Landscaping To
Reduce the Energy
Used In
Cooling Buildings
John H. Parker
ABSTRACT--Although
much less energyis expendedin the
United Statesfor spacecooling thanfor spaceheating, this
end use is significantin many areas of the countryand is
often the major factor in utility peak electrical demands
duringthe warmermonths.Vegetative
landscapingcan reduce
the energyrequiredto cool residences
byprovidingshadeand
wind control and throughmicroclimatecooling via evapotranspiration. "Precision landscaping"design conceptscan
optimizethe energysavings,particularlydttringutilitypeak
demandperiods. Energy analysesof an insulatedmobile
home show that landscapingcan reducecooling energyby
more than 50 percent during warm summerdays.Deciduous
treesare particularly ttseftdbecausethey allow solar radiation to reacha buildingduringthe heatingseason.
gainwill be via the walls and windows.
A primarycharacteristic
of precisionlandscaping
i•
the placementof trees and shrubsreasonablyclose to
the residence.This proximityplantingprovidesoptimal
shadingpatternsand also usessolarenergyfor evapotranspiration,thereby creating cool microclimatesdirectly adjacentto the walls and windows.The resultant
reductionin ambientair temperatures
and in incoming
solarradiationreducesthe rate of heattransferthrough
the walls and windows.A designthat maximizesthis
effect is a multi-layertree canopy with dense shrubs
locatedunderneathand immediatelyadjacentto the
walls and windows.
Peak Load Landscaping
The relationbetweenvariousenergyconservation
activities and electrical utility power demand patterns
(Shlachtmanand Parker 1981) is importantbut relatively unresearched.
A majorproblemof mostelectrical
utilities is supplyingpowerduring peak demandperiods, when the most expensiveequipmentor fuels are
often employed.Furthermore,consumerutility bills
significantlyincreasewhen new and muchmoreexpensive powerplants are built to meet future peaks. In
responseto these load-management
problems, many
government
agenciesand utilitiesthroughoutthe United
Statesare consideringthe impositionof "peak load
pricing" where higher rates are chargedfor electrical
consumption
duringpeak load periods.
In much of the United Statesand particularlyin the
so-calledsunbelt,the energyusedtbr air conditioning
makesup a large fractionof the peak electricalutility
loadsduringthe warmestperiodsof the summer.Consequently,a landscapedesignthat specificallyreduces
coolingrequirements
duringpeak load periodsis particularly effective.If peak load pricing is implemented,
the dollar savingsalso will be large.In fact, the widespreadadoptionof "peak load landscaping"
could significantlydelaythe buildingof new powerplants,thus
savingconsumerseven more money.
The centerof the seasonalpeak demandperiod for
southernFloridaoccurson August6. For otherareasin
the Southand probablyelsewherein the United States,
a datenearerAugust I wouldbe appropriate.Typically,
the daily peakdemandin the summeroccursduringlate
afternoon,when ambient temperaturesreach a maximum and when peoplecominghomefrom work turn on
their air conditioners.Consequently,
the anglesbetween
Inrecent
years,
interest
inurban
forestry
has
increased
significantly. A major justification for planting and
preservingvegetationin urban environmentsis climate
amelioration.This articlewill discussthe waysin which
vegetativelandscaping
can reducethe energyconsumed
in air-conditioning
a residence.Major designconcepts
that will be analyzed include precisionlandscaping,
peak load landscaping,
landscaping
for naturalventilation, shadingand microclimatecooling near the air
conditioner,
and indirectenergymaintenance
inputs.
PrecisionLandscaping
For Energy Conservation
A number of recent studies (DeWalle 1978, Parker
1979, Parker1981) haveanalyzedthe potentialof vegetative landscaping
for reducingthe energyusedin cooling a residence.Most have focusedon landscapedesignsthat maximize shadingbut minimizeobstructions
to summerbreezes.To optimize effectiveness,the concept of "precisionlandscaping"has been developed
(Parker 1979). It involves an analysisof the climatic
data of a specific locale as well as the thermal and
energyutilizationpatternsof the residenceunderstudy.
Landscaping
reducescoolingrequirements
by:
Blocking solar radiation from the building, the
adjacentground,and the foundation,
Creatingcool microclimatesnear the residenceby
evapotranspiration,
and
Either channelingor blockingair flows through
and around the residence.
In what follows, a precisionapproachwill be illustratedby designsand conceptsapplicableto residences
in hot, humid areas of the United States. Specific
climatic characteristics of Miami,
Florida, will be uti-
lized since they are representativeof hot, humid environmentsand have already been analyzed in several
previousstudies(Robinette 1977, U.S. Departmentof
Energy 1978). Nevertheless,the conceptsare generally
appropriatefor any geographicalarea with significant
coolingrequirements.It will be assumedthat the resi-
dencehasa well-insulated
ceilingso mostof the heat
82/JOURNAL
OFFORESTRY/February
1983
the sun and the windows on the west, southwest,and
south(unlessthereis a significantoverhang)sidesof a
buildingduringthe mid- to late afternoonon August6
designateprimary positionsfor the placementof trees
and shrubs for a residence in Miami.
