energy information document 1028

8/14/2019 Energy Information Document 1028

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Energy Information Document 1028

Chapter IX. Landscaping to Conserve Energy:A Guide to Microclimate Modification1

Alan W. Meerow and Robert J. Black2

INTRODUCTION

The worlds petroleum reserves are not a limitlessresource. Oil supplies continue to dwindle as theworlds population grows and more developing countries

establish an industrial base. As progress continues in thedevelopment of alternative energy sources, energyconsumption must be reduced through effectiveconservation practices.

In northernmost Florida, temperatures fall belowfreezing on an average of 15 to 25 days and in Miami,winter fronts with associated winds cause uncomfortablycool temperatures for short periods of time but Floridassubtropical climate reduces the demand for winterheating (Barrick and Black, 1984). However, Floridaslong, hot and humid summers create a high demand forair conditioning during 5 to 7 months of the year.

As much as 30 percent of the cost of keeping ahome comfortable could be saved by effective man-agement of the microclimate that surrounds it.Microclimate is the term used to define any small, localarea within which the effects of weather are bothrelatively uniform and easily modified. The "passive"methods of microclimate modification discussed in thischapter are simple, low-cost improvements that can

decrease the energy costs associated with maintaininginterior comfort.

Microclimate modification involves the best use ofstructural and landscape design elements to maximize ormoderate sunlight, shade and air movement. Structural

modifications involve the design of the house andassociated construction (walkways, fences, patios).Landscape modifications involve the use of plants tofurther increase or decrease the impact of sun and windupon the local environment.

STRUCTURAL ELEMENTS OFMICROCLIMATE MODIFICATION

When choosing or designing a new home, severaldecisions can be made that will strongly influence thedegree to which interior comfort will require heating or

air conditioning. The homeowner should incorporateeffective insulation in ceilings and walls, andweatherstrip around windows and doors, even if localordinances do not require such practices. Total energysavings of 50-60 percent can be realized if conservationpractices are followed in new home construction(Buffington, 1979).

In Floridas hot and humid climate, a house will bemore energy efficient if it is oriented with the long axis

1. This document is Chapter 9 of the Energy Information Handbook, Energy Information Document 1028, a series of the Florida Energy Extension Service,

Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Revised: August 1991.

2. Alan W. Meerow, Assistant Professor, Agricultural Research and Education Center, Ft. Lauderdale; Robert J. Black, Associate Professor, Department

of Environmental Horticulture, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville FL 32611.

The Florida Energy Extension Service receives funding from the Florida Energy Office, Department of Community Affairs and is operated by the

University of Floridas Institute of Food and Agricultural Sciences through the Cooperative Extension Service. The information contained herein is

the product of the Florida Energy Extension Service and does not necessarily reflect the views of the Florida Energy Office.

The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational

information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national

origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.

Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / Christine Taylor Stephens, Dean


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Chapter IX. Landscaping to Conserve Energy: A Guide to Microclimate Modification Page 2

running east-west (Figure 1). With this orientation, onlythe short walls of the house receive the direct morningand afternoon sun, thereby reducing the total heat loadon the structure. An east-west orientation can save asmuch as $75 to $100 per year in cooling costs alone forthe average home.

In the winter, when the sun is lower in the sky, thesouth-facing long wall receives the heating benefits ofsolar radiation. Divergence of up to 10 degrees in eitherdirection from this orientation is allowable tocompensate for the prevailing wind direction or otherspecific site requirements.

FigureFigure 1.1. In Floridas hot, humid climate, a house will be

more energy efficient if it is oriented with the long axis

running east-west.

Light colored materials reflect sunlight; darkmaterials absorb the radiation. A house with dark wallsand roof is less expensive to heat in winter, but morecostly to cool in summer. Light colored walls and roofslower cooling costs but increase the need for winterheating. In Florida, the use of light colored materials ismore cost effective and energy efficient, since thecooling season is considerably longer than the heatingseason.

