Designation: C1155 − 95 (Reapproved 2013)Standard Practice forDetermining Thermal Resistance of Building EnvelopeComponents from the In-Situ Data1This standard is issued under the fixed designation C1155; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (´) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers how to obtain and use data fromin-situ measurement of temperatures and heat fluxes on build-ing envelopes to compute thermal resistance. Thermal resis-tance is defined in Terminology C168 in terms of steady-stateconditions only. This practice provides an estimate of thatvalue for the range of temperatures encountered during themeasurement of temperatures and heat flux.1.2 This practice presents two specific techniques, thesummation technique and the sum of least squares technique,and permits the use of other techniques that have been properlyvalidated. This practice provides a means for estimating themean temperature of the building component for estimating thedependence of measured R-value on temperature for thesummation technique. The sum of least squares techniqueproduces a calculation of thermal resistance which is a functionof mean temperature.1.3 Each thermal resistance calculation applies to a subsec-tion of the building envelope component that was instru-mented. Each calculation applies to temperature conditionssimilar to those of the measurement. The calculation of thermalresistance from in-situ data represents in-service conditions.However, field measurements of temperature and heat flux maynot achieve the accuracy obtainable in laboratory apparatuses.1.4 This practice permits calculation of thermal resistanceon portions of a building envelope that have been properlyinstrumented with temperature and heat flux sensing instru-ments. The size of sensors and construction of the buildingcomponent determine how many sensors shall be used andwhere they should be placed. Because of the variety of possibleconstruction types, sensor placement and subsequent dataanalysis require the demonstrated good judgement of the user.1.5 Each calculation pertains only to a defined subsection ofthe building envelope. Combining results from different sub-sections to characterize overall thermal resistance is beyond thescope of this practice.1.6 This practice sets criteria for the data-collection tech-niques necessary for the calculation of thermal properties (seeNote 1). Any valid technique may provide the data for thispractice, but the results of this practice shall not be consideredto be from an ASTM standard, unless the instrumentationtechnique itself is an ASTM standard.NOTE 1—Currently only Practice C1046 can provide the data for thispractice. It also offers guidance on how to place sensors in a mannerrepresentative of more than just the instrumented portions of the buildingcomponents.1.7 This practice pertains to light-through medium-weightconstruction as defined by example in 5.8. The calculationsapply to the range of indoor and outdoor temperatures ob-served.1.8 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.9 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2C168 Terminology Relating to Thermal InsulationC1046 Practice for In-Situ Measurement of Heat Flux andTemperature on Building Envelope ComponentsC1060 Practice for Thermographic Inspection of InsulationInstallations in Envelope Cavities of Frame BuildingsC1130 Practice for Calibrating Thin Heat Flux TransducersC1153 Practice for Location of Wet Insulation in RoofingSystems Using Infrared Imaging1This practice is under the jurisdiction of ASTM Committee C16 on ThermalInsulation and is the direct responsibility of Subcommittee C16.30 on ThermalMeasurement.Current edition approved Nov. 1, 2013. Published March 2014. Originallyapproved in 1990. Last previous edition approved in 2007 as C1155 – 95(2007).DOI: 10.1520/C1155-95R13.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at

[email protected] For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13. Terminology3.1 Definitions—For definitions of terms relating to thermalinsulating materials, see Terminology C168.3.2 Definitions of Terms Specific to This Standard:3.2.1 building envelope component—the portion of thebuilding envelope, such as a wall, roof, floor, window, or door,that has consistent construction. — For example, an exteriorstud wall would be a building envelope component, whereas alayer thereof would not be.3.2.2 convergence factor for thermal resistance, CRn—thedifference between Reat time, t, and Reat time, t−n, divided byReat time, t, where n is a time interval chosen by the usermaking the calculation of thermal resistance.3.2.3 corresponding mean temperature—arithmetic averageof the two boundary temperatures on