Composite Wall R-Values | EGEE 102: Energy Conservation and ..The R-value of a composite wall is calculated by adding the effective R-values of each of the layers of the wall. . four layers—? inch drywall inside for aesthetic purposes, real insulation in between the studs, ? inch plywood sheathing outside, and wood siding as the final exterior finish. . Polyurethane Board per inch, 6.25.