Exterior insulating sheathing

An exterior insulating sheathing having a good vapour transmission consisting of a foam polystyrene ply and a second ply of fiberboard, said second ply having at least one side water resistant, said second ply being glued to said polystyrene ply throughout with a continuous layer of glue, so that said continuous layer of glue is sandwiched between said foam polystyrene ply and said second ply, as to continuously fasten said polystyrene ply to said second ply, to form a unitary product defining an exterior insulating sheathing, said exterior insulating sheathing having said at least one water resistant side of said second ply forming one of the exposed side of said exterior insulating sheathing, said exterior insulating sheathing being nailable and mountable on supporting beams and having a racking load according to ASTM E-72 of about 2100.+-.200 lbs, a minimum tensile strength of 65.+-.14 kiloPascal, a linear expansion at 97% relative humidity and 23.degree. C., of less than 0.40%.+-.0.02, a water vapour transmission of at least 96 nonagram per second per meter square (.sup.ng /Pa.s.m.sup.2), and a modulus of rupture of 0.6.+-.0.1 megaPascal (mPa).