a building envelopecomponent, weighted to account for non-steady-state heat flux.3.2.4 estimate of thermal resistance, Re—the working cal-culation of thermal resistance from in-situ data at any onesensor site. This does not contribute to the thermal resistancecalculated in this practice until criteria for sufficient data andfor variance of Reare met.3.2.5 heat flow sensor—any device that produces a continu-ous output which is a function of heat flux or heat flow, forexample, heat flux transducer (HFT) or portable calorimeter.3.2.6 temperature sensor—any device that produces a con-tinuous output which is a function of temperature, for example,thermocouple, thermistor, or resistance device.3.3 Definitions: SymbolsApplied to the Terms Used in ThisStandard:3.3.1 Variables for the Summation Technique: A = areaassociated with a single set of temperature and heat fluxsensors,C = thermal conductance, W/m2·K,CR = convergence factor (dimensionless),e = error of measurement of heat flux, W/m2,M = number of values of ∆T and q in the source data,N = number of sensor sites,n = test for convergence interval, h,q = heat flux, W/m2,R = thermal resistance, m2·K/W,s(x) = standard deviation of x, based on N−1 degrees offreedom,T = temperature, K,t = time, h,V(x) = coefficient of variation of x,∆T = difference in temperature between indoors andoutdoors, K,λ = apparent thermal conductivity, W/m·K, andx = position coordinate (from 0 to distance L in incrementsof ∆x),ρ = material density, kg/m3.3.3.2 Subscripts for the Summation Technique: a = air,e = estimate,I = indoor,j = counter for summation of sensor sites,k = counter for summation of time-series data,m = area coverage,n = test for convergence value.o = outdoor, ands = surface,3.3.3 Variables for the Sum of Least Squares Technique:Cρ= material specific heat, J/kg·K (Btu/lb·°F),Ymi= measured temperature at indoor node m for time i K,Fni= measured heat flux at interior node n for time i W/m2,λ = apparent thermal conductivity, W/m·K,Tmi= calculated temperature at indoor node m for time i K,qni= calculated heat flux at interior node n for time i W/m2,WTm= weighting factor to normalize temperature contribu-tion to Γ,Wqn= weighting factor to normalize heat flux contribution toΓ, andΓ = weighted sum of squares function.3.3.4 Subscripts for the Sum of Least Squares Technique:s = specific heat of value, “s,” J/kg·K4. Summary of Practice4.1 This practice presents two mathematical procedures forcalculating the thermal resistance of a building envelopesubsection from measured in-situ temperature and heat fluxdata. The procedures are the summation technique (1)3and thesum of least squares technique (2, 3). Proper validation of othertechniques is required.4.2 The results of each calculation pertain only to a particu-lar subsection that was instrumented appropriately.Appropriateinstrumentation implies that heat flow can be substantiallyaccounted for by the placement of sensors within the definedsubsection. Since data obtained from in-situ measurements areunlikely to represent steady-state conditions, a calculation ofthermal resistance is possible only when certain criteria aremet. The data also provide an estimate of whether the collec-tion process has run long enough to satisfy an accuracycriterion for the calculation of thermal resistance. An estimateof error is also possible.4.3 This practice provides a means for estimating the meantemperature of the building component (see 6.5.1.4) for esti-mating the dependence of measured R-value on temperature forthe summation technique by weighting the recorded tempera-tures such that they correspond to the observed heat fluxes. Thesum of least squares technique has its own means for estimat-ing thermal resistance as a function of temperature.5. Significance and Use5.1 Significance of Thermal Resistance Measurements—Knowledge of the thermal resistance of new buildings isimportant to determine whether the quality of constructionsatisfies criteria set by the designer, by the owner, or by aregulatory agency. Differences in quality of materials orworkmanship may cause building components not to achievedesign performance.3The boldface numbers in parentheses refer to the list of references at the end ofthis practice.C1155 − 95 (2013)25.1.1 For Existing Buildings—Knowledge of thermal resis-tance is important to the owners of older buildings to determinewhether the buildings should receive insulation or otherenergy-conserving improvements. Inadequate knowledge ofthe thermal properties of materials or heat flow paths within theconstruction or degradation of materials may cause inaccurateassumptions in calculations that use published data.5.2 Advantage of In-Situ Data—This practice provides in-formation