In contrastto some other energy-conserving
techniques,the coolingeffectsof vegetativelandscaping
are
maximizedduringperiodsof maximumtemperature
and
solarradiation.This statementhasbeencorroboratedby
recent quantitativecomparisons(Parker 1981) of the
temperaturesof walls of residences.with and without
vegetativeshading,duringdaytimehours.Thesestudies
have shownthat plantinglarge-canopied
trees on the
westside in combinationwith a hedgeplantedimmediately adjacentto the west wall can reduce west-wall
temperatures
by 28 degreesFarenheit(from 113øFto
85ø) duringvery hot, humidafternoonsin southFlorida. Figure 1 showsa representative
experimentinclud¸1983 SOCIETY
OFAMERICAN
FORESTERS
datagathered
at the specificsite.If winddirections
are
notconsistent
on a daily or seasonal
basis,theappropriatepositions
for specificplantswill notbe welldefined,
andpotentialenergysavingswill be less.
If air conditioningwill be usedonly minimally,low
branchesshouldbe prunedto allow the passageof
prevailing
summerbreezesthroughthehouse.Treesand
shrubsshouldbe positionedso as to funnelthe breezes
UTILITY
DEMAND
into the windows.
10s
A major conflict arises with a residencecooled
sometimes
by naturalventilationand sometimes
by air
conditioning.
Earlierstudies
of energy-saving
landscaping
havefocusedon heat lossvia air infiltrationduringthe
heatingseason(Mattinglyet al. 1976) but havegenerally ignoredheat gain via air infiltrationduringthe
coolingseason.Winds can significantlyincreasethe
heat gain of an air-conditioned
residenceby increasing
SHADING
BY
TREE/HEDGE
12
2
Noon
4
6
8
10
12
TIME OF DAY
Figure1. Surfacetemperatureof the westwall of a residence
with and withoutvegetativeshading:electric utility demand
on a warm summerday.
ing a typical electricalutility load curvefor a peak load
day. Clearly, trees preciselypositionedto shadewestfacing windowsduring the late afternoonon August6
(or August I in other areas), combinedwith closely
plantedshrubs,can dramaticallyreduceelectricalconsumptionby air conditionersduring this peak period.
Trees and shrubsplanted on the south side are also
importantbecauseof the considerablesolarradiationon
the lowersectionsof the southwall and the immediately
adjacentgroundduring August and Septemberafternoonswhen sun angleshavebecomelower.
Shading and Microclimate Modification
Near
the Air Conditioner
Of high priority for landscapingis the area immediately surroundingthe air-conditioningunit or units.One
or two treesshouldbe plantedfairly closeto the unit so
that after five yearsof growththeir canopieswill completelyshadethe air conditionerand the adjacentground
and walls duringmorningsand afternoonsof the entire
coolingseason.Care shouldbe takento ensurethat the
exhaust air flow of the air conditioner
the infiltration of warm water-ladenair throughcracks
aroundthe windowsand doors as well as wall joints.
Consequently,designsthat channelwinds towardan
air-conditionedresidencecan actuallyincreasethe energyconsumedin cooling.
One study(Steenet al. 1976)of the summercooling
load of a typicalair-conditioned
housein Floridaestimatedthat the total heat gain via infiltrationexceeded
the heatgainby radiationand conduction
throughthe
windowsand walls. This designconflictcan be alleviatedby a carefulplacementof shrubsandtreesso that
windsare channelledinto the dwellingwhen the windowsare open,but awayfrom it whenthe windowsare
closed.For example,in southernFloridathe prevailing
summerwindsare from the southeast.Consequently,air
infiltrationcan be reducedby locatingtall shrubsimme-
diately adjacentand north of east-facingwindowsor
west of south-facing
windows.When the windowsare
opened,thesesameshrubswill facilitatenaturalventilation. Figure2 illustratesthis shrubplacementconcept.
If a housewill be air-conditioned
duringmostof the
coolingseason,
shrubs
and1ow-canopied
treesshouldbe
usedto blockprevailingwinds.For example,if summer
winds are from the southeast, tall shrubs should be
positioned
on the southsidesof eastwindowsand the
east sides of south windows.
Recent anemometer
mea-
surements(Parker 1981) havedocumentedthat this type
of vegetative
arrangement
cansignificantly
reducevelocities of air impingingon the windows,and thus can
is not obstructed
by the vegetation.