Fencing is primarily used around homes to ensure

privacy or mark boundaries. Fencing also directlyinfluences the patterns of air circulation and airmovement can affect the energy efficiency of the home,depending on the season of the year, direction ofprevailing winds, and degree of dependence on airconditioning for home cooling. Air movement aroundthe home may raise energy consumption by increasingconductive heat loss (in winter) and heat gain (in

Figure 2. A solid fence on the north side of thehouse provides a barrier against cold winter winds.

summer) through walls and windows, and infiltration of

FigureFigure 3.3. Open fencing maximizes summer air flow.

outside air around the edges of windows and doors.

In Florida, winters prevailing winds are from the

north or northwest. Thus, a solid fence on the north sideof a house can provide a barrier against cold winterwinds (Figure 2). Southerly winds predominate duringthe warmer months of the year when effective aircirculation is generally desired. Open fencing (Figure 3),especially with bottom clearance, maximizes air flow andreduce reliance on air conditioning for cooling.However, if air conditioning rather than natural coolingis used to cool the home, prevailing summer windsshould be blocked or diverted away from the house toreduce warm air infiltration. In general, vegetationprovides greater flexibility in directing air circulation,and is a better choice than fencing used expressly forthis purpose.

In summer, large roof overhangs can help shadewindows and walls, as well as walkways adjacent to thehouse. Arbors or trellises over outdoor living areasincrease comfort and shade nearby walls. Decks shouldbe built with bottom clearance to allow air to circulatebelow the structure. If possible, driveways should belocated on the east or north side of the house to reduceheat buildup during warm afternoons. Solid surfacessuch as concrete and asphalt, which transfer a great dealof heat, should be kept to a minimum, and ground cover

plants or wood chip mulch used instead. Brickdriveways build up less heat than either asphalt orconcrete and produce less glare than concrete.

LANDSCAPE ELEMENTS FOR SHADE


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Overview

Plants provide the most economical means ofmodifying microclimate around a home, and represent aninvestment in future energy savings. Research in Floridahas shown that energy costs for air conditioning at

certain times of the day can be reduced more than 50percent by proper placement of the right plant materialsaround a residence.

Plants in the landscape interact directly with the twoprimary comfort factors of Florida weather: sun andwind. Summertime heat gain in a home can be reducedby using plants to 1) shade the residence from directsolar radiation, 2) either divert or channel air movementaway from or towards the house, and 3) create coolertemperatures near the home by evaporation of waterfrom plant leaves. Heating costs in winter can bedecreased by selecting and properly locating plants sothat 1) the amount of direct solar radiation received bythe home is maximized and 2) the effects of cold winterwinds are minimized.

In many areas of Florida, the wasteful practice ofcompletely clearing building sites of existing vegetationis being abandoned. Existing trees should beincorporated into new home design whenever possible.Mature stands of native vegetation can often provide thedesired energy saving shade and wind control that wouldotherwise require years to develop from new plantings ofnursery stock. Native plants are well adapted to local

climate and soils, and established natives will notordinarily require supplemental irrigation or fertilization.

Using Trees for Shade

Trees are the most valuable tools in landscaping forpassively increasing the interior comfort of a home.Once established, most landscape trees require littleperiodic maintenance, and represent an appreciatinginvestment in the homes value. Trees are both effectiveproviders of shade and modifiers of air movement. Atree species performance of these functions depends on

how tall it grows, whether or not the leaves stay on thetree all year, and the shape and density of the canopy.Lists of appropriate trees and how they rate for thesespecific qualities may be found in three otherpublications: 1) EES-40 -- "Landscaping To ConserveEnergy: Trees for North Florida," 2) EES-41 --"Landscaping to Conserve Energy: Trees for CentralFlorida," and 3) EES-42 -- "Landscaping to ConserveEnergy: Trees for South Florida." These publications areavailable at your county extension office.

Trees that grow 60 or more feet tall are capable ofcasting shadows over the roof of a typical single familyhouse. Unfortunately, new plantings of most large-growing trees require 20 or more years to reach full size.The homeowner is probably better off investing inquality ceiling insulation, attic ventilation, and

construction of radiant barriers (a layer of aluminum foilsituated in the air space between the roof and the atticinsulation) unless a preexisting tree canopy effectivelyshades the roof during the summer months (Fiarey,1984a; 1984b). Large trees overhanging the roof of ahouse also present the risk of damage from falling limbs,and the nuisance of clogging rain gutters with leaf andtwig litter. If a solar collector has been installed on theroof, careful attention must be paid to positioning shadetrees so that the efficiency of the collector is notreduced.

