side. South glass should
be a minimum of 7 percent of the house’s square footage for
a sun-tempered home and a maximum of 12 percent for fully
passive-solar designs. Don’t go wild on windows. North,
east and west windows should follow the “Window Allocation
Guidelines”. Exceeding the 12 percent guideline for south
windows may cause the home to overheat in summer, and may
allow excess heat loss from the windows during the night
and during long, cold, cloudy periods.

Coleman recommends choosing south glass carefully for your
climate. Many of the new low-emissivity (low-e) coatings
reduce heat loss and gain, but for south windows you do not
want glass that keeps out the solar heat. She recommends
south glass that has a Solar Heat Gain Coefficient (SHGC)
of at least 0.5. In warmer climates with properly designed
south overhangs, uncoated double-parre glass is preferred.

5. Design the roof overhangs to shade windows properly from
the high summer sun. Overhangs are key to successful solar
homes. Judkoff says the exact geometry of overhangs is
critical for balancing the need to admit maximum sunlight
in winter and minimize solar heat gain in summer.
Generally, the warmer and sunnier the climate, the deeper
the overhang should be. A 2-foot overhang nicely shades an
8- to 9-foot wall in most locations. Coleman recommends the
website Sustainable By Design to
help you design overhangs properly. If overhangs aren’t
possible, use insulated shutters to keep out the summer
sun.

6. Provide thermal mass (tile floors and brick or masonry
walls) in the south side of the house. Mass absorbs and
stores heat when sunlight strikes it or when its
temperature is lower than the air temperature. As the
room’s air temperature drops below the mass’ surface
temperature, heat is released and the air is warmed.
Temperatures indoors remain relatively stable and
comfortable, despite dramatic oscillations in outdoor
temperatures.

The mass in floors, framing, wallboard and furniture is
usually sufficient to accommodate the solar heat in
sun-tempered homes. But when you build a full passive solar
design with ‘up to 12 percent south glass, you need to add
extra thermal mass in the form of tile, concrete floors, or
masonry walls or planters. For optimal results, some mass
should be in direct contact with the incoming sunlight
throughout the day. The mass also should be distributed
throughout the house. Add about 7 square feet of
4-inch-thick mass for every 1 square foot of south glass
above the 7 percent minimum. One easy, inexpensive way to
add this mass is to choose a concrete slab-on-grade
foundation.

In addition to these solar design factors, follow these two
principles that apply to all energy efficient homes:

7. Insulate and seal the structure well. Careful attention
to detail is essential. Insulation should not be compressed
and air should not leak in. If you can’t get as much south
glass as you would like, adding extra insulation can result
in the same overall lower energy consumption. judkoff
recommends insulating at least to the level prescribed by
the International Energy Conservation Code or ASHRAE 90.2,
which are region-specific recommendations for the
building-envelope elements and mechanical systems. Choose
energy-efficient windows and consider using insulated
shades to keep heat from escaping at night (and to keep the
heat out in the summer), especially in cold climates.
Entryways separated from the main living space by an, inner
door are especially helpful in preventing cold air from
rushing in whenever the outside door is opened.

8. Correctly size the heating and cooling system. Many
solar homes require almost no additional heat, so you will
need a much smaller, less expensive heating system. A
woodstove or natural gas wall heater may be sufficient, or
you can connect baseboard radiators to your hot water
heater. The insulation and thermal mass features of a solar
home make it more resistant to summer overheating, and in
some climates you may get by with using ceiling fans
instead of air-conditioning.

Helpful Passive-Solar Design Tools

Many professional solar designers use powerful new software
like Energy-10, a computer program that allows designers to
predict the energy performance of various building-design
strategies, permitting a fine tuning of designs to achieve
optimal comfort, performance and economy. (Energy10 is
available from the Sustainable Buildings Industry Council.
(see Resources at the end of this article. ) Designers also use a Solar Pathfinder, a $255 device that
allows a budder or architect to fully assess the year-round
solar potential of a site in about 15 minutes.

Debbie Rucker Coleman prefers a simpler approach using
Passive Solar Design Strategies: Guidelines for Home
Building
by the Sustainable Buildings Industry Council
(see Resources at the end of this article.) . “Most homeowners would be able to
follow these guidelines, as the notebook is put together
very well. I like the program because it tells you the
percentage Of Your heating needs the sun is providing and
lets you compare strategies such as adding insulation or
increasing south glass. You can complete the work sheets
manually or with the computer software version.”

