A method for stabilizing a slab foundation comprises a narrow, vertical moisture barrier assembly spaced outside and surrounding the perimeter grade beam of the slab foundation by a predetermined distance, and extends to preferably five feet below surface grade. The narrow trench for the barrier is preferably less than three inches wide. A membrane formed of a synthetic composition forms a moisture barrier against one wall of the narrow trench, and is held in place by back fill. The moisture barrier is intended for use in expansive soils that are subject to shrinking and swelling.