THERMAL INSULATION - Plea-arch
notePLEANO T ES2Passive and Low Energy Architecture InternationalDESIGN TOOLS AND TECHNIQUESTHERMAL INSULATIONAndras Z61d and Steven SzokolayIN ASSOCIATION WITH THE UNIVERSITY OF QUEENSLAND DEPT. OF ARCH ITECTURE &,REPRINTED BY RESEARCH, CONSULTING AND COMMUNICATIONS, KANGAROO VALLEY, NSW
THERMAL INSULATION . Andras Zlild and Steven V. Szokolayall righ.ts reservededited by sv Szokolayfirst published 1997Reprinted by R.C.&C. 2001PLEA : Passive and Low Energy Architecture InternationalIn assocla!ion withDepartment of Architecture, The Univel'sity of QueenslandBrisbane 4072Andras Zold is professor of Building Physics at the Tecllntcai University of Budapest(Hungary), chairing the Department of Building Energetics. He is author of numerous bookson energy conservation, energy economics In buildings and he serves on both national andintematlonal standards committees.Steven V. Szokoiay was Director of the Architectural Science Unit and later Head ofDepartment of Architecture at the University of Queensland-now retired. He has a dozenbooks and over a hundred research papers to his credit. He is a consultant to various UNbodies and served on numerous government committees. He is currently President of PLEA.The manuscript of this pubficatlon has been refereed byJohn Martin Evans. Director. Research Centre for Habitat and E;nergy,University of Buenos Aires.andOr. Nick Baker, Director, The Martin Centre, Department of ArchitectureThe University of GambrtdgeOriginally printed by The University of Queensland Printery, Manager ian TaylorReproduced by R.C. & C., NSWISBN 0 86776 671 9
PREFACEThe Directorate of PLEA (Passive and Low Energy Architecture}International recognised the lack of inexpensive but authoritative texts onvarious topics relating to sustainability, to the design of buildings forpassive environmental control. In response to this need it was decided toproduce and publish a series of PLEA Notes, under the overall title:Design Tools and Techniques.The first in the series: Solar geometry has been published last year andthe present Thermal Insulation is Note 2 in the series. Note 3, onThermal comfort is in preparation and it is likely to be published later thisyear.Further titles in preparation are:Radiative and evaporative coolingControl of open spacesEnergy issues in sketch designDaylighting.Any suggestions for further topics to be covered, offers of contribution, aswell as comments and criticisms would be welcome and should beaddressed to me, as series editor, at''P 0 Box 851, Kenmore, 4069, Australiafax No: 61 7 3378 1051 ore-mail: S.Szokolay@mailbox.uq.edu.au.These Notes are available at the same address.Steven V. SzokolayJanuary 1997.
CONTENTS1Basics and 0101116171818202021222324242525252626Transport phenomena in generalBalance conditionsDesign objectives and approachesConductivity: declared and design valuesMultidimensional heal flow: thermal bridgesCalculation methodThermal pointsResuhanl transmittanceRadiant heattransfetRadiant heal exchange: terrestrial bodiesRadiative balance of opaque elementsNon-steady one-dimensional heat flowPeriodic heal flowHeal storage cepacily2728282931323435363739404142Moisture transport3.13.23.33.43.53.63.73.83.945page 5Theory and refinements2.12.22.32.42.52.62.72.83The role of insulationThermal quantitiesHeatflowSurface conductatlce atld resistatlceCavitiesTransmittance: U-valueInsulating materials .Envelope conductanceTemperature gradientsTransmittance calculationsRools at1d exposed floorsSlabs on ground and basementsTransparent insulationPracticalities: Where to put the insulation?1.14.1 Flat roofs1.14.2 Pitched rools and attics1.14.3 Floors1.14.4 Walls: external insulation1.14.5 External surfaces: absorptatlce and emittance1.14.6 Walls: cavily insulation1.14.7 Walls: internal insulation1.14.8 Some common mistakesWhat does thermal insulation give?1.15.1 Thermal insulation and necessary air change1.15.2 Fabric protectionSomeconclusionsDalton's lawCharaclersistics of moist airSorptionConvective moisture transportVapour diffusion through wallsVapour pressure distribution in cross sectionDesign approachGraphic analysisMoisture transport through flat roofsEconomics of insulationReferencesData sheetsIndexWor1 ed examples434346464748495054555859-6566ex.t U-value of multilayer waHex.2 Temperature gtarfientex.3 U-vaJue to limit inner surface temperatureex.4 lnsulauon to satisfy a requirementex.5 Locating a point of specified temperatureex.6 U-value of a composite wallex.l U-value of a pitched roofex.8 Modifyir J U-value for added insulationex.9 CalculatiOn of ground lossesexamples 1, 2, 3, 4 correcledex. t 0 Modify U-value for thermal pointsex.tt Resultant U-value calculationex. 12 Prediction of condensation risk211121213141516171730343551
