An environmental barrier composition and method of use for forming a structural barrier system to seal structural openings such as doors and windows is provided. The composition includes a liquid rosin that is silyl-terminated polyether, a filler material, and a curing catalyst. The method includes applying the composition to a prepared through-hole in combination with flashing material at least along a bottom surface of the through-hole and installation of a window or door into the prepared through-hole to form a barrier system in which a continuous film can be easily inspected, is waterproof, meet air barrier requirements of ASTM E2178, has elastomeric properties, allows substrate expansion, contraction, and movement, does not shrink or lose volume as it cures, and cures rapidly with sufficient adhesion to transition flashing to incorporate into a designed drainage plane and allow integration of a continuous air barrier system where the barrier is at least semi-vapor-permeable.