Walls & Ceilings Architect/October 2007

ASHRAE Parallel Path Flow Method When it’s time to calculate the heat ﬂow of insulated building envelope assemblies, there are three different methods of varying complexity devised by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). These methods can all be found in Chapter 25 of the ASHRAE Handbook of Fundamentals. The ﬁ rst and simplest is the isothermal planes method. This is used when cross-sections have continuous, homogeneous layers. The second method, the parallel path ﬂow method is used when cross-sections have structural and cavity areas and when components have similar thermal resistance. The third method, the modiﬁ ed zone method is used with steel framed assemblies. These assemblies have cross-sections with both structural and cavity areas. Calculating heat ﬂow can be as simple as adding thermal resistance in a system with homogeneous layers, as with the isothermal planes method, or as complex and complicated as the modiﬁed zone method. With the complexity of the modiﬁed zone method, it’s a good idea to use the free online calculator provided by the Oak Ridge National Laboratory (www.ornl.gov/sci/roofs+walls/ calculatores/modzone/index.html). Structural components are highly conductive and create thermal bridges. For example, metals conduct 300 to 1,000 times more heat than most building materials. The thermal impact of a metal stud in a framed cavity is greater than the actual surface area of the stud, so metal has an exaggerated effect on heat transfer out of proportion to its physical size. This stresses the importance of choosing the proper insulation assembly. INSULATION ASSEMBLIES The type of insulation assembly to use depends on the material used for the external walls of the building. External walls are typically concrete block, concrete tilt-up, metal, curtain walls (no cavities) or masonry façade (brick, block or concrete panels with insulateable cavities). Concrete block and tilt-up walls With both types of concrete walls, insulating sheathing can be installed either on the interior or exterior of the concrete, with foam plastic insulation board being the most common choice of sheathing material. The location of the sheathing depends on the regional climate and the sheathing material used. EIFS EIFS refers to the Exterior Insulation and Finishing System, which resembles traditional stucco. When installing an EIFS, the manufacturer’s installation instructions must be followed. EIFS is a highly energy efﬁcient cladding. Steel Stud Cavity Walls The most common wall assembly is the steel stud cavity wall, which includes a masonry façade. To improve the thermal performance and increase cavity condensation control, the designer can specify exterior insulating sheathings which increase cavity surface temperatures and improve energy efﬁ ciency as well. Incorporate exterior air barriers which also function as wind barriers to ASHRAE Isothermal Planes Method 28 | Walls & Ceilings Architect | October 2007

Table of Contents Feed for the Digital Edition of Walls & Ceilings Architect/October 2007

Contents Trade News Silver Certiﬁed Airtight The Men of Steel Thinking Thermal Firestop 101 ICFs Create a Tight Envelope The Finish Line The Green Thumb Cracking the Code

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