CONTEXTThermal insulation is one of the most important techniques In the hands ofthe architect for providing indoor thermal comfort with minimal energy use.Insulation can drasticalfy reduce, in some cases almost eliminate heatflow through the building envelope. Its role is most significant in climateswhere the thermal problem is constant for long periods of time, such as1)2)in cold climates, in heated buildings, where heat loss is to bereducedIn warm climates, in air conditioned buildings, where heat gain isto be minimisedIn climates where cold winters alternate with hot summers, insulation maybe beneficial in both seasons.However, in climates with large diurnal variations, or in any climate wherethe building can satisfactorily operate 'free running', I.e. without heating orcooling, the significance of insulation is much reduced. Here several otherthermal control strategies may be used.In hot-dry climates the thermal mass of the building can be relied on toeven out the large diurnal variation of temperatures. In fully crossventilated buildings of warm humid climates the role of insulation isreduced to practically nil, as the indoor air cannot be lower in temperaturethan the outdoor, so there is no temperature difference. Exceptions are allenvelope elements exposed to solar radiation, particularly the roof. Herethe heat flow is driven NOT by the outdoor-indoor air temperaturedifference, but by tihe difference between the sol-alr temperature of theouter surface (which expresses the combined effect of air temperatureand solar radiation) and the indoor alr. For such elements insulation isadvisable.Undoubtedly, insulation is paramount from the point-of- view of energyconservation. It is in consistently cold, or consistently hot climates wherebuildings use the most energy for heating or air conditioning. And it isunder such circumstances that thermal insulation is most effective.The present Note deals with thermal insulation only. Future Notes will bedevoted to other techniques of passive thermal control.Some may find the present Note too theoretical, abstract and short ofactual application examples. This is however quite deliberate. The authorsbelieve that rather than presenting ready-made solutions for a largenumber of different problems, the principles must be clarified, as if oneknows these principles, one can derive the solution for any Situation. Thebest procedure for any situation is to check what the heat trasfermechanism is and select the appropriate insulation accordingly.3
Symbols and nl(gtl\g)AH(Wtm K)conductanceOBT dry bulb temperatureCNim'lradiant emissionEshape factorF,(W/rn')global (solar) irradianceG(JIK)heat storedH(g/kg)saturation absolute humidityHs(m'lh)volume now in ventilationL(rr .m'ltPa)ppermeance(W)heat now rate (nux)a(m'K/W)resistanceR(m KIW)Ra-a air-to-air resistance(m'K/W)surfaceresistance,oulsideRoo surtace resistance, inside(m'K/W)Rsl(N.s/g)vapour resistanceRv(%)relative humidayRHstorage quantityschange in storage quantilyt.Sabsolute temperatureT(s)time constantTcU-value, thermal transmittance CN/m K)uresultant U-valuerNJm KlUR(g/h)water vapour productionwc cJWBT wet bulb trremperatureAcc q-. bb tancecorrection to ensityStefan-Boltzmann constanttransmiltancemoisture content (10 solids)u66(1 41)(m)breadth, lhid ness(m)effective ttid ness(J/kgK)specific heat capacity(g/m')vapour concentration(m)heightconvective surface conductance (W/m K)(Wim K)inside suriace conductance(Wfm K)outside surtace conductance(Wtm K)radiation coefficient(W/mK)linear heat loss coefficient(m)lenglh 't.t.t"'wmass flow rate (e.g. vapour flux)partial pressure of water vapoursaturation vapour pressureoverall heat loss coefficientenvelope conductanceventilation conductancetemperatureair temperaturetemperature insidetemperature outsiderelative temperature scalesol-air temperaturetemperature differencewavelength (radiation)(n m.kPa)(W/mK)(kg/rn'}CNfmZK )(%)4(glh)(Pa)(Pa)(WIK)(WIK)CNIKl("C)c·c1c·c c·qc·q(K)(flm)
