Anyone who lives in a home with a sun-space will tell you that the sunspace is themost enjoyable room in the house. Manytimes the homeowner’s only regret is thatthe sunspace is not larger. Although aes-thetics often drive the decision to add asunspace or include one in a new homedesign, sunspaces can also provide sup-plemental space heating and a healthyenvironment for plants and people. In fact, a well-designed sunspace can pro-vide up to 60% of a home’s winter heatingrequirements.

This publication will address basic ele-ments of sunspace design; design consid-erations for supplemental space heating,growing plants, and use as a living space;design guidelines including siting, heatdistribution, and glazing angles; andmajor sunspace components includingglazing options, thermal mass, insulation,and climate controls. A list of sources formore information is also provided.

Basic Elements

In a basic design, sunlight passes throughglass or other “glazing” and warms thesunspace. The glazing is either vertical (astypical windows are installed) or sloped at an angle. To moderate temperatureswings, massive materials (e.g., masonryor water) can be used to absorb the heatand store the sun’s thermal energy. Atnight or during extended periods ofcloudy weather, this “thermal mass”releases the heat it holds to warm the inte-rior of the sunspace. Ceiling, wall, founda-tion, and window insulation in thesunspace retard heat loss at night and dur-ing cold weather. Climate-control featuresinclude operable windows, vents, andfans to keep the sunspace from overheat-ing and to circulate the warm air to otherparts of the house.

Design Considerations for Different Functions

Sunspaces serve three main functions: theyare a source of auxiliary heat, they providespace to grow plants, and they are enjoy-able living areas. The design considera-tions for these functions are very different,and although it is possible to build a sun-space that will serve all three functions,some compromises will be necessary.

If the primary function of the room is onlyto provide heat, you can maximize heatgain by using sloped glazing, few plants,little thermal mass, and insulated,unglazed end walls. If the winters aresunny in your area, carefully sized ther-mal mass will prevent extreme overheat-ing during the day. In practice, sunspacesare rarely built to serve only as heaters,because there are less expensive ways toprovide solar heat.

Sunspace Basics

CLEARINGHOUSE

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This New England home uses a sunspacewith overhead glazing. The sunspace can beisolated from the rest of the house to protectliving spaces from temperature extremes.

This document was produced for the U.S. Department of Energy (DOE) by the National Renewable Energy Laboratory (NREL), a DOE national laboratory.The document was produced by the Technical Information Program, under the DOE Office of Energy Efficiency and Renewable Energy. The Energy Efficiencyand Renewable Energy Clearinghouse (EREC) is operated by NCI Information Systems, Inc., for NREL/DOE. The statements contained herein are based oninformation known to EREC and NREL at the time of printing. No recommendation or endorsement of any product or service is implied if mentioned by EREC.

Printed with a renewable source ink on paper containing at least 50% wastepaper, including 10% postconsumer waste

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If the space will mainly be used as agreenhouse, remember that plants needfresh air, water, lots of light, and protec-tion from extreme temperatures. Green-houses consume energy through thegrowth processes of plants and the evapo-ration of water: one pound of evaporatingwater uses about 1000 Btu that would oth-erwise be available as heat. Plants requireoverhead glazing (i.e., glazing in the roof),which complicates construction and main-tenance, and glazed end walls, which arenet heat losers. The bottom line is that asunspace designed as an ideal horticul-tural environment is unlikely to havemuch energy left over for supplementaryspace heating.

Most people want touse their sunspacesas year-round livingareas, so sunspacesshould have mini-mum glare and onlymoderate humidity.Carefully sized ther-mal mass will greatlyimprove comfort levels by stabilizingtemperatureextremes. Thermalmass materialsshould be placed indirect sunlight andshould not be cov-ered with rugs, furniture, or plants.Movable windowinsulation oradvanced glazingsminimize nighttimeheat losses andgreatly improve comfort.

Sunspace Design Guidelines

Passive solar structures are conceptuallysimple, but sunspace designers andbuilders must pay close attention to detailto ensure maximum performance and reli-ability of the structures.

Computer software is now available tohelp establish design and performance cri-teria for specific passive solar projects likesunspaces. This software makes it rela-tively easy to avoid making uninformed,

potentially costly, and disappointing deci-sions about a sunspace addition. Somesources for software are identified at theend of this publication.

SitingA sunspace must face south. Due south isideal, but 30̊ east or west of due south isacceptable. If your project is a retrofit, con-sider how the new addition will look onthe south side of your house. If the southside faces the street, the design must bewell integrated into the home to avoid a“tacked-on” look. And, you will need toprotect your family’s privacy. If the southside of your house faces the backyard, pri-vacy may be less of an issue.

