## 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 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.

Modified Zone Method Calculator - Input Form

Property Dimension
(in)
Material
Notes
Interior
Finish
Layer 1 di1= Layer 1 is the visible layer on the interior surface of the wall. If only one layer is present set material to "none" and thickness to "0.0" for layer 2.
Layer 2 di2=
Stud Flange L = Stud thickness (t) is given as sheet-metal gage number rather than inches. A larger number is a thinner material.
Depth dc=
Thickness t =
Spacing s =
Cavity Insulation dc Cavity thickness is same as stud depth.
Exterior
Finish
Layer 3 de3= Layer 1 is the visible layer on the exterior surface of the wall. If only two layers are present set material to "none" and thickness to "0.0" for layer 3. If only one layer is present set material to "none" and thickness to "0.0" for layers 2 and 3.
Layer 2 de2=
Layer 1 de1=

References:
• ASHRAE 1997 - 1997 ASHRAE Handbook of Fundamentals. Atlanta - pp. 24.11 and 14.12.
• J.Kosny, J.E. Christian - " Reducing the Uncertainties Associated with Using the ASHRAE Zone Method for R-value Calculations of Metal Frame Walls" - ASHRAE Transactions 1995 v.101, Pt 2.
• E. Barbour, J. Goodrow, J. Kosny, J.E. Christian - "Thermal Performance of Steel-Framed Walls" - prepared for The American Iron and Steel Institute by NAHB Research Center, Nov. 21.1994.
• ASHRAE Research Project 785-TRP - " Building Insulation Thermal Anomalies " prepared by Enermodal Engineering Limited and Oak Ridge National Lab. Feb. 1996.