THERMAL INSULATIONPart 1 BASICS AND PRACTICALITIES1.1The role of lnsulalionA bLilding Is essentially o space surrounded by the ouHcflng envelope:floor, walls and roof as the main elements, which may Include subelements, such as windows, doors, etc. If the Interior spoco Is kept at otemperature different from that outdoors (by heating or cooling), heat willflow through the envelope from the wormer to the cOlder side.rfr IFig.UA leaking bucketIn winter, heating Is provided by some energy-using Installation. This heatinput must equal the heat loss, if the Indoor temperature is to bemaintained. The analogy of a leaking bucket (Fig.l.1) may help tovisuof.se the piocess. Cteo y. H the water level (the indoor temperature) is tobe mdntOinod. the water flow rote from the top (the heat input) must bethe some os the water flowing out through the hOles (the heal loss throughthe envelope), The water level Is analogous with temperature. If the sum of·all leaks Is greaiEII than the flow from the lop, the water level (the indoortemperature) wDI drop. Conversely, when the bucket Is empty (the inside isat lhe same temperature as the outside) and we wont to liMtho bucket (heatup the building), the flow from the top (the heat input) must be greater thenthe leaks (the heat foss),If energy Is to be conserved, or the healing cost Is to be kept down, thenthe heal loss through the envelope should be reducedThis Is the task of thermal Insulation.Cleak plugging').In summer, If the Interior Is cooled by on ene gy-bosed !Niollollon (eg.mechanical cOOling) then there wit be on Inward heat flow,le. heat gain,which Is port of the cooing lood. This con also be controled by lnsuotlon.Before we con oHempt to contrOl such heat flows, the heal transferprocesses themselves must be understood.1.240"Cto cFig.1.2 Temperaturepoint and intervalThermal quantitiesHeal is a form of energy, appearing as molecular motion In substances oras radiation In space. If Is measured in the some units os any other form ofenergy: the Sl unU being the joule (J). The mulllples kJ (kiloJoule 1CXXl J)and MJ (megojoule 1 000 000 J) ore often used,Temperature con be considered os a symptom of the presence of heat ina substance; It Is o measure of the thermal state of that substance. TheCelsius scale tokes the freezing point of water as the starling point, OOCand the boling point (under normal atmospheric cond liOns) as lOO"C.The total absence of heat Is the starting point of the absolute temperatureor Kelvin scale. The Intervals of thiS scale ore the some as of the Celsiusscale, but the starting pOint is the 'absOlute zero', -273.15 C. In notationthe folowing convention is adopted (Fig. 1.2): a particular poSition on the scale (1): c a temperature difference or interval. regardless of Its position on thescale (t.t): K5
THERMAL INSULAT10NSpecific heat capacity (c) of a substance expresses the relationshipbetween heat and temperature: it is the amount of heat energy thatcauses unit temperature increase of a unit mass of the substance.measured in units of J/kg.K. Some typical values are:brick, concrete 800- 1000 J/ kg.Kdry air1005 J/ kg.Kwater4176 J/ kg.K' ··For values of ather materialS see Data sheet 1.1.3Heat flowThe magnitude of heat flow can be measured two ways:a) as heat flow rate (Q), or heat flux, ie. the total flow In unit time througha defined area of a body or space, In units of J/s. which Is a watt (W)b) as heat flux density (or density of heat flaw rate), le. the rate of heatflow through unit area of a body or space, in W/m2 The muHiple kW,(kilowatt) Is often used for both quantnies).Heat can be transmitted by three processes: conduction, the propagation of heat within a body, le. the spreadingof molecular movement throughout an object or objects in contact convection, in a narrow sense the transfer of heat from a sOlid bodysurface to a fluid (gas or liquid) or vice-versa, but In a broader sense itmay mean the transport of heat by a carrying fluid from one saidsurface to another. at some distance away radiation: