Reducing the uncertainties associated with using the ASHRAE zone method for R-value calculations of metal frame walls

Several authors report the accuracy of the ASHRAE zone method of R-value calculation of metal frame walls as unsatisfactory. A series of more than 1,000 two-dimensional computer simulations was conducted for several metal frame wall configurations. Several wall design parameters, such as stud spacing, stud (depth) size, stud flange size, stud metal thickness, thermal resistance of cavity insulation, and thermal resistance of exterior sheathing, were considered during modeling. This allowed the influence of each above-mentioned designing parameter on metal frame wall thermal performance to be assessed. Wall R-values calculated by the ASHRAE zone method were compared with the results of the computer simulation. The comparison showed that the differences in the thermal calculations are caused by the metal stud zone area estimation. Based on results of finite-difference thermal modeling and regression analysis, a new, more precise technique of estimating zones of thermal anomalies caused by metal studs for metal frame walls was developed. The effects of several wall design parameters were calculated. An improved method of R-value predictions for metal frame walls is proposed.