Preliminarytemperaturemeasurements
indicatethat
the blockingof direct solarradiation,coupledwith the
evaporativecooling by the vegetation,can reducethe
ambientoperatingtemperatureof the unit by 6 or 7
degreesFarenheit.Thus, this strategicplantingof only
one or two treescan increasethe operatingefficiencyof
the air conditionerby as muchas 10 percentduringthe
warmestperiods.
!
I
Landscapingfor Wind Control
Trees and shrubscan be positionedto influencethe
movement
of air throughandarounda residence
(White
1954, Parker 1981). It should be noted that wind patternsat a specificresidentialsiteare oftenmuchdifferent and more variable
than those recorded at a local
weatherstation(particularlyif otherbuildingsand structures are in the immediate area). Consequently,landscapingfor wind controlgenerallyshouldbe basedon
Figure 2. Shrub valve system:shrub placementfacilitates
breezeswhen windowis open, reducesinfiltrationwhen
dow is closed.
February1983/JouRN^[OVFOR[STaY/83
lessen warm air infiltration. The same shrubs can also
shadethe adjacentwalls and windows.
Material
Plant Species
It is important to use plant specieswhich do not
requirelargeamountsof water,fertilizer, or pesticides.
Table 1 showsthe result of a recent analysis(Parker
1982) of the annual indirect energy costs for lawns,
shrubs,and trees in a Florida landscape.The energy
contained
in each of these maintenance
materials
was
significantlylarger than that in the gasolineused in
mowing the lawn. Native trees and shrubsresistantto
droughtand diseaseand appropriateto the site (especially soil) can sharply reduce these indirect energy
inputs.
Although both evergreenand deciduoustrees are
effectivein reducingcoolingneeds,deciduousspecies
shouldbe plantedon the southsideso that passivesolar
heating (via radiation through south-facingwindows)
can occurduring the winter. In southernFloridathis is
not a major consideration,since the heating seasonis
extremelybrief. Shrubsnear southand eastwalls could
be deciduousto allow for some passivesolar heating
and to facilitatebreezesduring mild weather.
Energy Conservation Landscaping Of
A Mobile
Table 1. Annual indirect energy costs per square meter
for lawns,shrubs,and trees in a Floridaiandscapefi
Home
In order to establishthe validity of these energyconservinglandscapedesignconcepts,a detailedstudy
was made on a mobile home that is a daycarecenteron
the campusof FloridaInternationalUniversityin Miami
(Parker and Parker 1979, Parker 1981). The doublewidth mobile home has insulation values of about 7 for
Lawn
Shrub
Fertilizer
Water
Pesticide
Gasoline
Total
29,900
' Fordetails,see Parker(1982).
Home
83,000
warm summer days.
Average
Averagerate
Reduction
Time periodand
ambient
of energy
in air
landscapecondition temperature consumption conditioning
Kilowatt-hours
Morning
(9 A.M.--NOON)
No landscaping
Landscaping
øF
per hour
Percent
90.0
91.0
5.56
2.28
58.9
93.8
93.9
8.65
3.67
57.6
Afternoon
(NOON--6P.M.)
No landscaping
Landscaping
that are comparableto those of many concrete-block
single-familydetachedresidences
in Florida. Both patternsand levelsof air-conditioningconsumption
by the
totally unlandscapedchild-care center were monitored
duringvery warmsummerdays.Thenan energyconservationlandscape
plan wasdesignedandinstalledduring
the summer of 1978.
Table2 showsa brief summaryof the data obtained
(Continuedon page 105)
the vegetationaffectsexchangeof heat betweena house
Sites
and its environment.
Home heatexchangeoccursthroughthreebasicprocesses:air infiltration, heat conductionthroughwalls,
Influence Heating
And Cooling Energy
roofs, and floors, and transmission of solar radiation
throughglass.Air infiltrationis the passage
of air into a
structurethroughjoints, pores,cracks,and otheropenings. Such flows result from pressuredifferencesbe-
David R. DeWalle, Gordon M. Heisler,
tween inside and outside air. Pressure differences in turn
and Robert E. Jacobs
ABS2;RACT
-- Experiments
with
small
mobile
homes
inPennsylvaniaindicatedthat shadeof treescan significantlyreduce
solar heatingand that by loweringwind speedsforestscan
lesseninfiltration of outsideair. In one deciduousstand in
summer,coolingenergyneedswere75percentlessthanin the
open. In winter shadingis counterproductive,
offsettingsavingsfrom reducedinfiltration of coM air. In the deciduous
stand, savingsin winter heating energywereonly 8 percent,
and with greatershadingin a densepineforestheatingenergy
needsrose 12 percent. Forestsand windbreaksare especially
effectivewith poorly sealedhousesand in windy weather.On
forested sites in most of the United States, energy use can
probablybe lessenedby manipulatingforest growthto allow
the sun to strike the house in winter. On open sites windbreaksand carefully locatedshade trees wouM lessenyearround energy use.
may be causedby the force of the wind and by differences in temperaturebetween inside and outside air.
