A weatherproofing backer for installing windows and doors is described as having a foam layer and an adhesive layer. The adhesive is pressure-activated and is self-sealing about attaching hardware. The foam is self-setting and, in use, molds to the flange of the window unit. When installed, the foam layer is compressed between the inner window frame and rough opening and between the window flange and the wall surface adjacent the rough opening. The weatherproofing backer is designed to extend beyond the flange of the window unit and thereby receive the weatherproofing membrane thereover. The weatherproofing backer, then acts as a sealed bridge between the membrane and the window unit precluding entry of air and water. Both the foam and the adhesive have fire retardants in the formulation thereof. A tape form with a self-skinned foam layer and a sheet form, with polymeric covering web are provided.