Trees can provide valuable shading of sidewalls,particularly in older homes where walls have littleinsulation and retrofitting is prohibitively expensive.Small (up to 25 ft.) or medium size (25-40 ft.) treesperform this function well and dont grow out of bounds.Fast growing trees (e.g., silk tree: Albizia julibrissin)can be planted at the same time as slower growingspecies to provide a temporary solution to shadingproblems. Such trees can be removed later as the slowergrowing trees approach their mature heights. Care mustbe taken that these vigorous, temporary trees do notshade or otherwise compete with the slower growing,more permanent landscape elements.

Eastern and western exposures accumulate the mostheat during the long days of summer. Tree shadingshould thus be maximized on these sides of the house.South-facing walls also benefit from tree shade.Although southern exposures may be relatively free ofdirect radiation in June, by August the sun has droppedsufficiently in the sky to cause significant heat loadincrease in the afternoon. Windows are the most directroute for sunlight to enter the home, and trees (orshading devices such as awnings) should be positionedto shade them throughout the day.

The outdoor compressor/condenser unit of the airconditioning system uses less energy if it and thesurrounding area are shaded from direct sun during theentire day. A tree can shade the unit when the sun isoverhead, while nearby shrubs can provide protectionduring the early morning and late afternoon hours.However, care must be taken not to block theconditioners air flow ("short circuiting"). If the warmdischarge air is prevented from escaping, the intake air


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temperature will be raised, causing the unit to operateless efficiently.

In winter, the sun is low in the southern sky.Southern exposures of a home can receive the cost-free,energy saving benefits of passive solar heating, provided

that deciduous trees are used along the southernexposures (Figure 4). Deciduous trees shed their leavesin the fall, and are bare during the coldest months of theyear. However, in south Florida, where winters are shortand mild, greater benefits are received by usingevergreen trees along the southern exposures to reduceheat build-up and lower air conditioning costs.

Evergreen trees maintain their leaves throughout the

FigureFigure 4.4. Deciduous trees on the southern exposure of

the home provide shade in summer, but allow winter

sunlight to passively heat the home.

year. There are two types: broad-leaved (e.g., Southern

magnolia, Magnolia grandiflora; American holly, Ilex

opaca), and needle-leaved (e.g., pines, Pinus spp.; cedarsand junipers, Juniperus spp.). Evergreens that are broad-leaved provide dense shade year round, and are most

useful as shade trees in the southern third of Florida. Bycontrast, the shade cast by needle-leaved trees is sparseand more open, though pruning can, in some cases,stimulate a denser canopy to develop. Where allowancefor air circulation must be balanced with degree ofshading, more open canopied trees may be preferredwhen air circulation and the degree of shading must beoptimized for maximum energy savings and comfort.Tree shape also influences the amount of shade cast(Figure 5).

Trees planted close to the home begin to provideshade sooner than those planted at greater distances.The benefits of new shade trees should be obtainedwithin five years. To accomplish this goal, a distance ofseven to twenty feet from tree to sidewall isrecommended. Lot size and ultimate height of the tree

chosen directly influence this distance. Trees plantedcloser also shade for a longer period of time during theday, and over a greater part of the hot season. Theshadow of a tree planted ten feet from the home movesacross the target surface four times slower than a treeplanted twenty feet away (Parker, 1978; 1983a; 1983b).Vegetation close to the residence also lowers the airtemperature near the home, reducing the heat conductedthrough the walls.

The correct placement of trees chosen to shade the

FigureFigure 5.5. The shape of a tree canopy strongly influences

the shade pattern that is cast.

home involves consideration of the angle of the sunsrays, the mature height and width of the tree canopy, andthe height of the structure to be shaded. Preciseguidelines on determining shade patterns can be found inthe following three publications: 1) EES-49 --"Landscaping to Conserve Energy: Shade Patterns inNorth Florida," 2) EES-50 -- "Landscaping to ConserveEnergy: Shade Patterns in Central Florida," and 3) EES-48 -- "Landscaping to Conserve Energy: Shade Patternsin South Florida." These publications are available at

your county extension office.

