The present invention provides polymer modified aqueous urea formaldehyde resin (UF resin) binder compositions useful in making a treated glass mat, e.g., for roofing shingles, wherein the polymer modifier is an multistage aqueous emulsion acorn copolymer comprising one protuberant polymer stage containing phosphorous acid groups and one or more other polymer stage comprising an addition copolymer incompatible with the protuberant polymer stage, wherein the multistage aqueous emulsion copolymer has a measured Tg of from −60 to 25° C., or, preferably from −30 to 12° C. and, further wherein the weight ratio of the total of monomers used to make the one or more other polymer stage to the total amount of monomers used to make the protuberant polymer stage ranges from 3:1 to 50:1, or, preferably, from 3:1 to 30:1 or, more preferably, from 3:1 to 20:1, or, even more preferably, from 8:1 to 12:1.