Abstract: A method of fabricating a polymer coated mat is disclosed to form a moisture barrier. The method includes moving a substrate mat along a processing path and heating the substrate mat to a predetermined temperature. A curtain of molten polymer is extruded through a slot die onto a surface of the heated substrate mat as the mat moves along the processing path to create a composite mat comprising a thin polymer film coating on at least one side of the substrate mat. The composite mat is then cooled in a controlled manner to cure and solidify the polymer film. The resulting composite mat may be used as a construction wrap, or it may be incorporated into other products such as an underlayment. A novel asphalt shingle may be fabricated using the composite mat in lieu of a traditional asphalt saturated mat or other substrate.