R-Values and Thermal Mass

7/29/2019 R-Values and Thermal Mass

1/5

From: Joaquin Karcher foneearth@taosnet

From: JoaquinKarcher [email protected]]Sent: Saturday, anuary 8, 2005 t 1:36AMTo: Charly Rupprecht; Jim Hallock'; 'JoeTibbets'Subject: Clearifications to the R-Value debate rom JoaquinThermal Mass and R-Value: Making Senseof a Confusing IssueFrom EBN Volume 7, No. 4 -- April 1998

Page of5

UnderstandingHeat Transfer"Mass-EnhancedR-Value"When is Mass-EnhancedR-Value Significant?Do We Need Mass-EnhancedR-Value Ratings?Final Thoughts

Sidebar: Effective R-value of Agriboard Panels

THE ADVERTISEMENT PROMISESR-30 from a lightweightmasonryblock wall system.Log homeproduct iteratureclaims that log walls insulateas well as fiberglassbecauseof the thermalmass.Salesmenat atrade show argue hat a new fiber-cementbuilding systemachievesR-28 even houghthe"tested"R-valuecomes n at only R-l6.What'sgoing on here?Do theseclaims of "effective R-values"that greatly exceed he widely publishedR-values or high-massmaterialshold up? Just what effect does hermal masshaveon the energyperformanceof an exterior wall system?The issueof thermal massand ts effect on the energyperformanceof buildings is one of the most confusing ssues acing designers, uilders, andbuyersofbuildings today.This article tries to sortout thesemysteries,providing enoughbackgroundon thephysicsof heattransfer o understand he relationshipbetween hermal storageand heat flow, and thenexplainingwhen this information is relevantand how it should be used n building design. This articledoesnot address he useof thermal mass nsidea building, where t can store heat (or coolth) and evenout temperature luctuations.UnderstandingHeat TransferHeat flows by threemechanisms:conduction,convection,and radiation.Conduction s the molecule-to-molecule transferof kinetic energy(onemoleculebecomesenergizedand, n turn, energizesadjacentmolecules).A cast-iron skillet handle heatsup becauseof conduction hrough the metal. Convection sthe transfer of heat by physically moving the molecules rom one placeto another.Hot air rises;heatedwater thermosiphons;our forced-airheatingsystemswork by moving hot air from one placeto another.Radiation is the transfer of heat throughspacevia electromagneticwaves(radiantenergy).A campfirecan warrn you even f there s wind betweenyou and the fire, because adiation s not affectedby air.With buildings, we refer tq heat flow in a numberof different ways. The most commonreferences "R-value," or resistance o heat flow. The higher the R-valueof a material,the better t is at resistingheatloss(or heatgain). U-factor (or "U-value," as t is often called) s a measureof the flow of heat--thermaltransmittance--throughamateial, given a difference n temperatureon either side. n the inch-pound(I-

file://C:\Documents and Settings\HP_Administrator\Desktop\backup\Flandouts\R-Values.htm 1412009


2/5

' From: Joaquin Karcher foneearth@taosnet Page of5

P) systern, he U-factor is the numberof Btus (British ThermalUnits) of energypassing hroughasquare oot of the material in an hour for every degreeFahrenheitdifference n temperatureacross hematerial (Btu/ft2hriF). In metric, it's usually given in wattsper squaremeterper degreeCelsius(wlm2iC).R-values aremeasuredby testing aboratories,usually in somethingcalled aguardedhot box. Heat flowthrough the layer of material can be calculatedby keepingone side of the materialat a constanttemperature, ay 90;F (32;C), and measuringhow much supplementalenergy s requiredto keep heother side of the material at a different constant emperature, ay 50;F (10;.C)--all this is defined n greatdetail in ASTM (American Society of Testingand Materials)procedures.The result is a steady-stateR-value ("steady-state"because he difference n temperatureacross he material is kept steady).R-valueand U-factor are the inverse of one another:U : 1/R.Materials that are very good at resisting he flow ofheat(high R-value, ow U-factor) can serveas nsulationmaterials.So far, so good.Materials haveanotherproperty that can affect their energyperformance n certain situations: eatcapacity. Heat capacity s a measureof how much heat a material can hold. The property is mostsignificant with heavy, high-thermal-massmaterials.As typically used n energyperformancecomputermodeling, heat capacity s determinedper unit areaof wall. For each ayer in a wall system, he heatcapacity s found by multiplying the densityof that material,by its thickness,by its specific heat(specificheat s the amount of heata materialcan hold per unit of mass).Water hasa heatcapacityof 1Btu/lb.1F .2kJkg&), whilemostbuildingmaterials rearound0.2to0.3 Btu/lb.;F 0.8 o 1.3kJlkg;K).If thereare various ayers n the wall, total heatcapacity s found by addingup the heat capacities oreach ayer (drywall, masonryblock, and stucco, or example). n the following section,we will examinehow the heatcapacity of materialscan affect the energyperformanceof buildings."Mass-Enhanced -Value"When peoplerefer to the "masseffect" or "effectiveR-value,"they aregenerallyreferring to the abilityof high-massmaterials,when used n certain ways, to achievebetter energyperformance han would beexpectedf only the commonly accepted steady-state)R-valueor U-factor of that materialwereconsidered.Let's take a look at a typical use of one of thesehigh-massmaterials n a wall system.Whenone side of the wall is wanner than the other side, heat will conduct rom the warm side nto the materialand graduallymove through it to the colder side. f both sidesare at constant emperatures--sayheinsidesurfaceat 7 5iF (a2p) and the outside surfaceat 32iF (18;C)--conductivitywill carry heatout ofthe buildingat an easilypredictedrate. As describedabove, his steady-state eat flow is what most testprocedures or determining R-value measure.In real-life situations,however, the inside and outside emperatures re not constant. n fact, in manypartsof the country, the driving force for conductiveheatflow (remember,heat alwaysmoves fromwarmer to colder) can changedramaticallyor even reverseduring the courseof a day. On a summerafternoon n Albuquerque,New Mexico, for example, t might be 901F 32p) outside,and the outsidewall surface--becauset hasa dark stucco--mightbe even hotter. It's cooler nside, soheatconducts romthe outside surfaceof the wall inward. As night falls, however, t cools down outside.The airtemperaturemay drop to 501F 10;C). The driving force for heat flow changes.As the temperaturedifference across he wall is reversed, he heat low is also reversed--drawingheatback towards heoutside of the building. AS a resultof this modulating heat low through a high-heat-capacitymaterial,less heat rom outside he building makes ts way inside.Under theseconditions, he wall hasaneffective hermalperformance hat is higher than the steady-stateR-value isted in books (suchas