Infrared wavelegths of the electromagnetic radiationspectrum, referred to as 'radiant heat', le. the heat transfer betweenopposing surfaces, which wiD take place through space, even throughvacuum.All three processes occur in heat transfer through the building envelOpeand each can be contrOlled by different means.The magnitude of conducflon heat flow rate between two points of asolid (or stagnant fluid) depends on four quantities:a) the cross-sectional area (A) through which the heat can flow. takenperpendicular to the direction of the heat flow, given In m 2b) the thickness (or 'breadth', b) of the body, ie. the length of heat nowpath, given in mc) the temperature dffference between the twa paints (eg. two oppositesurfaces of on envelope element), At t.-t. given inKd) a characteristic of the molena!. known as conductivity (1.), in olderliterature referred to as the 'k-volue', measured as the heat flow rotethrough unit area. with unit temperature difference, between twopoints unit distance apart, as W.m/m2K W/m.K The value of 1. variesbetween 0.02 W/m.K for a very good insulating material and almost400 W/m.K for a highly conductive metal, such as copper (see Datasheet 1).The heat flow rote (Q) Is proportionate to the area. the temperaturedifference and the conductivity, but lnversety proportionate to thethickness (b 'breodth1.AQ-A b (t.-t;). 1.1)If both 10 and t1 are constant (or assumed to be constant) we considersteadv-state heat flow. In real Situations one or both the.se temperaturesvary, which causes a non-steady heat flow.6
THERMAL INSULATIONA special case of this is the periodic heat flow. when a certain cycle (eg.the 24-hour day) repeats Itself with little change. ThiS will be mentioned Insection 1.7 and discussed in detail in section 2.12-13. However. of leastwith lightweight structures the steady-state assumption con still be usedto provide a snapshot of the momentary heat flow. or an estimate of theoverage heat flow ovetQ full cycle,Whilst conducNvlty Is a property of the material. regardless of shape andSize, conductance (C) is the corresponding property of a defined body, abuilding element. such os a slob. lfs unit Is W/m2K.The reciprocal of conductance is resistance ( rl ) in m 2K/W, proportionateto the thickness and inversely proportionate to the conductivity.b. 1.2)R· Note that the -ltv ending implies a material property, whilst 1he -onceending the property of a defined body, such as a slab or a wall.In a multilayer construction (eg. a wall of n layers), the heat will fiOY!through the layers in sequence, thus the resistances of these layers ore "inse(!es', therefore additive.tt. 1.3)lJ· lit d1.4Surface conductance and resistanceWhilSt conductance Is a measure of heat transfer through an elementfrom surface-to surface, the surface hself also offers a resistance to heatflow. These surface resistances ore denoted rl0 or rl"" for inside andoutside surfaces respectively. The reciprocal of surface resistance is thesurface conductance (h), so rl .!. and rl., .:!. Surface conductanceIIih0ta ce Is a lumped parameter, including radiant and convective components:In the us the terms film coefficient or film conductanceFig.1.3 Surface conduc n h h his increased by arr ftow(den tedare often used with the same meaning os surfaceconductance.llf)The magnitude of surface resistance depends on the position of thesurface, on the direction of heat flow and on air movement (Figs. 1.3 and1.4). The lost can be natural, due to temperature difference, or forced,eg. due to wind. Standard values ore given in Data sheet 2.1.5Fig.1.4 and reducedif the air is still.·CavitiesWithin a matertal layer heat Is propagated by conduction. If the bLildingelement includes a cavity or air space. the heat transfer through this ismore complex: mainly by convection and radiation. only to a minorextent by conduction. .In cavities more than a few miUimetres thick. convection currents willdevelop a nd substantially increase the heat transfer (fig.1.5). Theresistance o f a cavity will depend primarily on its position (vertiCal in a wall,horiZontal in a roof) and the direction of heat flow. The width of the cavitywill make very little difference beyond about 25 m m . The radiant heattransfer may be significant from the wormer to the cooler surface facingthe cavity: normally about 2/3 of the total heat flux. 11 occurs even if the7
