A gypsum board comprises a set gypsum layer having a first face and a second face. An uncoated fibrous mat is affixed to at least one of the faces. The mat comprises a non-woven web bonded together with a resinous binder. The web comprises glass fiber consisting essentially of a major portion composed of chopped continuous glass fibers having an average fiber diameter ranging from about 8 to 25 .mu.m and optionally a minor portion consisting essentially of at least one of small diameter glass fibers having a fiber diameter of at most about 13 .mu.m and microfibers having an average fiber diameter ranging from about 0.05 to about 6.5 .mu.m. The board is exceedingly durable and has a high resistance to water absorption, rendering it particularly useful for exterior insulation systems.