MODIFIED ZONE METHOD CALCULATOR
R-VALUE CALCULATION FOR METAL STUD WALLS WITH INSULATED CAVITIES
The Modified Zone Method was developed in 1994 by Dr. Jan Kosny as a part of the NAHB/ORNL project. It was designed to help in calculations of clear-wall R-values for light-gage steel-framed walls with insulated
cavities containing 2x4 and 2x6 C-shape steel studs with solid webs. The Modified Zone Method is recommended for R-value calculations in steel stud walls by the 1997 ASHRAE Handbook of Fundamentals [ASHRAE 1997].
The Modified Zone Method is similar to the Parallel Path Method and the ASHRAE Zone Method. All three methods are based on parallel-path calculations - the difference being how the metal stud thermal bridge zone of influence (w in the figure below) is estimated. In the Parallel Path Method w is assumed to be equal to the length of the stud flange - L. In the ASHRAE Zone Method w is assumed to be equal to the length of stud flange plus twice the total thickness of all finish material layers on the thicker side (can be exterior or interior). In the Modified Zone Method w is determined by the equation
w = L + zf * (total thickness of all finish material layers on thicker side)where zf is dependent on the ratio between thermal resistivity of finish material and cavity insulation, size (depth) of stud, and thickness of finish material layers.
The accuracy of the Modified Zone Method was verified by detailed finite-difference computer modeling of over 200 metal frame walls with insulated cavities [Kosny 1995]. The Modified Zone Method results were within +/- 2 percent of the detailed computer modeling results for all cases.
For six 3.5 and 6-in. stud steel-framed wall assemblies tested by ORNL R-values calculated by the Modified Zone Method were compared with values calculated with most popular calculation methods. The Modified Zone Method gave the most accurate R-value predictions. In addition, R-value results for 15 metal stud walls tested by Holometrix [Barbour 1994] and Modified Zone Method calculations [Kosny 1995, ASHRAE Research Project 785-TRP-1996] differ by an average of 6.5 percent. This technique was found to be the simplest and most accurate R-value calculation method for calculating clear-wall R-values for light-gage steel stud walls with insulated cavities.
To estimate the R-value for a steel-stud wall using the Modified Zone Method fill out the form below. If you are unsure of dimensions or resistivity values there is another version of this form that allows you to pick from predefined lists.