THERMAL INSULA110Ncavity Is evacuated, as radiation travels also through vacuum. Radianttransfer will depend on surface qualities: emittance and absorptance ofthe two surfaces. In practice it is sufficient to employ a cavity resistance(Rc) value which combines convective and radiant components, butdistinguishing cavities between ordinary building materials and with lowemittance surfaces (eg. aluminium foil). Cavity resistqr: e values are givenin Data sheet 2.1.6Transmittance: U-va lueHeat transmission through an envelope element takes place from the airinside the building to the air outside (or vice-versa). The air-to-airresistance (R 0 0 ) will be the sum of the resistance of the element itself andthe two surface resistances. The reciprocal of this air-to-air resistance isthe transmittance, or U-vatue. The unit of measurement is the same as forconductance (W /m 2 K), the difference being that conductance is takenfrom surface to surface, whilst the U-value from air to air (Fig.l .6).Fig.1.5 Convectionair current in cavity. 1.4)The transmission heat flow is proportionate to the U-value, the area (A) ofthe element, perpendicular to the direction of heat flow and thedifference between the outdoor and indoor temperatures. 1.6)Fig.1.6 Conductanceand transmittanceThe convention here adopted is that always the t1 is subtracted from thet 0 , as in winter this gives a negative difference, a negative Q, indicatingheat loss. A positive Q indicates heat gain, eg. in air conditioning loadcalculations.Air has a very low conductivity, as long as it is still. The best insulators arestructures which encapsulate air with as little material as possible.Insulation is provided by the air, the material is there in the form ofbubbles, or fibres only to keep the air still.1. 7Insulating materialsIf the definition of 'insulation' is the control of heat flow, then three forms ofinsulation can be distinguished:reflective, resistive and capacitive insulation.Reflective insulation, as mentioned above, can be employed where thedominant heat transfer mechanism is radiation. The only practicalreflective insulating material is aluminium foil, which has ·a lowabsorptance and low emittance (approx. 0.02- 0.05). It can be used onits own, but more frequently laminated with building paper, either single ordouble-sided, or even to a plasterboard (resulting in a ' foil-backed'plasterboard. To be effective, it must face a cavity or air space. Variousmultiple-foi
Thermal insulation is one of the most important techniques In the hands of the architect for providing indoor thermal comfort with minimal energy use. . cooling, the significance of insulation is much reduced. Here several other thermal control strategies may be used.
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Thermal insulation materials fall into the latter category. Thermal insulation materials may be natural substances or man-made. If the density of insulation is low, the air or gas voids are comparatively large and this makes for the best insulation for low to medium temperatures where compression and/or vibration is not a factor.
Industrial Insulation Phase 2 1 Insulation - Terms, Definitions & Formula Revision 2.0, August 2014 Introduction Thermal resistance, thermal conductivity and thermal transmitting are some of the terms used in the insulation industry. They are a measurement of different factors which make up an insulating product. When choosing and insulation
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Case study: insulation for refrigerators Insulation reduces energy cost, but has a cost itself Total cost is the cost of insulation plus the cost of energy lost by hear transfer through walls Objective function: minimize total cost given: x thickness of insulation C. M cost of insulation/mass T temp. di . across insulation C. E cost of .
paper no.1( 2 cm x 5 cm x 0.3 mm ) and allowed to dry sera samples at 1: 500 dilution and their corresponding at room temperature away from direct sun light after filter paper extracts at two-fold serial dilutions ranging that stored in screw-capped air tight vessels at – 200C from 1: 2 up to 1: 256.
