Aqueous binder compositions for fiber mats include a urea-formaldehyde resin and an aqueous emulsion of a copolymer prepared by polymerization of a monomer mixture including styrene, an alkyl(meth)acrylate, acrylonitrile, and an unsubstituted or substituted acrylamide. Even when used in relatively low concentration, the copolymer provides improved physical properties to the fiber mat. The fiber mats can be used in composite flooring, asphalt roofing shingles, siding, and dry wall.