I'm renovating my bathroom. What's the best insulation for walls, especially if they're exterior?

For exterior bathroom walls, fiberglass batts or mineral wool are common choices, offering good R-value and some moisture resistance. Always use a vapor barrier on the warm-in-winter side of the wall assembly to prevent condensation within the cavity, which causes mold and rot. Beyond insulation, ensure you have proper moisture-resistant drywall (like green board or cement board) and a correctly sized exhaust fan vented to the exterior. If you're opening up walls, that's the time to address any existing moisture issues and check for rot. A professional can help size your fan and ensure the vapor barrier is installed correctly, especially in basements where moisture dynamics can be tricky.