file://C:\DocumentsndSettings\HP_AdministratoilDesktop\backup\Flandouts\R-Values.htm1412009


3/5

From: Joaquin Karcher foneearth@taosnet Page of5

ASHRAE's Handbook of Fundamentals).This dynamicprocesss what somepeoplecall the "masseffect."Another cofirmon scenario s when the outside emperature luctuatesbut nevercrosseshe indoorsetpoint emperature. n this case, he direction of heat flow neverchanges, ut the thermal lag or timedelay in heat flow can still be beneficial by delaying the peakheatingor cooling load. For example, fthe outdoor temperature n Miami peaksat 95;F (35iC) at 5:00 on a summer afternoon,but it takes eighthours for the heat o travel through the wall, the effectof that peaktemperaturewon't be felt inside thebuilding until the middle of the night. Becausemost cooling equipmentoperatesat higher efficiency ifthe outdoor air temperature s lower and because ighttime thermostatsettingsmay be higher (at least ncommercial buildings), potentially significant savingscan result. Not only cantotal cooling energybereduced,but peak oadscan also be reduced.This can ead to smaller (and esscostly) mechanicalsystemsand lower demandcharges or electricity. This time lag effect can saveenergyand money,butnote that it does not affect the total amountof heat lowing through the wall.As noted above, he amount of heat flow through a wall is reducedby the use of thermal masswhen thetemperaturesluctuateabove and below the desired ndoor temperature, o under theseconditionsamaterial might have a "mass-enhanced" -valuethat is greater han its steady-stateR-value.To estimatethis mass-enhanced -value for a given high-massmaterial n aparticular climate,researchers t OakRidge National Laboratorymeasure he thermalperformanceof a high-masswall under dynamicconditions, n which the temperatureon one side of the wall is kept constantand the temperatureon theother side s made o fluctuateup or down. With this measuredheat low under dynamic conditionsas abasis, hey then usecomputermodeling to arrive at steady-statewall R-values hat would be required oachievecomparableoverall energyperformanceunder various climate conditions. Thoseresultsarewhat we are calling the "mass-enhanced -values" for the high-massmaterialunderthe modeledconditions.(Others efer to this as theffictive R-value,a term that canbe misleading.)The multiplierobtainedby dividing the mass-enhanced -value of a material n a given climate by its steady-stateR-value s referred o by Oak Ridge researchers s heDynamicBenefit or MassiveSystemsDBMS).When is Mass-EnhancedR-Value Significant?The masseffect is real. High-masswalls really can significantly outperform low-masswalls ofcomparablesteady-stateR-value--i.e., hey canachievea higher "mass-enhanced -value." BUT (andthis is an mportant "but"), this mass-enhanced -value s only significant when the outdoortemperatures ycle aboveand below indoor temperatureswithin a24-hour period.Thus, high-masswalls are most beneficial in moderateclimates hat have high diurnal (daily) temperatureswingsaroundthe desired ndoor setpoint.Nearly all areaswith significant cooling loadscan benefit from thermal mass n exteriorwalls. Thesunny Southwest,particularly high-elevationareasof Arizona, New Mexico and Colorado,benefit themost from the masseffect for heating. n northernclimates,when the temperatureduring a24-hotnperiod in winter is always well below the indoor temperature,he masseffect offers almost no benefit,and the mass-enhanced -value is nearly identical to the steady-stateR-value. The ASHRAE Handbookof Fundamentals ists "meandaily temperature ange"data or hundredsof U.S. climates n the chapteron climate data.Thesevaluescan be helpful in figuring out how significant mass-enhanced -valuemight be for a particular climate, but they do not tell the whole story; also significant is the percentageof days during the heating and cooling seasonswhen the outdoor temperaturecycles aboveand belowthe indoor temperature.Do We Need Mass-EnhancedR-Value Ratinss?

