Sunspace Basics
Anyone who lives in a home with a sunspace
will tell you that the sunspace is the most enjoyable room in the house. Many
times the homeowner's only regret is that the sunspace is not larger. Although
aesthetics often drive the decision to add a sunspace or include one in a new
home design, sunspaces can also provide supplemental space heating and a healthy
environment for plants and people. In fact, a well-designed sunspace can provide
up to 60% of a home's winter heating requirements.
This publication will address basic
elements of sunspace design; design considerations for supplemental space
heating, growing plants, and use as a living space; design guidelines including
siting, heat distribution, and glazing angles; and major components including
glazing options, thermal mass, insulation, and climate controls. A list of
sources for more information have also been provided.
Basic Elements
In a basic design, sunlight passes
through glass or other glazing and warms the sunspace. The glazing is either
vertical (as typical windows are installed) or sloped at an angle. To moderate
temperature swings, massive materials (e.g., masonry or water) can be used to
store the sun's thermal energy and absorb the heat. At night or during
extended periods of cloudy weather, this "thermal mass" releases the
heat it holds to warm the interior of the sunspace. Ceiling, wall, foundation,
and window insulation in the sunspace minimize heat loss at night and during
cold weather. Climate control features include operable windows, vents, and
fans to keep the sunspace from overheating and to circulate the warm air to
other parts of the house.
Design Considerations for
Different Functions
Sunspaces serve three main functions:
they are a source of auxiliary heat, they provide space to grow plants, and
they are enjoyable living areas. The design considerations for these functions
are very different, and although it is possible to build a sunspace that will
serve all three functions, some compromises will be necessary.
If the primary function of the room is
only to provide heat, you can maximize heat gain by using sloped glazing, few
plants, little thermal mass, and insulated, unglazed end walls. If the winters
are sunny in your area, carefully sized thermal mass will prevent extreme
overheating during the day. In practice, sunspaces are rarely built to serve
only as heaters, because there are less expensive ways to provide solar heat.
If the space will mainly be used as a
greenhouse, remember that plants need fresh air, water, lots of light, and
protection from extreme temperatures. Greenhouses consume energy through the
growth processes of plants and the evaporation of water: one pound of
evaporating water uses about 1000 Btu that would otherwise be available as
heat. Plants require overhead glazing, which complicates construction and
maintenance, and glazed end walls, which are net heat losers. The bottom line
is that a sunspace designed as an ideal horticultural environment is unlikely
to have much energy left over for supplementary space heating.
Most people want to use their sunspaces
as year-round living areas, so sunspaces should have minimum glare and only
moderate humidity. Carefully sized thermal mass will greatly improve comfort
levels by stabilizing temperature extremes. Thermal mass materials should be
placed in direct sunlight and should not be covered with rugs, furniture, or
plants. Movable window insulation or advanced glazings minimize nighttime heat
losses and greatly improve comfort.
Sunspace Design Guidelines
Passive solar structures are
conceptually simple, but sunspace designers and builders must pay close
attention to detail to ensure maximum performance and reliability of the
structures.
Computer software is now available to
help establish design and performance criteria for specific passive solar
projects like sunspaces. This software makes it relatively easy to avoid
making uninformed, potentially costly, and disappointing decisions about a
sunspace addition. Some sources for software are identified in the Source List
at the end of this publication.
Siting
A sunspace must face south. Due solar
south is ideal, but 30 degrees east or west of due south is acceptable. If
your project is a retrofit, consider how the new addition will look on the
south side of your house. If the south side faces the street, the design must
be well integrated into the home to avoid a "tacked-on" look. And,
you will need to protect your family's privacy. If the south side of your
house faces the backyard, privacy may be less of an issue.
