Minimizing Structural Steel’s Impact On Building Envelope .

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Thermal Bridging Solutions:Minimizing Structural Steel’s Impacton Building Envelope Energy TransferThis document is the product of the joint Structural Engineering Institute (SEI) /AmericanInstitute of Steel Construction (AISC) Thermal Steel Bridging Task Committee, inconjunction with the SEI’s Sustainability Committee’s Thermal Bridging Working Group.More information on the work of the committee and on the topic in general can befound at www.seisustainability.org and www.aisc.org/sustainability respectively.SEI / AISC Thermal Steel Bridging Task Committee MembersJeralee AndersonJames D'Aloisio (Chair)David DeLongRussell Miller-JohnsonKyle OberdorfRaquel RanieriTabitha StineGeoff WeisenbergerUniversity of WashingtonKlepper, Hahn & HyattHalcrow YollesEngineering VenturesKlepper, Hahn & HyattWalter P MooreAmerican Institute of Steel ConstructionAmerican Institute of Steel ConstructionA Supplement to Modern Steel Construction, March 2012

Thermal BridgingReducing energy usage in buildings has becomeone of the most widespread goals in the constructionindustry. Efforts to reduce building energy use aretypically focused on the mechanical, electrical andglazing systems and not the structural system.However, one area where structural designers canreduce energy consumption is thermal bridging. For theprototype 3-story, 9,000 sq ft steel-framed, masonry cladstructure evaluated in this study as much as 4% in annualenergy savings can theoretically be realized if thermalbridging were to be accounted for in the design.Thermal bridging refers to the loss of buildingenergy through thermal conductivity of elements that“bridge” across the insulation of a wall or roof enclosureof a conditioned (i.e., heated or cooled) space whenthe outside temperature is warmer or colder than theinterior space. While all structural framing materialscontribute to thermal bridging, this document will onlyfocus on strategies, solutions and improved details toaddress thermal bridging specific to steel members.Historically SpeakingWhile the amount of energy loss due to thermalbridging may be significant, not many U.S. structuralengineers are currently considering in their buildingdesigns. The lack of thermal bridging considerationsappears to be due to fundamental misconceptions aboutthe level of impact that a structural engineer’s everydaydesign decisions can have on the thermal efficiency of astructure. There are several reasons for this.First, there is the unspoken premise held bymany structural engineers that their sole purpose isto design an economical system to provide for thebuilding's structural integrity. Energy efficiency is seenas the responsibility of others—architects, mechanicalengineers, envelope consultants, energy modelers, andothers who understand thermal issues. The thinking isthat structural integrity, serviceability, and durability arethe areas of focus for the structural engineer.Second, there does not seem to be a compellingargument to do things differently. Do structural detailsreally make a significant difference in the overall energyperformance of a building? How could a thin steel platethat extends through the insulation plane of an exteriorwall cause very much heat loss? Shouldn't there be hardnumbers about how much money will be saved in theoccupants’ utility bills, so that a real-world comparisoncan be done between the savings and the cost ofmodified details using new materials and products toaddress the issue of thermal bridging?Finally, if structural engineers are to move fromthe tried-and-true structural details—half-inch steelplates and angle legs extending out to supportmasonry, continuous steel canopy and balcony beamscantilevering out from the interior structure through2 the building wall (see Figure 1), and steel-to-steelconnections anchoring rooftop grillages down throughthe roof insulation—what are the alternative details thatcan be used with a similar level of confidence? Can theprofession be comfortable with a detail that introducesplastic materials into the compressive stress zone of aconnection, a facade support that uses intermittentlyspaced support elements rather than continuous ones,or a design detail that interrupts the steel structure atthe point of maximum stress by inserting complicated,proprietary manufactured component?Reducing heat flow within the building envelopehas benefits that extend beyond reducing energy use,such as minimizing the potential for condensation onsurfaces. Also, colder interior surfaces can make peoplefeel colder than the ambient air termperature, causingthem to raise the temperature of the room or plugin an electric heater to feel comfortable. These areconsiderations that the Committee will be addressingin the future.Figure 1: Infared scan of structural steel balcony beamcantilevering out from a structure.Moving ForwardThe goal of this document is to begin to address thesequestions with the understanding that a comprehensiveperspective on the issues surrounding thermal bridgingwill take time to evolve. This is a new perspectiveon the evaluation of structural systems which will beguided by developments in other countries, a betterunderstanding of building envelope performance, andthe ever-increasing importance of managing our energyresources. This document discusses approaches toaddress thermal bridging issues in steel-framed structuresthat can currently be evaluated and implemented.A Supplement to Modern Steel Construction