Simple Passive-Solar Solutions

So what are the worst-case consequences if you follow the
basic principles, but don’t want to perform your own
detailed energy analysis or hire a professional to do it
for you? Less-than-optimal amounts of south-facing windows
may require running an auxiliary heating system more, or
may necessitate adding another log to the woodstove or
putting on a sweater. Undersized mass will cause winter
daytime overheating, which is easily remedied by opening
windows.

A less-than-perfect passive-solar design can be remedied
after construction. What you cannot easily change later is
the basic shape, design and orientation of the house.
That’s why a well-insulated, south-facing structure is the
essential foundation for a comfortable passive-solar home.


Solar Resources

Books and Plans

The Solar House: Passive Heating and Cooling by Dan Chiras.

Sun-inspired Home Plans by Debbie Rucker Coleman. Available
from Energetic Design: www.sunplans.com

The Best of Fine Homebuilding: Energy Efficient Building
by Fine Homebuilding.

Solar Energy: Today’s Technologies for a Sustainable Future
by Burke Miller.

Videos

The Solar-Powered Home with Rob Roy. An 84-minute
video examining basic principles, components, set up and
system planning for an off-grid home. Features tips from
America’s leading experts in the field of home power.
Available from Earthwood Building School. www.cordwoodmasonry.com.

Solar Organizations

American Solar Energy Society. Publishes Solar Today
magazine and sponsors an annual national meeting. Also
publishes an online catalog of publications and sponsors
the National Tour of Solar Homes. www.ases.org.

Center for Renewable Energy and Sustainable Technologies
(CREST). Online source of renewable energy solutions.
www.crest.org.

Florida Solar Energy Center. Offers design guidelines for
building homes in hot, humid climates. www.fsec.ucf.edu.

Alliance to Save Energy. Excellent information on saving
energy and money. www.ase.org.

Northeast Sustainable Energy Association. Green-building,
clean-energy and Sustainable transportation site. www.nesea.org

National Renewable energy Laboratory, Center for Buildings and Thermal Systems. Key players in research and education
on energy efficiency and passive-solar heating and cooling.
www.nrel.gov/buildings/ high
performance.

Sustainable Buildings Industry Council. This organization
has a terrific website with information on workshops,
books and publications, and links to many other
international, national and state solar-energy
organizations.


Using Passive-Solar Heating Instead of Gas

Most Americans remain unaware that we have already consumed
nearly half of our known of Gas domestic natural gas.

Passive-solar heating is vital to our energy future as the
two dominant players in the home-heating market, natural
gas and home heating oil, are being depleted. Most
Americans remain unaware that we have already consumed
nearly half our domestic natural gas. In Texas, energy
companies drill 17 new gas wells a day — 6,400 wells
each year — just to maintain current production levels,
say Randy Udall, director of the Community Office for
Resource Efficiency in Aspen, Colorado, and Steve Andrews,
an energy analyst. The United States is now the world’s
largest importer of natural gas, most of which comes from
Canada.

But just as in Texas, gas production in western Canada is
declining. Twenty new wells are drilled every day just to
keep production from failing, causing Canadians to worry
about future supplies for their own country. Consequently,
some Canadians are balking at increasing their export of
natural gas to their energy-hungry neighbor to the south.
Importing natural gas from any but our land-linked
neighbors will be costly and difficult.

Like natural gas, oil production in the United States has
been dropping since 1972. Many energy analysts predict
global oil production will peak between 2005 and 2010. Then
demand will begin to outstrip supply, sending prices up, up
and away.

Passive-solar heating provides warmth and comfort, and can
replace declining fossil fuel resources economically, with
a fraction of the environmental impact. Using passive-solar heating instead of gas makes environmental sense. Passive-solar
heating is crucial not only to individuals but to the
well-being of the nation and the world economy.


A Passive-Solar Homes Tour

More than 800 solar homes will be showcased on October 5,
2002, as the American Solar Energy Society hosts the 2002
National Tour of Solar Homes.

On the ASES tour, homeowners and businesspeople will
explain how their photovoltaic and passive-solar systems
work, and how solar energy can maximize your home’s energy
efficiency and cut your water heating bills. For a tour
near you, visit the ASES website at American Solar Energy Society.