Because the sun is low in the sky in thewinter, any obstacle over 10 feet (3 meters)tall within 15 feet (4.6 meters) of the southglazing is likely to block solar gain. If thesunspace will be shaded only in the earlymorning or late afternoon, there is nomajor cause for concern. It is important,however, that the space receive direct sun-light between 10:00 a.m. and 3:00 p.m. Donot plant trees near the south glazing, andseriously consider removing existing treesfrom the area. Contrary to prior opinion,even deciduous trees that lose their leavesin the winter are capable of blocking the

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Sunspaces serve

three main functions:

they are a source

of auxiliary heat,

they provide a place

to grow plants, and

they are an enjoyable

living space.

South

These drawings illustrate a few of the ways to attach a sunspace to a house.

March 21 orSept. 21

June 21Dec. 21

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In northern latitudes, the sun is much lower in the sky in the winterthan in the summer. For this reason, vertical glazing receives muchmore solar gain in the coldest part of the year. Sloped glazing takes in the most solar radiation in the summer.

Energy Efficiency Comes FirstBefore you add a sun-space, it makes economicsense to improve theenergy efficiency of yourhome. Caulking, weather-stripping, insulating, andother energy-efficiencyimprovements will pay forthemselves quicklythrough lower heating andcooling bills.

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A mature, well-

formed deciduous

tree will screen more

than 40% of the

winter sunlight

passing through its

branch structure.

sun. In fact, a mature, well-formed decid-uous tree will screen more than 40% of thewinter sunlight passing through its branchstructure.

If you have a choice, locate the sunspaceso that the walls of the house serve as oneor both end walls of the sunspace (toreduce heat loss) and the addition is adja-cent to kitchens, dining areas, children’splayrooms, and family living areas occu-pied during the day and early evening.

Heat DistributionWarm air can be blown through ductworkto other living areas. It can also move pas-sively from the sunspace into the housethrough doors, vents, or open windowsbetween the sunspace and the interior liv-ing space. Strategically placed openings in the common wall can distribute thewarmed air from the sunspace to thehouse by the “thermosiphoning” circula-tion of the air. In a thermosiphon, warmair rises in the sunspace and passes intothe adjoining space through an opening,and cool air from the adjoining space isdrawn into the sunspace to be heated.

The minimum opening in the com-mon wall should beabout 8 square feet(0.7 square meters)per 100 square feet(9.3 square meters)of glazing area. If thedesign calls for twoopenings—one highin the sunspace andone low—the mini-mum area for eachopening is approxi-mately 2.5 squarefeet (0.2 squaremeters) per 100 square feet (9.3 square meters)of glazing, with 8 vertical feet (2.4 meters) of sepa-ration between open-ings. Again, these are rules of thumbthat should berefined through com-puter modeling or

confirmed with local experts. An uninsu-lated masonry wall between the houseand the sunspace will also transfer someheat into the living space by conduction.

Glazing: Sloped or Vertical?Although sloped glazing collects moreheat in the winter, many designers prefervertical glazing or a combination of verti-cal and sloped glazing. Sloped glazingloses more heat at night, can be coveredwith snow in the winter, and can causeoverheating in warmer weather. Verticalglazing can maximize gain in winter,when the angle of the sun is low, and yieldless heat gain as the sun rises toward itssummer zenith. A well-designed overhangmay be all that is necessary to shade theglazing in the summer.

Compared with sloped glazing, verticalglazing is less expensive, easier to installand insulate, and not as prone to leaking,fogging, breaking, and other glazing fail-ures. Vertical glazing is often more aes-thetically compatible with the design ofexisting homes.

Sunspace Components

GlazingGlazing is the clear or translucent materialthat allows sunlight to enter and warm thespace. Glass is the most common glazingmaterial, and many sunspace builderschoose glass for its durability, clarity, andappearance. However, plastic glazings canbe cheaper, stronger, lighter, and easier towork with—making them popular choiceswith the 20% of homeowners who buildtheir own sunspaces. Some plastics eventransmit solar energy more effectivelythan glass. On the down side, plastics

Sunspaces incorporate thermal mass to absorb solarheat. Solar-heated air can then be used to heat thehouse, either passively through openings in the common wall or by blowing through ductwork intoother parts of the house.

This passive solar mountain home uses vertical glazing to collect the sun’s energy.

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Operablewindows

Insulated roof

Double-glazed

window

Vents with dampers

2” rigid insulation

Masonry wall

Water-filled, dark-colored containers

scratch more easily,expand and contractmore in response to temperatureextremes (makingthem harder to seal),and generally areless durable thanglass.

Deciding on whichglazing to use is onlythe first step in thedecision-makingprocess, however.Advances in glazingtechnology make itpossible for designersto fine-tune perfor-mance by choosingglazings that meetthe specific needs oftheir projects.