Because the sun is low in the sky in the
winter, any obstacle over 10 feet (3 meters) tall within 15 feet (4.6 meters)
of the south glazing is likely to block solar gain. If the sunspace will be
shaded only in the early morning or late afternoon, there is no major cause
for concern. It is important, however, that the space receive direct sunlight
between 10:00 a.m. and 3:00 p.m. Do not plant trees near the south glazing,
and seriously consider removing existing trees from the area. Contrary to
prior opinion, even deciduous trees that lose their leaves in the winter are
capable of blocking the sun. In fact, a mature, well-formed deciduous tree
will screen more than 40% of the winter sunlight passing through its branch
structure.
If you have a choice, locate the
sunspace so that the walls of the house serve as one or both end walls of the
sunspace (to reduce heat loss) and the addition is adjacent to kitchens,
dining areas, children's playrooms, and family living areas occupied during
the day and early evening.
Heat Distribution
Warm air can be blown through ductwork
to other living areas. It can also move passively from the sunspace into the
house through doors, vents, or open windows between the sunspace and the
interior living space. Strategically placed openings in the common wall can
distribute the warmed air from the sunspace to the house by the "thermosiphoning"
circulation of the air. In a thermosiphon, warm air rises in the sunspace and
passes into the adjoining space through an opening, and cool air from the
adjoining space is drawn into the sunspace to be heated.
The minimum opening should be about 8
square feet (0.7 square meters) per 100 square feet (9.3 square meters) of
glazing area. If the design calls for two openings--one high in the sunspace
and one low--the minimum area for each opening is approximately 2.5 square
feet (0.2 square meters) per 100 square feet (9.3 square meters) of glazing,
with 8 vertical feet (2.4 meters) of separation. Again, these are rules of
thumb that should be refined through computer modeling or confirmed with local
experts. An uninsulated masonry wall between the house and the sunspace will
also transfer some heat into the living space by conduction.
Glazing: Sloped or Vertical?
Although sloped glazing collects more heat in the winter, many designers
prefer vertical glazing or a combination of vertical and sloped glazing.
Sloped glazing loses more heat at night and can cause overheating in warmer
weather. Vertical glazing allows maximum gain in winter, when the angle of the
sun is low, and less heat gain as the sun rises toward its summer zenith. A
well- designed overhang may be all that is necessary to shade the glazing in
the summer. Compared with sloped glazing, vertical glazing is less expensive,
easier to install and insulate, and not as prone to leaking, fogging,
breaking, and other glazing failures. Vertical glazing is often more
aesthetically compatible with the design of existing homes.
Sunspace Components
Glazing
Glazing is the clear or translucent
material that allows sunlight to enter and warm the space. Glass is the most
common glazing material, and many sunspace builders choose glass for its
durability, clarity, and appearance. However, plastic glazings can be cheaper,
stronger, lighter, and easier to work with--making them popular choices with
the 20% of homeowners who build their own sunspaces. Some plastics even
transmit solar energy more effectively than glass. On the down side, plastics
scratch more easily, expand and contract more in response to temperature
extremes (making them harder to seal), and generally are less durable than
glass.
Deciding on which glazing to use is only
the first step in the decision-making process, however. Advances in glazing
technology make it possible for designers to fine-tune performance by choosing
glazings that meet the specific needs of their projects.
Historically, manufacturers have used
multiple layers of glass to improve the insulating value of a window. In
addition to making the unit more energy efficient, extra layers of glass also
increased the weight and bulk--as well as the price--of the unit. However,
today's low-emissivity (low-e) coatings--thin, invisible metal or metallic
oxide films--have revolutionized the glazing industry.
Low-e coatings are applied to the
surface of glazings or to films suspended in the airspaces between the panes
of glass. They reduce radiant heat loss and gain and dramatically improve a
window's insulating value. For example, double-glazed, low-e windows are about
as energy efficient as triple-glazed windows using regular glass, but they
cost and weigh less. Note that there have been reports that windows with less
than 70% visible light transmittance might support plant growth.
