This invention relates to a low fiber, plyable facer suitable for use in the construction industry, particularly for insulation board manufacture, comprising a dry preformed fiber mat containing a binder for the fibers, preferably a preformed glass mat, coated with a prefoamed composition which contains a thixotropic polymer latex, a foam sustaining amount of a surfactant and a flame retarding and/or strengthening amount of a mineral filler and also to the use and process for the preparation of the above as well as to a siding underlayment or insulation board having a foamed, thermosetting resin core which is surfaced with said facer as a product for commercial use.