A compression seal assembly that can be installed on a wall without requiring access to both sides of the wall. The compression seal comprises a core member, a split washer, a gasket, and a first nut. The core member includes a cylindrical body and a flange rigidly attached to the cylindrical body. The cylindrical body includes a smooth cylindrical portion separating the flange from a first set of threads. The split washer is helical shaped and has a first end and a second end. The split washer is ramped through the hole until the split washer is entirely on the first side of the wall. A gasket is then inserted onto the smooth cylindrical portion adjacent the second side of the wall, and the first nut is threaded onto the first set of threads. As the first nut is tightened, the split washer, the wall and the gasket are compressed between the flange and first nut.