When argon, sulphur hexafluoride, carbon
dioxide, or other gas fills with higher insulating values than air are
included between glazings, the energy efficiency of windows is further
improved. Although the extra layers of glazing and low-e coatings lower total
light transmittance somewhat, the reduction is more than offset by the
increased amount of heat remaining in the room. Other new window technologies
include spectrally selective coatings (the next generation of low-e films)
that reject heat while admitting light, electrochromic glazings that lighten
and darken as small electric currents are applied and removed, and "superwindows"
that combine a number of features (e.g., low-e coatings, gas fills, and
insulating frames and spacers) into one unit.
If you decide to use overhead glazing in
the roof of your sunspace, invest in one of the glazing systems developed
specifically for this purpose. Overhead glazing has a reputation for leaking,
but excellent sealing systems are now on the market. Invest in a good
system--this is not a place to cut corners. In some areas, building codes
require that you use plastic glazing or tempered or laminated glass in
overhead and sloped glazing sections for safety reasons.
Which glazing system is most appropriate
for your project depends on your budget and the climatic conditions at your
site. For more detailed information on current and future glazing options,
contact the Energy Efficiency and Renewable Energy Clearinghouse (see Source
List).
Thermal Mass Considerations
Water is the most efficient thermal
mass, because it holds the most heat per unit of volume. Anything that will
not leak will work to hold the water, and designers and homeowners have used
everything from plastic jugs to 55-gallon (208-liter) drums to specially
designed (and often very attractive) containers.
Masonry materials (brick, concrete, or
stone) are also good choices for thermal mass. Although they store only about
half as much heat as water, they can also support the structure, form the
floor of the space, and serve as the wall between the house and the sunspace.
Masonry is most effective in 4- to 6-inch (10- to 15-centimeter) thicknesses.
If walls are built with concrete blocks, the holes in the blocks must be
filled with concrete.
The surfaces of thermal mass materials
should be dark colors of at least 70% absorptance. Black has about a 95%
absorptance rate, deep blue has about 90%, and deep red approximately 86%.
Nonstorage materials should be lighter colors so they will reflect light to
the thermal mass not located in the sun. Thermal storage materials can be
located in the floor and in the north, east, and west walls of the sunspace.
When masonry floors and walls are the
only thermal storage materials in the space, 3 square feet (0.3 square meters)
of 4-inch-thick (10-centimeter-thick) masonry surface per square foot of south
glazing is probably adequate. When water in containers is the only heat
storage medium used, the recommended ratio is 3 gallons (11.3 liters) per
square foot (0.09 square meter) of glazing. These are only rules of thumb and
should be confirmed by modeling your project on a computer or checking with a
design or building professional in your area who is familiar with local design
practices.
Insulation
To maximize comfort and efficiency, it
is important that your sunspace be well insulated. The perimeter of the
sunspace's foundation wall or slab should be insulated down to the frost line
(i.e., the depth at which frost penetrates the soil) and underneath the slab
if it is appropriate in your area. If you live in a very cold climate,
insulate the east and west walls of the sunspace rather than glazing them.
Always insulate the sections of exterior walls that are not glazed. Check with
solar specialists in your area or the resources cited in the Source List for
guidance on your particular project.
Although overhead glazing can be
beautiful, an insulated roof provides better thermal performance. When the
highest part of the structure is well insulated, heat loss in winter is
reduced, and the summer sun will not strike the interior wall and cause
overheating. Instead, skylights can be used to provide some overhead light for
plants. And, if they are the type that open, skylights offer a way to vent
excess heat. Skylights are available with advanced glazings that reduce
radiant heat loss to the night sky.
Window coverings, shades, and other
forms of movable insulation help trap the warm air in the sunspace both after
the sun has set and during cloudy weather. When closed during extremely hot
days, window coverings can help keep the sunspace from overheating.
Thermally isolating the sunspace from
the house at night is important. Large glass panels, French doors, or sliding
glass doors between the house and the sunspace will maintain an open feeling
without the heat loss associated with an open space.
