933 The Ashrae Zone Method

The ASHRAE Zone Method should be used for U-factor calculation when highly conductive elements are present in the wall that do not fit the geometry or spacing criteria in the above parallel path correction factor table. The Zone Method, described in the ASHRAE Handbook of Fundamentals, is an empirically derived procedure which has been shown to give reasonably accurate answers for simple wall geometries. It is a structured way to calculate the heat transmission through a wall using both series and parallel paths.

The Zone Method takes its name from the fact that the conductive element within the wall influences the heat transmission of a particular region or "zone." The zone of influence is typically denoted as "Zone A," which experiences a significant amount of lateral conduction. The remaining wall section that remains unaffected is called "Zone B." The width of Zone A, which is denoted "W," can be calculated using the following equation:

= 13.75 inches

Figure 9.7 Zones A and B.

Where "m" is the width of the metal element at its widest point and "d" is the shortest distance from the metal surface to the outside surface of the entire component. If the metal surface is the outside surface of the component, then "d" is given a minimum value of 0.5 (in English units) to account for the air film.

For example a 3-1/2-inch R-11 insulation is installed between 1.25-inch wide metal studs 16 inches on-center. The assembly is sheathed on both sides with 1/2-inch gypsum board. The variable "m" is the widest part of the stud, which is 1.25 inches. The variable "d" is the distance from the metal element to the outer surface of the wall, which in this case, is the thickness of the gypsum wall board, 0.5 inches. The width of Zone A is:

WA = 1.25 + (2 x 0.5) = 2.25 inches The width of Zone B is:

Figure 9.7 shows the boundaries of the zones described above.

Actual step-by-step procedures for performing the calculations for Zone A and Zone B can be found in the

Figure 9.7 Zones A and B.

ASHRAE Handbook of Fundamentals as well as other publications.5

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