Heat Transfer through Building Envelopes: How It WorksStructural steel elements are integral to the buildingenvelope or building enclosure. Accordingly, designersneed to have a basic knowledge of how envelopeswork, especially the thermal impact of steel elementspenetrating the envelope.Conduction, Convection, RadiationHeat transfer can occur through a building envelopein three ways: conduction, convection, and radiation.Convection is the transport of heat energy in air thatflows through the envelope. This can be a significantsource of building energy loss if the envelope does nothave an effective air barrier system in the envelope.Radiation is responsible for very little heat transferacross the envelope, but radiation on the exteriorsurface of a building in the form of solar gain, or heatloss on a cold, clear night can be very significant. Also,on the inside, warm bodies (such as humans) radiateheat to colder surfaces— such as exterior walls cooledby heat loss through conductive materials.Conduction—the flow of heat through materials—isresponsible for the majority of the heat flow throughalmost all functional building envelopes, and is theprimary concern in terms of thermal bridging. Resistingconductive heat flow is usually accomplished by the useof insulation materials.R-Values and U-FactorsThere are two units for measuring an assembly’s heatflow properties: R-value and U-factor. An assembly’s R-valueis a measure of its resistance to heat flow. The normalconvention in the U.S. is to express the R value per inchof material, with the units hr·ft2·ºF/Btu. It was originallydeveloped to compare different types of thermal insulation,but it has become the generally accepted measure of allmaterials, not just insulation, as well as a metric for completeenvelope assemblies. The inverse of the R-value is theU-factor (U 1/R), which is a measure of the ability of anassembly to transfer heat, expressed in the conventionalU.S. units Btu/hr·ft2 ·ºF. The R-value and U-factor for anassembly depend on the materials contained in theassembly and their geometry. Each material has an intrinsicthermal conductivity, k (Btu/hr·ft·ºF). For some materials,this k value may vary significantly with temperature, butfor most common building materials the properties arerelatively constant for the range of temperatures normallyexperienced by buildings.The use of the R value to “rate” assemblies is morecomplicated and requires consideration of the threedimensional paths that heat can take through theassembly. The commonly used “effective R-value” isan imprecise term that is used differently for differentpurposes. For example, a material with higher thermalmass or capacity (that is, the ability to store thermalenergy), such as brick, may have a low R-value (asmeasured in a steady state condition) but transfers heatat a lower rate when temperatures fluctuate, such asbetween warm days and cold nights.Serial vs. Parallel Conductive Heat PathsConductive heat flow through a building envelopeassembly, such as a wall, can occur either in series orparallel, similar to the flow of electricity.In a series heat path, heat moves progressivelythrough one material, then the next, and so on. Seriesheat flows occur when the building materials are layersin adjacent planes like a sandwich: for example, a wytheof brick, then a layer of rigid insulation, then anotherwythe of block in a wall assembly. For such systems,the total R-value of the assembly can be determinedby simply adding the R-values of the individual layers,times their thickness.A parallel heat path occurs when a plane of materialis interrupted, or “bridged,” by another material thathas different thermal properties. For example, a steelplate passing through a layer of rigid insulation formsa parallel heat path. Parallel heat paths are morecomplicated to evaluate than a series path. In a steadystate system, if the bridging material is well connectedthermally on both sides, the effective R-value of the totalarea can be calculated by tallying the algebraic sums ofthe materials’ areas times their U-factors, divided by thetotal area, and inverting the result. This is the formula:Reff Atotal / [(A1*U1) (A2*U2)]For example, for a 10 foot square area of one inchof expanded polystyrene insulation (say R-4 per inch)bridged (penetrated from one side to the other) by a¼-in.steel plate, 10 ft wide, (R-0.0031 per inch), theeffective R-value of the total wall area would be:(14,400) / [(30) / (0.0031) (14,370) / (4)] 1.1So, within the range of assumptions and limitationsof this formula, the effective R-value for the plane ofinsulation in the wall would drop from R-4 to R-1.1 withthe addition of the steel plate.Quantification of Energy LossAlthough quantitative and easy to use, the formulafor calculating an effective R-value, based on parallelheat paths across an insulation plane, has significantlimitations that make it a poor model for the actual energylost in buildings. The main limitation is the assumptionof fully effective thermal transfer of the materials outsideof the insulation thickness—both interior and exterior. Ahighly conductive material can only transfer heat to itsfull potential if the heat energy can be brought to it onThermal Bridging one side, and can have it all drawn away on the otherside. For this reason, the formula should be interpretedas the “maximum reduction of R-value” only.Knowing the amount of energy that a building losesthrough a thermal steel bridge is important. However,it is difficult to manage what cannot be observed andmeasured. Fortunately, methods exist to help qualifyand quantify the issue.3

