Abstract: Disclosed are several embodiments of a water-tight plug which are particularly suitable to close and seal holes drilled through the exterior wall surface of a house or mobile home during installation of insulation materials. The disclosed plugs include fluid-tight sealing means provided adjacent a head of the plug which is sandwiched during plug installation between a surface around the periphery of a hole and the plug head. In addition, the plugs include a projection from the plug head extending into the hole and a means for radially outwardly expanding at least a portion of the projection against the interior periphery of the hole and locking the projection in its radially expanded position to lock the plug in position.