file://C:\DocumentsndSettings\HP_Administrator\Desktop\backup\Flandouts\R-Values.htm1412009


4/5

From Joaquin Karcher [oneearth@taosnet Page of5

Clearly, high-massmaterials used n exterior walls perform better han would be expectedbasedsolelyon their steady-stateR-values. But the actual hermalperformances highly dependenton where thebuilding is located. Manufacturersof thesematerials ightly want to take credit for this improvedperformance,but how canthat be done n away that doesn'texaggerate erformance or partsof thecountry where the mass effectbenefit ust isn't there?"Right now, we don't havea system hat forcespeople o deal with calculations n a constantw&y," saysBruce Wilcox, P.E., of the Berkeley SolarGroup, who has done extensivemodeling of masseffectsfor the PortlandCement Associationandothers.All sorts of claims are being made about mass-enhanced -value(usually called "effective R-value")with little standardization.The first stepneeds o be consensus n how the masseffect shouldbeaccounted or in testing and modeling. Jeffrey Christian at Oak Ridge National Laboratoryhasbeendeveloping and refining the method of dynamicthermal analysisand simulation describedabove.This isthe most extensiveeffort to date o quantiff the mass effect. Christian'sgroup,with the help of Bruce Wilcox and others,also developed hermalmass ables or the Model EnergyCode n the late 1980s hat can be used oaccount or the thermal massbenefits of high-massbuilding materials n wall systems.The next step, suggestsChristian,might be to formalizethe testingand simulationprocedures hroughdevelopmentof ASTM standards.Establishmentof an ASHRAEcommittee o address he mass effect may alsobe in the works. To ensure hat suchstandardswould beapplied n a consistentmanner,Wilcox suggests hat applicable ndustriesmight haveto setup some sortof council, perhapsmodeledafter the National FenestrationRatingCouncil (NFRC), which enforcesconsistent eporting of window energyperformance.Sucha "ThermalMass Rating Council" mightoverseestandards elating to how masseffect and mass-enhanced -valuearereported.Wilcox remainsleery of the whole concept of mass-enhanced -value--not hat the effect exists,but whether t canbeused clearly with building materials."I don't know if there'sany way to make t a propertyof thematerial," he told EBN, "It's a property of the system."Thereare a lot of questions o sort out, suchashow many climates need o be modeled:are six enough,as Oak Ridge researchers aveused,or do weneed2}? Would such a system ake credit for time delays n heat ransfer,or ust actualreductions n theamount of heat hat moves through? Who will pay for all theresearch o make this happen?Are the industries hat sell high-massmaterials arge enough o support a Thermal MassRating Council and the additional researchneededonthese ssues?Final ThoughtsHigh-massbuilding materialscan offer significant energybenefits n exterior walls. The benefitmay beprimarily in the shifting of peak oad conditions or in an actualreduction n overall heatgain or loss.Thesebenefits arehighly dependentupon wherethe building is located,how it is designed,and how it isoperated.How we should give credit--in terms of energyperformance--forhigh-massbuilding materialsis still very much open for debate.Until standardized rocedures or determining he regionalsignificanceof the masseffect are widely applied, here will likely be continued confusionandcontinuedexaggeration egarding he energybenefits of thermal mass.Oak Ridge researchers ndcompaniessuch as Agriboard Industriesarehelping to bring these ssues nto public awareness, ut agreatdeal of work remains o be done.-Alex Wilson

For more information:Jeffrey Christian

file://C:\Documents and Settings\HP_Administrator\Desktop\backup\Flandouts\R-Values.htm1412009


5/5

. From: JoaquinKarcher oneearth@taosnet

OakRidgeNational LaboratoryBldg3l47, P.O.Box 2008OakRidge,TN 37831-6070423574-5207, 23-574-9338fax)Barry SullivanAgriboard ndustries, .C.P.O.Box645Fairfield,1A 525565 5 472-0363, 15 472-008 (fax)PortlandCementAssociation5420OldOrchardRoadSkokie, L 600778471966-6200BruceWilcox,P.E.BerkeleySolarGroup58 CalvertCourtPiedmont,CA946ll5 0 6017475, 5 0 6017415 (fax)[email protected])

Page5 of5

file://C:\DocumentsndSettings\HP_Adminishator\Desktop\backup[landouts\R-Values.htn1412009