Abstract: Building materials of the type containing an impermeable membrane, as for example, an insulation material, or a wall board, or ceiling tile are provided. The impermeable membrane is first formed upon a sheet of paper which gives structural rigidity to the membrane, and is then used as an adhesive to hold the paper onto a porous body by heating the membrane to a softened condition and pressing it against the porous body. The membrane is preferably made in two steps wherein a first application of a thermoplastic latex is put on the paper and then a second application of latex is applied to seal pores in the film produced by the first application of latex. A fire retardant i.e. either a flame retardant salt in the paper or a halogen liberating resin coating on the outside surface is provided to liberate flame extinguishing gases to be held in place by the vapor barrier.