about thermal performance that is based on mea-sured data. This may determine the quality of new constructionfor acceptance by the owner or occupant or it may providejustification for an energy conservation investment that couldnot be made based on calculations using published design data.5.3 Heat Flow Paths—This practice assumes that net heatflow is perpendicular to the surface of the building envelopecomponent within a given subsection. Knowledge of surfacetemperature in the area subject to measurement is required forplacing sensors appropriately. Appropriate use of infraredthermography is often used to obtain such information. Ther-mography reveals nonuniform surface temperatures caused bystructural members, convection currents, air leakage, andmoisture in insulation. Practices C1060 and C1153 detail theappropriate use of infrared thermography. Note that thermog-raphy as a basis for extrapolating the results obtained at ameasurement site to other similar parts of the same building isbeyond the scope of this practice.5.4 User Knowledge Required—This practice requires thatthe user have knowledge that the data employed represent anadequate sample of locations to describe the thermal perfor-mance of the construction. Sources for this knowledge includethe referenced literature in Practice C1046 and related workslisted in Appendix X2. The accuracy of the calculation isstrongly dependent on the history of the temperature differ-ences across the envelope component. The sensing and datacollection apparatuses shall have been used properly. Factorssuch as convection and moisture migration affect interpretationof the field data.5.5 Indoor-Outdoor Temperature Difference—The speed ofconvergence of the summation technique described in thispractice improves with the size of the average indoor-outdoortemperature difference across the building envelope. The sumof least squares technique is insensitive to indoor-outdoortemperature difference, to small and drifting temperaturedifferences, and to small accumulated heat fluxes.5.6 Time-Varying Thermal Conditions—The field data rep-resent varying thermal conditions. Therefore, obtain time-series data at least five times more frequently than the mostfrequent cyclical heat input, such as a furnace cycle. Obtain thedata for a long enough period such that two sets of data that enda user-chosen time period apart do not cause the calculation ofthermal resistance to be different by more than 10 %, asdiscussed in 6.4.5.6.1 Gather the data over an adequate range of thermalconditions to represent the thermal resistance under the condi-tions to be characterized.NOTE 2—The construction of some building components includesmaterials whose thermal performance is dependent on the direction of heatflow, for example, switching modes between convection and stablestratification in horizontal air spaces.5.7 Lateral Heat Flow—Avoid areas with significant lateralheat flow. Report the location of each source of temperatureand heat flux data. Identify possible sources of lateral heat flow,including a highly conductive surface, thermal bridges beneaththe surface, convection cells, etc., that may violate the assump-tion of heat flow perpendicular to the building envelopecomponent.NOTE 3—Appropriate choice of heat flow sensors and placement ofthose sensors can sometimes provide meaningful results in the presence oflateral heat flow in building components. Metal surfaces and certainconcrete or masonry components may create severe difficulties formeasurement due to lateral heat flow.5.8 Light- to Medium-Weight Construction—This practice islimited to light- to medium-weight construction that has anindoor temperature that varies by less than 3 K. The heaviestconstruction to which this practice applies would weigh 440kg/m2, assuming that the massive elements in building con-struction all have a specific heat of about 0.9 kJ/kg K.Examples of the heaviest construction include: (1) a 390-kg/m2wall with a brick veneer, a layer of insulation, and concreteblocks on the inside layer or (2) a 76-mm (3-in.) concrete slabwith insulated built-up roofing of 240 kg/m2. Insufficientknowledge and experience exists to extend the practice toheavier construction.5.9 Heat Flow Modes—The mode of heat flow is a signifi-cant factor determining R-value in construction that containsair spaces. In horizontal construction, air stratifies or convects,depending on whether heat flow is downwards or upwards. Invertical construction, such as walls with cavities, convectioncells affect determination of R-value significantly. In theseconfigurations, apparent R-value is a function of meantemperature, temperature difference, and location along theheight of the convection cell. Measur