MODIFIED ZONE METHOD CALCULATOR

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
clearwall Rvalues for lightgage steelframed walls with insulated
cavities containing 2x4 and 2x6 Cshape steel studs with solid webs. The
Modified Zone Method is recommended for Rvalue 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 parallelpath 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 + z_{f} * (total thickness of all finish material layers on thicker side)where z_{f} 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 finitedifference 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 6in. stud steelframed wall assemblies tested by ORNL Rvalues calculated by the Modified Zone Method were compared with values calculated with most popular calculation methods. The Modified Zone Method gave the most accurate Rvalue predictions. In addition, Rvalue results for 15 metal stud walls tested by Holometrix [Barbour 1994] and Modified Zone Method calculations [Kosny 1995, ASHRAE Research Project 785TRP1996] differ by an average of 6.5 percent. This technique was found to be the simplest and most accurate Rvalue calculation method for calculating clearwall Rvalues for lightgage steel stud walls with insulated cavities.
To estimate the Rvalue for a steelstud wall using the Modified Zone Method fill out the form below. If you want to examine an option that is not available in the predefined lists in this form there is another version of this form that allows you to enter your own dimensions and resistivity values.