Using Shrubs and Vines for Shade

Shrubs can provide effective barriers to earlymorning and late afternoon sunlight on eastern andwestern exposures respectively. Small-leaved, open-branched shrubs provide shade without unduly restrictingair movement for passive cooling. Espaliered shrubs,shrubs trained to grow horizontally against a wall block


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a great deal of sunlight before it strikes and heats up thewall (Figure 6).

Vines are especially useful for shading homes when

FigureFigure 6.6. Espaliered shrubs can insulate a wall from heat

build-up.

small lot size restricts the use of shade trees. Vines areeither self-supporting or twining. Self-supporting vinescling to a surface by either pad-like holdfasts (e.g.,

Virginia creeper: Parthenocissus quinquefolia) or aerial

roots (e.g., trumpet vine: Campsis radicans). Self-supporting vines are not recommended for woodstructures because they may trap moisture that can leadto decay of the wood. On brick or concrete blockhomes, a fast growing, self-supporting vine caneffectively prevent the sun from heating a wall (Figure6) (Parker, 1981). Twining vines (Confederate jasmine:

Trachelospermum jasminoides; painted trumpet:

Clystostoma callistegioides) climb by means of stems ortendrils that require some form of support. By providinglattice-type support or a trellis, twining vines can be usedto shade walls, windows, and outdoor living spaces(Figure 7). As with shade trees, only deciduous vinesare recommended for southern exposures on structures in

north Florida, to allow winter sun to passively heat thehome.

LANDSCAPE ELEMENTS FOR WINDCONTROL

Winter

In Florida, winds prevail from the north in winter.These northerly winds often sweep cold, arctic air intothe state, the effects of which are felt most strongly inthe Panhandle and north-central counties.

A home loses a greater amount of heat on a cold and

FigureFigure 7.7. Twining vines provided with climbing support

shade an outdoor living space, and block the sun from

nearby windows and walls.

windy day than on an equally cold but still day. About1/3 of the heat loss from a home is by transfer throughthe ceilings and walls (conduction). Wind increases theloss of heat from the outside surfaces of those samewalls and from the roof by sweeping the warm air away(convection). Cold air infiltration through spaces aroundwindows and doors also increases reliance on costlyhome heating systems. The use of windbreaks andfoundation plantings substantially reduces the heat

robbing action of winter winds.

In Florida, windbreaks situated on the north,northwest and, to a lesser extent, northeast exposures ofthe home can provide significant energy savings duringthe winter heating season. The height and foliagedensity of trees used in windbreaks directly influencetheir effectiveness as wind barriers. Evergreen trees withdense canopies provide the most complete protection.However, extremely dense or solid windbreaks tend toconcentrate their effects over a much shorter distance


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than those of moderate texture. A multi-layered canopyof shrubs and trees of moderate density planted in two tofive rows (Figure 8) is the most effective windbreakdesign, but even a single row of trees provides somewindbreak action. Windbreaks significantly reduce windvelocity for a distance equal to 10 times the height of

the trees, less significantly to 20 times the height. Thegreatest amount of protection occurs within a distance offive times the height of the windbreak.

A dense planting of shrubs close to the north andnortheastern walls of the home creates a "dead air" spacethat has insulating properties (Figure 8). By reducing airmovement in the immediate vicinity of the walls, thisdead air space also reduces secondary heat loss by coldair infiltration through cracks and window spaces.

Evergreen shrubs of dense texture (e.g., Podocarpus

nagi, Pittosporum tobira), closely spaced together,

provide this type of protection effectively. These sameshrubs keep north sidewalls cool in the summer viatranspiration and shading during early morning and lateafternoon.

FigureFigure 8.8. Windbreaks and foundation plantings willweaken the effects of cold winter winds.

