Generally, the bathtub drain and overflow pipes require a large hole in the subfloor. Instead of trying to fill the huge opening with sealant, build an insulated box and seal it. The existing floor joists form two sides of the box. Add blocking between the joists to close the other two sides. Make a cover for the bottom with exterior grade plywood. The cover should have two pieces that meet at the drain pipe. Cut a notch in the meeting edge of each piece so it will fit around the pipe.
The Box
The existing floor joists form two sides of the box. Add blocking between the joists to close the other two sides. Make a cover for the bottom with exterior grade plywood or its equivalent. The cover should have two pieces that meet at the drain pipe. First, cut the plywood to the correct size. Locate and drill a hole slight larger in diameter than the drain pipe. Saw the plywood into two pieces by cutting through the hole.
Sealing
Before insulation, run a generous bead of sealant around joint where the framing meets the subfloor and in the corners where the joists meet the blocking.
After insulation, staple gasket to the face of the floor framing that forms the box. Screw the plywood cover over the gasket.
Run another bead of sealant where the drain penetrates the plywood. Or install a flat flexible gasket like the one on the next page.
Option
The box can also be installed before the plumbing is in place. Cut an extra large hole in the subfloor where the bathtub drain will go. Build the box as described above, except cover it with a single piece of plywood. Once the cover is fastened, caulk all the cracks where framing meets from the inside. (The extra large hole in the subfloor allows you to get the caulking gun below floor level.) The plumber can drill a hole for the drain and seal the gap around the pipe.