Infrared ImagingInfrared (IR) cameras images can provide a quickvisual assessment of the heating or cooling energy lostthrough a building envelope. The cameras detect anddisplay the infrared radiation signatures from surfaces,called thermographs. For materials with an EmissivityFactor (also known as E-Factor, which is the ratio ofabsorbed vs. reflected/transmitted energy) of 1.0, this isdirectly proportional to the surface temperature of theobject. For materials with a factor significantly less than1.0 (such as unpainted metals and glass), the signaturesinclude a portion of IR waves reflected off the surface.Accurate IR building thermography requires certainconditions, such as a significant temperature differentialbetween the inside and outside of a building. Surfacesshould not be recently exposed to sunlight, as thiswill skew the results due to solar radiation heating.The E-Factor of the building materials should beknown so compensations can be made. Since surfacetemperatures can be affected by both heat conductionand convection through the building envelope, thedifference in interior versus exterior air pressure shouldbe taken into account.An IR scan of a completed and occupied building canbe useful feedback on a building’s performance, mainlyto identify problems that should be avoided next time.However building envelope commissioning can includeIR scanning as part of the verification process to ensurethat the envelope was constructed in accordance withthe design. (See Figure 2)Energy ModelingA precise and accurate energy model of a buildingtakes into account the actual three-dimensional detailsof the building, including all the materials’ thermaltransfer properties and thermal mass, and the type andeffectiveness of the air barrier system. It also requires aprecise set of predictions of operational usage, buildingoccupancy, and weather conditions that the buildingwill experience. With such a model, different structuralbuilding envelope details can be modeled and iteratedto optimize their performance characteristics. Althoughprecise models are clearly useful, they require enormouscost and effort.Most building energy modeling assessmentsperformed today consider greatly simplified buildingsystems. They use broad-brush assumptions aboutmechanical systems, occupant usage, climate data, aswell as the overall insulation and air leakage performanceof typical wall, roof, and fenestration assemblies. Thisapproach makes it feasible to perform the modelingwithin the constraints of the design and constructionFigure 2: Brick in contact with warmsteel shelf angles, which stand out in infrared images.project, but it does not directly consider the effect of “hotspots” or discrete conditions—such as steel bridgingdetails—that are more prone to heat transfer. In addition,much of the modeling performed today is done whenthe building design is nearly complete or completed,precluding the model from providing feedback on thestructural design. ASHRAE (American Society of Heating,Refrigeration, and Air Conditioning Engineers) TechnicalCommittee 4.4’s new document 1365-RP, “ThermalPerformance of Building Envelope Details for Mid- andHigh-Rise Buildings” provides thermal performance dataon 40 common steel and concrete building envelopedetails for mid- and high-rise construction.Energy modeling can also be completed on individualsections of buildings, such as windows or other targetedareas, using software programs. These programs model theindividual elements of the building section and calculatethe heat transfer across the section based on their thermalproperties in accordance with the laws of thermodynamics.They can provide a reasonable calculation of an effectiveR-value of a section of a building envelope, especiallywhere a “hot spot” has an effect. This R-value can thenbe used in an overall building energy model. This is theapproach taken in this document to clearly identify areasand alternative approaches to addressing bridging instructural steel details. Typically, the role of the energymodeler does not fall to the structural engineer. Thisresponsibility usually falls on the architect or mechanicalengineer/subcontractor.Thermal Bridging and the CodesBuilding envelope considerations for energy efficiencythat take into account thermal bridging are now beingevaluated for inclusion in codes and standards for bothbaseline and high-performance green buildings. Three4 key publications, the International Green ConstructionCode (IGCC) and ASHRAE 189.1 and 90.1, offer someinsight on how codes may potentially adopt thermalbridging provisions.A Supplement to Modern Steel Construction