Summer

In Florida, summer breezes prevail from the southand southeast. In north Florida, breezes during the "dogdays" of July and August originate from the south orsouthwest; in south Florida they largely remainsoutheasterly (Barrick and Black, 1984). How best touse plants to interact with summer air movement is

largely determined by the means with which the home iscooled.

For a home in which air conditioning is used onlyminimally, trees and shrubs should be strategicallysituated to channel cooling breezes toward the windows

(Figure 9). Low-branching trees should be avoided onthe southeastern and/or southwestern exposure, or thelow branches removed (Figure 10). Plants used to shadewindows from the sun should be far enough away to notrestrict air movement. Shrubs near the windows can bepositioned to further funnel moving air into the house(Figure 9). If shrubs are be used to provide low shadefor exposures facing prevailing summer winds, usespecies that have small leaves and an open branching

pattern (e.g., Glossy abelia: Abelia X grandiflora in

northern Florida; Thryallis: Galphimia glauca in southor central Florida). Winter wind barriers on the north

and northwest sides of the home also deflect coolingbreezes from the south back toward the house in thesummer (Figure 8).

During the 5 to 7 months of Floridas uncomfortablywarm temperatures, some residents find it impossible tostay cool without air conditioning, regardless of the cost.Wind movement around the home during the coolingseason substantially raises the energy costs for airconditioning by increasing the infiltration of hot, humidoutside air around windows, doors, and through cracks.Studies of air conditioned homes in Florida havedetermined that heat gain by infiltration is actuallygreater than gain by conduction and radiation throughwalls and windows (Steen et al., 1976).

Shrubs and trees should be positioned around the airconditioned home to divert the prevailing southernbreezes away from the house (Figure 12), the exact

FigureFigure 10.10. To allow air movement around passively

cooled homes, low-branching trees should be avoided

on the southeastern and/or southwestern exposure, or

the low branches removed.


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opposite of what would be desired for a passively cooled

FigureFigure 9.9. Around the passively cooled home, trees and

shrubs should be positioned to direct breezes toward

windows.

home. A multi-layered summer windbreak should bedesigned along the southern exposures and away fromthe home. The tallest components of the windbreakshould be the closest to the house. In this way, a "windramp" can be created that will channel the breezes overthe home. Along and close to the walls that face thedirection of summer winds, a foundation planting ofshrubs should be used to create a dead air space that willreduce or eliminate warm air infiltration. Deciduousshrubs, or at least more open-branched species thanrecommended for north-facing foundation plantings,should be used on the south side to allow the sun topassively heat those exposures in winter.

During the mild transitional months of fall and earlyspring, natural ventilation is desirable, even in homesthat are air conditioned during the peak of the hotseason. The south-facing foundation shrubs can bepruned in September to permit air movement, and then

allowed to fill out again the following spring, if suchpruning does not disrupt the aesthetic integrity of thelandscape. Shade trees positioned between windows andprevailing summer winds should be low-branching(Figure 11) to provide maximum protection against airmovement. Additional tall shrubs can be placed nearbyon the windward side of east and west windows.

FigureFigure 11.11. If the home is air-conditioned, shade trees

positioned between windows and prevailing summer

winds should be low-branching to provide maximum

protection against air movement.

COOLING EFFECTS OF TRANSPIRATION

Plants release water through pores in their leaves.The evaporative loss of this water is called transpiration.As hot air passes over the surface of the leaves, themoisture absorbs some of the heat and evaporates. Theair surrounding the leaf surface is thus cooled by theprocess. This interaction is called evaporative cooling,

and air temperatures surrounding vegetation can belowered by as much as 9 degrees F (5 degrees C) by itseffects.

The greater the amount of leaf area in the landscape,the greater the cooling effects of transpiration. The useof plants for shade and wind control instead of structuralfeatures such as fences and arbors thus provides anadditional benefit toward maintaining thermal comfortduring Floridas long summer. Air temperatures nearshade trees and foundation shrubs will be considerablylower than open areas, resulting in lower heat gains

through nearby walls or windows. If summer breezesare channeled through and across vegetation, theircooling capacity will be increased.