Wind velocity reductionsby vegetationcan reduceair
infiltrationsignificantly.
Shade
from
trees can lessen heat conduction
waysof reducingenergyneedsfor spaceconditioning.
Managementof trees and other landscapevegetation
offers one approach.By modifying the microclimate,
ß
and
radiationtransmission
into a home primarilyby reducing solar radiation reaching exterior surfaces.Wind
velocity reductionsby vegetationalso reduceheat conduction, by lesseningheat convectedto and from exterior surfacesof buildings.
Informationon the use and management
of treesand
foreststo reduce energy requirementsfor home space
conditioning
hasbeendevelopingin a numberof studies
dating back to about 1940 {see reviews by DeWalle
1980 and Heisler 1977). This article summarizesseveral
experimentsto determineeffects of forest sites on energy use for summer cooling and winter heating of
mobile homes. Forest sites are of particular interest
Rising
energy
costs
have
caused
homeowners
toseekbecause houses are often
84/JOURNAL
OFFORESTRY/February
1983
108,800
Table 2. Rate of electrical energy consumptionfor air
conditioning the daycare center during a number of
the ceiling, floor, and walls--thermal characteristics
Forest
Tree (5 yearsold)
.......
BTUs per square meter .......
3,500
5,800
22,600
18,400
18,400
18,400
6,700
84,600
42,000
1,300
located in forests for esthetic
reasons.The informationfrom theseexperimentsalso
indicates some effects of windbreaks and shade trees on
energy use.
¸1983 SOCIETY
OFAMERICAN
FORESTERS
Landscapingto Reducethe Energy Usedin CoolingBuildings(frompage84)
evergreen
windbreak
forreducing
residential
energy
consumption.
ASHRAE
beforeand two yearsafter the vegetativelandscaping.
Trans.85, 16 p. New York.
The resultsare the averages
of largenumbersof experiPARKER,
J. 1979.Precision
landscaping
forenergyconservation.
P. 151-156in
Proc.Natl. Conf. Technology
for EnergyConservation.
Tucson,Ariz.
mentaldata pointsgatheredduringperiodsof similar
PARKER,
J. 1981. Uses of Landscapingfor EnergyConservation.
Rep. to
climatic conditions.Overall, the energy used in air
FloridaGovernor'sEnergyOffice, 102 p. Tallahassee.
conditioning
duringwarmsummerdayswasreducedby
PARKER,
J. 1982. An energyand ecologicalanalysisof alternateresidential
landscapes.J. Environ. Syst. 11:271-288.
about58 percent.Otherdata(Parker1981)indicatethat
PARKER,
D., and J. PARKER.1979. Energyconservation
landscaping
as a
comparable
savingswouldbe achieved
duringthe entire
passive
solarsystem.
P.471-474in Proc.Fourth
Natl.Passive
SolarConf.
KansasCity, Mo.
coolingseason.
ROBINFIlE,G. 1977.Landscape
Planningfor EnergyConservation.
EnvironClearly,vegetative
landscaping
can be an extremely
mentalDesignPress,Reston,Va., 224 p.
effectiveenergy-conservation
technique.The magnitude
SHL^CHTMAN,
P., and J. PARKER1981. Peak load energyconservation.
P.
19-29 in Proc. 4th Miami Internatl. Conf. AlternativeEnergySources,
of the savingssuggests
that trees and shrubsreduce
Miami Beach, Fla.
coolingrequirements
not only by shadingbut alsoby
STEEN,J., W. SHRODE,
andE. STUART.1976. P. 212 in Basisfor Developreducingwarmair infiltrationand creatingcool micromentof a Viable EnergyConmrvation
Policyfor FloridaResidents.
Fla.
StateEnergyOff., Tallahassee.
climatesimmediatelyadjacentto the building. Future
U.S. DEPARTMENT
OFENERGY
1978.Optionsfor Passive
EnergyConservation
attempts
to improveenergyefficiencyshouldnot focus
in Site Design.Wash.. D.C. 285 p.
WHITE,R. 1954 Effectsof lanct•cape
development
on the naturalventilation
on thebuildingalone,but shouldincludelandscaping
as
of buildings
andtheiradjacent
areasTexasEng.Exp. Sin.,Res.Rep.45.
an integralcomponent. ß
16 p. CollegeStation.
THE AUTHOR--JohnH. Parker is an associateprofessorin the
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February1983/JOURNAL
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