Climate Controls
Overheating can kill plants and make the
sunspace unlivable. To control overheating, some designers place operable
vents at the top of the sunspace where temperatures are the highest and at the
bottom where temperatures are the lowest. For times when you are not home to
open vents manually, thermostatically controlled motors can be installed to
automatically open them.
If passive (i.e., nonmechanical)
circulation is not possible or practical, fans with thermostatic controls can
be used to circulate air to the rest of the house. Other types of climate
controls include shades or movable window insulation that can be operated with
electric timers or sensors.
An Investment in Future
Enjoyment
Few home improvements offer the
aesthetic appeal and practical paybacks that a carefully designed and
constructed sunspace can. Although you may be tempted to tackle the endeavor
on your own, it is money well spent to consult with a solar engineer,
architect, or contractor. They will provide feedback, as well as a computer
analysis of your design. Remember: It is much less expensive to make changes
on paper than to alter a sunspace once it is built. And after your sunspace is
finished, you can enjoy it for years to come.
Source List
There are many groups that can provide you
with more information on sunspaces. The following organizations are just a few
that can help you with your sunspace questions.
Organizations
American Society of Heating,
Refrigerating, and Air-Conditioning Engineers (ASHRAE)
1791 Tullie Circle, NE
Atlanta, GA 30329
(404) 636-8400
ASHRAE publishes the "Handbook of
Fundamentals" that details heat transfer, light transmittance, and shading
properties of different window types and materials.
American Solar Energy Society (ASES)
2400 Central Avenue, Unit G-1
Boulder, CO 80301
(303) 443-3130
FAX (303) 443-3212
ASES is a nonprofit educational
organization founded in 1954 to encourage the use of solar energy technologies.
ASES publishes a bimonthly magazine, "Solar Today," sponsors the
annual National Solar Energy Conference, has regional chapters throughout the
United States, and offers a variety of solar publications through its catalogue.
National Center for Appropriate
Technology (NCAT)
3040 Continental Drive
Butte, MT 59701
(406) 494-4572
NCAT's publication "Solar Greenhouses
and Sunspaces--Lessons Learned" describes the experiences of sunspace
owners and builders during the Department of Energy's Appropriate Technology
Small Grants Program.
The Energy Efficiency and Renewable
Energy Clearinghouse (EREC)
P.O. Box 3048
Merrifield, VA 22116
(800) 363-3732
EREC provides free general technical
information to the public on the many topics and technologies pertaining to
energy efficiency and renewable energy.
Software
BuilderGuide
Passive Solar Industries Council (PSIC)
1511 K Street, NW
Suite 600
Washington, DC 20005
(202) 628-7400
PSIC offers workshops around the country
on the BuilderGuide computer program and guidelines for passive solar building
and remodeling projects. Climate-specific guidelines are available for more than
2,000 cities and towns around the United States. PSIC also provides the building
industry with practical, useful information on the use of passive solar
technologies in buildings. PSIC developed the Passive Solar Design Strategies:
Guidelines for Home Builders workshops and the BuilderGuide software.
WINDOW Software
Elizabeth Finlayson
MS 90-3111
Lawrence Berkeley Laboratory
Berkeley, CA 94720
(510) 486-7179
(technical questions)
FAX (510) 486-4089
(software orders)
With the U.S. Department of Energy,
Lawrence Berkeley Laboratory developed WINDOW software to help manufacturers and
building professionals optimize the thermal and daylighting performance of
window systems.
Reading List
"The Passive Solar Energy Book,"
E. Mazria, Rodale Press, 1979.
"Sunspaces: New Vistas for Living and
Growing," P. Clegg and D. Watkins, Garden Way Publishing, Storey
Communications, 1987.
"The Sunspace Primer: A Guide to
Passive Solar Heating," R. W. Jones and R. D. McFarland, Van Nostrand
Reinhold Company, 1984.
- U.S. Department of Energy (DOE)
- National Renewable Energy Laboratory
(NREL)
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