The present disclosure relates generally to flame retarding building materials and methods for making them. More particularly, the present disclosure relates to flame retarding building materials that have both flame retardant character and desirable water vapor permeability values. In one embodiment, the disclosure provides a flame retardant vapor retarding membranes comprising: a building material substrate sheet having a melt viscosity of about 1 Pa·s to about 100,000 Pa·s at about 300° C. at 1 rad/s; and a polymeric coating layer disposed on the building material substrate layer, wherein the coating layer has a melt viscosity of about 1 Pa·s to about 100,000 Pa·s at about 300° C. at 1 rad/s.