Historically, manufacturers have usedmultiple layers of glass to improve theinsulating value of a window. In additionto making the unit more energy efficient,extra layers of glass also increased theweight and bulk—as well as the price—ofthe unit. However, today’s low-emissivity(low-e) coatings—thin, nearly invisiblemetal or metallic oxide films—have revo-lutionized the glazing industry.

Low-e coatings are applied to the surfaceof glazings or to films suspended in theairspaces between the panes of glass. Theyreduce radiant heat loss and gain and dra-matically improve a window’s insulatingvalue. For example, double-glazed, low-ewindows are about as energy efficient astriple-glazed windows using regular glass,but they cost and weigh less. Note thatthere have been reports that windowswith less than 70% visible light transmit-tance might not support plant growth.

When argon, sulphur hexafluoride, carbondioxide, or other gas fills with higher insulating values than air are includedbetween glazings, the energy efficiency ofwindows is further improved. Althoughthe extra layers of glazing and low-e coat-ings decrease the amount of light cominginto the room, the reduction is more thanoffset by the increased amount of heatretained in the room.

Other new window technologies includespectrally selective coatings (the nextgeneration of low-e films) that reject heatwhile admitting light, electrochromicglazings that lighten and darken as smallelectric currents are applied and removed,and “superwindows” that combine anumber of features (e.g., low-e coatings,gas fills, and insulating frames and spac-ers) into one unit.

If you decide to use overhead glazing inthe roof of your sunspace, invest in one ofthe glazing systems developed specificallyfor this purpose. Overhead glazing has areputation for leaking, but excellent seal-ing systems are now on the market. Investin a good system—this is not a place to cutcorners. In some areas, building codesrequire that you use plastic glazing or tem-pered or laminated glass in overhead andsloped glazing sections for safety reasons.

Which glazing system is most appropriatefor your project depends on your budgetand the climatic conditions at your site.For more detailed information on currentand future glazing options, contact theEnergy Efficiency and Renewable EnergyClearinghouse (see Source List).

Thermal Mass ConsiderationsWater is the most efficient thermal mass,because it holds the most heat per unit ofvolume. Anything that will not leak willwork to hold the water, and designers andhomeowners have used everything fromplastic jugs to 55-gallon (208-liter) drumsto specially designed (and often veryattractive) containers.

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House

Wintersun

Headwallandoverhang

Knee wall

Grade

Glazing

Optionalnorth wall

Optionalsloped glazing

Summersun

House roof (if eave is on south side)

Roof stops here forsloped south wall

Housefoundation

Roof (glazing optional)

A well-designed overhang may be all that is necessaryto shade a vertically sloped interior sunspace wall.

Vertical glazing is

less expensive and

easier to install and

insulate, and it is

not as prone to the

leaking, fogging,

breakage, and other

glazing failures

often associated with

sloped glazing.

This well-designed passive solar home uses a sunspace to collect solar energy, and a tile floor and a masonry half-wall to store the heat. This leaves the space open to the rest of the house.

Photo UnavailablePlease contact the Energy Efficiency and

Renewable Energy Clearinghouse at (800) DOE-EREC to receive a printed

copy of this publication

5

Masonry materials (brick, concrete, orstone) are also good choices for thermalmass. Although they store only about halfas much heat as an equal volume of water,they can also support the structure, formthe floor of the space, and serve as thewall between the house and the sunspace.Masonry is most effective in 4- to 6-inch(10- to 15-centimeter) thicknesses. If wallsare built with concrete blocks, the holes in the blocks must be filled with concrete.

The surfaces of thermal mass materialsshould be dark colors of at least 70%“absorptance.” Absorptance is the amount of solar radiation absorbed by a surfacematerial. Black has about a 95% absorp-tance rate, deep blue has about 90%, anddeep red approximately 86%. Nonstoragematerials should be lighter colors so theywill reflect light to the thermal mass notlocated in the sun. Thermal storage mate-rials can be located in the floor and in thenorth, east, and west walls of the sunspace.

When masonry floors and walls are theonly thermal storage materials in thespace, 3 square feet (0.3 square meters) of4-inch-thick (10-centimeter-thick) masonrysurface per square foot (0.09 square meter)of south glazing is probably adequate.When water in containers is the only heatstorage medium used, the recommendedratio is 3 gallons (11.3 liters) per squarefoot (0.09 square meter) of glazing. These

are only rules of thumb and should beconfirmed by modeling your project on a computer or checking with a design orbuilding professional in your area who isfamiliar with local design practices.

InsulationTo maximize comfort and efficiency, it isimportant that your sunspace be wellinsulated. The perimeter of the sunspace’sfoundation wall or slab should be insu-lated down to the frost line (i.e., the depthat which frost penetrates the soil) andunderneath the slab if it is appropriate inyour area. If you live in a very cold cli-mate, insulate the east and west walls ofthe sunspace rather than glazing them.Always insulate the sections of exteriorwalls that are not glazed. Check with solarspecialists in your area or the resourcescited in this publication’s Source List forguidance on your particular project.

