This research program evaluated the flexural capacity of a metal building wall system with rigid board foam insulation sandwiched between C- and Z-section girts and through-fastened steel panels. Vacuum box tests were conducted to simulate wind suction on the wall system, and distinct failure modes were observed. The metal panel pulled over the screw heads for wall systems without insulation, however when rigid board insulation was added, the insulation acted as a washer and girt failure or screw fracture was observed. Screw bending and fracture were common in the specimens with the thickest rigid board insulation and for locally stocky cross-sections because a concentrated moment could be developed in the fastener. In these cases wall system capacity was decreased by the presence of rigid board insulation. For wall system specimens with locally slender girts, rigid board insulation did not influence girt capacity because girt deformation under load was primarily in the cross-section and not at the through-fastened connections.