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t wall R-value(whole-wall R-value),independent of the type of 2001). These standards establish thermal performance v wall system and construction materials. requirements for building envelope components, which are The following list of thermal performance terms were more focused on in-cavity R-value,or nominal R-value of the introduced: insulation.For thermal calculations,the ASHRAE Handbook Center-of-cavityR-value.: Sum of wall material R-values of Fundamentals (ASHRAE 2001) recommends using the calculated at a point in the center of a wall cavity.This R-value parallel-path method for wood framing and the modified-zone doesn't include framing materials. method for steel-frame walls(Kosny, Christian 1995). CEC Clear-wall R-value:R-value for the wall area containing Title 24 thermal requirements for steel-frame wall assemblies only insulation and necessary framing materials for a region are based on the zone method(ASHRAE 2001,Kosny,Chris- with no windows,corners,or connections between other enve- tian 1995A).IECC standard requirements are based mostly on lope elements such as roofs,foundations,and walls. results of ASHRAE or DOE research projects.However,they Framing factor:Framing factor is the ratio of the area of are very often modified as a result of requests from companies all structural members (studs and top and bottom plates or producing different building materials,consulting companies, tracks in case of steel framing)to the total wall area. or trade associations. The common denominator for all Interface details:A set of common structural connections prescriptive thermal requirements coming from ASHRAE, between the exterior wall and other envelope components, IECC, and CEC,is the fact that they all recognize only stud such as wall/wall (corners), wall /roof, wall/floor, window material,stud spacing,and stud depth.This leads to unrealis- header,window sill,door jam,door header,and window jamb, tically low framing factors(9.4%for stud spacing 16-in. o.c. that make up a representative residential wall. or 40-cm,and 6.3%for stud spacing 24-in.o.c.or 61-cm). Whole-wall R-value: R-value estimation for the whole In the case of hot-box tests performed by North American opaque wall including the thermal performance of the "clear labs,the high of the wall assembly is most-often 8-ft(2.44-m). wall"area with insulation and structural elements and typical The natural choice for the width of the wall specimen is 8-ft envelope interface details,including wall/wall(corners),wall (2 44-m.),since it can accommodate both 16-in.and 24-in.stud /roof,walUfloor,wall/door,and wall/window connections. spacing(41-cm.and 60-cm.respectively).The top and bottom In keeping with the data presented by California Energy plates (or tracks in case of steel framing) are part of the test ihmiCommission.and.ASHRAE reports,all wall assemblies in this specimens,which yields framing factors of 14%for stud spac- report have framing factors close to 25%(CEC 2001A,CEC ing 16-in.(40-cm)o.c.and 11%for stud spacing 24-in.(61-cm) 2001B). It is well known that the presence of framing o.c.Figure 1 shows the traditional 8-ft by 8-ft(2.44-m by 2.44- members(like wood or steel profiles)reduces the R-value of m)wall assembly used by ORNL for hot-box testing. a wall system.The measure of this effect is known as the fram- ing effect coefficient,I,ofawall,which is calculated using the During 2001 and 2003,CEC and ASHRAE projects esti- following simple expression that contains clear-wall R-value, mated the framing factor in current low-rise residential build- Rcw and the center-of-cavity R-value, R . ings(CEC 2001B,Carpenter,Schumacher 2003).It was found that in Californian low-rise residential buildings approxi- R w mately 27%of the total wall area is occupied by framing and f= [I - Rn]' 100 (1) the average framing factor in walls nationwide is approxi- mately 25%. This number includes the framing used around The US residential construction market is dominated by windows and doors, structural reinforcement, and corners framing.In the case of wood framing,this means that in Cali- wood-frame construction. Steel framing represents only a very small fraction of that market. However, steel-framed forma,27%of the opaque wall area is made of solid wood. technologies offer many advantages like termite resistance, The Residential Energy Services Network (RESNET), dimensionaLstability,and lightweight construction,and mate- which is widely-used for design and code-approval purposes, rials that can be recycled.The main disadvantage is the high does not address the intense thermal bridging generated by thermal conductivity of steel.According to the American Iron architectural and structural components with increased and Steel Institute, steel-framed home construction has amount of framing members as well as insulation imperfec- increased 300%in the US and Canada since 1998.(AISI] tions (RESNET 2003). Building load calculation programs Clear wall R-value(Kosny, Desjarlais 1994)is the most like Manual J(Rutkowski 2005)don't incorporate these ther- widely-used thermal performance measure of wall assem- mal anomalies. Previous ORNL research demonstrated that blies. Clear-wall R-value can be measured using a hot-box about 10 to 15%of the US residential energy consumption is facility(ASTM 2006),and it represents R-value of the system generated by thermal bridging(about 0.8 Quad a year),which tireof structural,insulating,and finish materials. ASHRAE 90.1 is not normally included in building loads analysis, sizing and 90.2,theInterational Energy Conservation Code(MCC), HVAC equipment, and whole-building energy consumption and Title 24 of the California Energy Commission (CEC) calculations(Kosny,Christian 1995B,Kosny,Syed 2004).In established energy performance standards for buildings this paper,this theoretical gap is addressed for most common (ASHRAE 19989, ASHRAE 1993, IECC 2003, ASHRAE wood and steel-framed wall technologies. 2 Buildings X