To maximize the effects of evaporative cooling,increase the amount of plant cover around the home.Use turf and/or ground covers to their fullest potential inthe landscape, as alternatives to paved surfaces such asasphalt or concrete. Many ground covers require lessmaintenance than turf grasses, and can be used as energysaving alternatives to large expanses of lawn. Suggested


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plant materials and guidelines for using ground covers

FigureFigure 12.12. Multi-layered windbreak of shrubs and trees

positioned around air-conditioned home divert southern

breezes away from house.

may be found in the following three publications: 1)EES-37 -- "Landscaping to Conserve Energy: GroundCovers for North Florida," 2) EES-38 -- "Landscaping toConserve Energy: Ground Covers for Central Florida,"and 3) EES-39 -- "Landscaping to Conserve Energy:Ground Covers for South Florida." These publicationsare available from your county extension office.

SUMMARY

In order to reduce energy needs for summer cooling:

1. Maximize use of ground covers and turf; limitamount of dark paved areas.

2. Maximize shading on roof by overhead canopytrees (provided that solar panels are not shaded).

3. Maximize shading on east, west, and south wallswith shade trees. Shadows on the south, southeast,and southwest exposures are shorter than shadowscast on west and east exposures; position trees andshrubs accordingly.

4. Use trellises in combination with ornamental vinesfor eastern, western, and southern exposures.

5. If air conditioning is used only minimally, maxi-mize the beneficial effects of prevailing coolingwinds.

Select and prune trees to allow air flow to thehouse.Avoid dense plantings away from the house thatwould act as a windbreak to cooling breezes.Avoid the use of solid foundation plantings thatwould create a dead air space.

6. If you rely on air conditioning:

Shade the outside compressor unit from directsun.Select trees to block air flow to the house.Situate a windbreak at some distance from thehouse to divert or block prevailing winds awayfrom the home.

Reduce warm air infiltration with a solid foun-dation planting that creates a dead air space nearthe house in the face of prevailing winds.

To minimize energy needs for winter heating:

1. Use deciduous trees, shrubs and vines on south,southeast and southwest locations except in southFlorida.

2. Maximize the use of evergreen plant materials infoundation plantings to create an insulating dead airspace along the northern exposure of the house.

3. Create a windbreak for the north and northwesternexposures of the home using evergreen trees andshrubs in a multi-layered canopy, preferably in 2- to5-foot rows.

With careful attention to these guidelines, the Floridahomeowner can improve the value of his or her homewith landscaping and realize substantial energy savingsthroughout the year.

BIBLIOGRAPHY AND REFERENCESFOR FURTHER READING

Barrick, W.E. and R.J. Black. Florida Climate Data.1984. Florida Energy Extension Service PublicationEES-5, University of Florida, Gainesville.

Buffington, D.E. 1979. Economics of Landscaping

Features for Conserving Energy in Residences.Proceedings of the Florida State HorticulturalSociety 92:216-220.

Fairey, P.F. 1984a. Radiant Energy Transfer and

Radiant Barrier Systems in Buildings. Design Note6, Florida Solar Energy Center, Cape Canaveral.

Fairey, P.F. 1984b. Designing and Installing Radiant

Barrier Systems. Design Note 7, Florida SolarEnergy Center, Cape Canaveral.

Parker, J.H. 1978. Precision Landscaping for Energy

Conservation. Proceedings of National Conferenceon Technology for Energy Conservation. Tucson,Arizona.


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Parker, J.H. 1981. A Comparative Analysis of the Role

of Various Landscape Elements in Passive Cooling

in Warm, Humid Environments. Pp 365-368 inProceedings of the International Passive and HybridCooling Conference, Miami.

Parker, J.H. 1983a. The Effectiveness of Vegetation on

Residential Cooling. Passive Solar Journal. 2:123-132.

Parker, J.H. 1983b. Landscaping to Reduce the Energy

Used in Cooling Buildings. J. Forestry 81:82-84,

105.

Steen, J.W. Shrode and E. Stuart. 1976. Basis for

Development of a Viable Energy Conservation

Policy for Florida Residents. Florida State EnergyOffice, Tallahassee.

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