Although overhead glazing can be beauti-ful, an insulated roof provides better ther-mal performance. When the highest partof the structure is well insulated, heat lossin winter is reduced, and the summer sunwill not cause overheating. Instead, sky-lights can be used to provide some over-head light for plants. And, if they are thetype that open, skylights offer a way tovent excess heat. Skylights are availablewith advanced glazings that reduce radi-ant heat loss to the night sky.

Window coverings, shades, and otherforms of movable insulation help trap thewarm air in the sunspace both after thesun has set and during cloudy weather.When closed during extremely hot days,window coverings can help keep the sun-space from overheating.

Thermally isolating the sunspace from thehouse at night is important. Large glasspanels, French doors, or sliding glassdoors between the house and the sunspacewill maintain an open feeling without theheat loss associated with an open space.

Climate ControlsOverheating can kill plants and make thesunspace unlivable. To control overheat-ing, some designers place operable vents

Tile floors and masonry walls provide thermal mass to store solar heat. Doors between sunspaces and other living areas can be opened to allow warm air to circulate.

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Few home

improvements offer

the aesthetic appeal

and practical

paybacks that a

carefully designed

and constructed

sunspace can.

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Source List

There are many groups that can provide you with moreinformation on sunspaces. The following organizationscan answer the more technical questions you or yourbuilder may have.

OrganizationsAmerican Society of Heating, Refrigerating, and

Air-Conditioning Engineers (ASHRAE)1791 Tullie Circle, NEAtlanta, GA 30329(404) 636-8400ASHRAE publishes the Handbook of Fundamentals that detailsheat transfer, light transmittance, and shading properties ofdifferent window types and materials.

American Solar Energy Society (ASES)2400 Central Avenue, Unit G-1Boulder, CO 80301(303) 443-3130FAX (303) 443-3212ASES is a nonprofit educational organization founded in 1954to encourage the use of solar energy technologies. ASES pub-lishes a bimonthly magazine (Solar Today), sponsors the annualNational Solar Energy Conference, has regional chaptersthroughout the United States, and offers a variety of solar pub-lications through its catalogue.

National Center for Appropriate Technology (NCAT)3040 Continental DriveButte, MT 59701(406) 494-4572NCAT’s publication Solar Greenhouses and Sunspaces—LessonsLearned describes the experiences of sunspace owners andbuilders during the Department of Energy’s Appropriate Tech-nology Small Grants Program.

The Energy Efficiency and Renewable Energy Clearinghouse (EREC)

P.O. Box 3048Merrifield, VA 22116(800) 363-3732EREC provides free general technical information to the publicon the many topics and technologies pertaining to energy effi-ciency and renewable energy.

SoftwareBuilderGuidePassive Solar Industries Council (PSIC)1511 K Street, NW, Suite 600Washington, DC 20005(202) 628-7400PSIC offers workshops around the country on the Builder-Guide computer program and guidelines for passive solarbuilding and remodeling projects. Climate-specific guidelinesare available for more than 2000 cities and towns around theUnited States. PSIC also provides the building industry withpractical, useful information on the use of passive solar tech-nologies in buildings. PSIC developed the “Passive SolarDesign Strategies: Guidelines for Home Builders” workshopsand the BuilderGuide software.

Reading ListThe Passive Solar Energy Book, E. Mazria, Rodale Press,1979.

Sunspaces: New Vistas for Living and Growing, P. Cleggand 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.

at the top of the sunspace where tempera-tures are the highest and at the bottomwhere temperatures are the lowest. Fortimes when you are not home to openvents manually, thermostatically con-trolled motors can be installed to automat-ically open them.

If passive (i.e., nonmechanical) circulationis not possible or practical, fans with ther-mostatic controls can be used to circulateair to the rest of the house. Other types ofclimate controls include shades or mov-able window insulation that can be oper-ated with electric timers or sensors.

An Investment in Future Enjoyment

Few home improvements offer the aes-thetic appeal and practical paybacks that a carefully designed and constructed sun-space can. Although you may be temptedto tackle the endeavor on your own, it ismoney well spent to consult with a solarengineer, architect, or contractor. They willprovide feedback, as well as a computeranalysis of your design. Remember, it ismuch less expensive to make changes onpaper than to alter a sunspace once it isbuilt. And after your sunspace is finished,you can enjoy it for years to come.

WINDOW SoftwareWith the U.S. Departmentof Energy, LawrenceBerkeley Laboratory devel-oped WINDOW softwareto help manufacturers andbuilding professionalsoptimize the thermal anddaylighting performanceof window systems.

For more information, contact:Elizabeth FinlaysonMS 90-3111Lawrence Berkeley LaboratoryBerkeley, CA 94720 (510) 486-7179 (technical

questions) FAX (510) 486-4089 (software

orders)