International Green Construction Code (IGCC). TheIGCC, a product of the International Code Council,is scheduled for final publication in March 2012.The IGCC states that the “building thermal envelope” shall exceed requirements in the InternationalEnergy Conservation Code (IECC) by 10%. ASHRAE Standards. ASHRAE 189.1, Standard forthe Design of High-Performance, Green BuildingsExcept Low-Rise Residential Buildings, containstables that list maximum U-values for various envelope assemblies and minimum R-values for insulation. ASHRAE 1365-RP, “Thermal Performance ofBuilding Envelope Details for Mid- and High-RiseBuildings” may affect how the ASHRAE standardsaddress the issue and potentially incorporate thresholds. Completed 2011, the project’s objective was toprovide thermal performance data—both indexedsurface temperatures and thermal transmittance—for 40 common building envelope details for midand high-rise construction, using three-dimensionalfinite-element analysis heat-transfer software (thermal transmittance was calculated for clear field,linear and point anomalies). If and how the results ofthis study will result in thermal transmittance requirements being incorporated into future versions ofASHRAE 90.1, Energy Standard for Buildings ExceptLow-Rise Residential Buildings,—and presumably189.1—is yet to be determined.While none of these publications currently incorporatethermal transmittance requirements of steel elementsthat bridge the building envelope, the potential forfuture recommendations and guidance is certainly there,especially as opportunities to increase energy efficiencyin buildings are pursued. In fact, at a recent meetingof the ASHRAE Standing Standards Project Committee90.1 (SSPC 90.1), the Envelope Subcommittee identifiedseveral topics for further consideration and development;among them is thermal bridging. Proposals are expectedto be developed in early 2012.Alternative Materials and Various RestrictionsThe incorporation of alternative steel or non-steelstructural materials can provide benefits in areas wherethermal steel bridging situations are encountered.Stainless SteelStainless steel has a different metallurgical chemistrythan carbon steel resulting in an R-value about threetimes that of carbon steel. Material costs for stainlesssteel are approximately four times that of A992 structuralsteel. However, with limited, strategic use of the material,additional costs can be minimized. The use of stainlesssteel members or stainless steel bolts in areas ofthermal bridging concern can significantly reduce heattransfer. Material and construction costs for the generalcontractor by adding in stainless steel to the connectionassembly can result in incremental upfront constructioncosts to the project. Welding processes need to beappropriately specified for material compatibility and toreduce potential for stress corrosion cracking.Alternative MaterialsThe use of unconventional “alternative” materials suchas fiber reinforced polymers (FRP), or recently introducedproprietary design-delegated elements, comes with issuescommon to new technologies. In many cases, a prescriptivecode-based acceptance procedure is not available forthese technologies and alternative compliance must bedemonstrated. In some cases the material may appear tobe prohibited, such as the use of FRP for masonry support,requiring the use of an alternate analysis to justify use ofa “non-combustible” material. A foam layer in a façadeconnection load path, for example, may be successfullyemployed on the building exterior, by treating the foamas insulation for code life safety requirements. However,because code reports, UL assembly, and loss preventioncriteria are not yet able to address the building solutionsneeded for structural thermal breaks in many cases,special approvals will likely be necessary. Fire protectionanalyses and time-tested technology reporting from othercountries are recommended approaches that have seensome stateside success. Note that the AISC Specificationfor Structural Steel Buildings (ANSI/AISC 360-10) does notcurrently address any non-steel assemblies as they relateto handling thermal bridging conditions.The mechanical properties of the elements usedare an important aspect

Conduction, Convection, Radiation Heat transfer can occur through a building envelope in three ways: conduction, convection, and radiation. Convection is the transport of heat energy in air that flows through the envelope. This can be a significant source of building energy loss if the envelope does not

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