Optimizing performance by continuously insulating and sealing building envelope, composite structural insulated panel (C-SIP) integrates 6 functions — exterior sheathing, superior insulation, air barrier, vapor retarder, moisture barrier, and structural walls — into one product. Wall panels consist of fiberglass reinforced thermoset polymer skins bonded to rigid polyurethane foam and will not delaminate. With R-value of 26, 4 in. foam core will not settle, shrink, or lose insulating value.