Bathrooms present unique challenges for shoe molding because of constant moisture exposure. Between showers, baths, splashing sinks, and steam, bathroom floors regularly get wet. Standard wood or MDF shoe molding can swell, warp, and eventually rot in this environment if not properly protected — or if the wrong material is chosen.
PVC shoe molding is the clear best choice for bathrooms. It is completely waterproof, will not rot or swell, and does not support mold growth. It can be painted to match your baseboards and will last indefinitely regardless of moisture exposure.
Solid wood shoe molding can be used in bathrooms if it is sealed on all surfaces (including the back and bottom) with a waterproof finish such as exterior-grade polyurethane or a marine-grade sealant. However, even well-sealed wood will eventually fail in a consistently wet environment. PVC is the more practical long-term choice.
Never use MDF shoe molding in a bathroom. MDF absorbs water like a sponge and swells permanently. Even a single mopping can damage MDF trim at floor level.