Maintenance · 5 min

Prevent Mold and Mildew After a Remodel

Mold in a San Diego bath is almost always a ventilation problem or a waterproofing problem, not a cleaning problem. Get the fan, the slope, and the membrane right during the remodel and the post-remodel headaches disappear.

What you'll learn

  • How to size a bathroom exhaust fan for a San Diego bath (CFM by square footage and duct length)
  • The difference between a vented fan and a recirculating fan, and why it matters in a steamy bath
  • The waterproofing layers that go behind the tile in a new shower and what each one does
  • The 20 minute ventilation habit that prevents 90 percent of bathroom mold growth

Step by step

  1. Size the exhaust fan right. A 50 to 80 square foot bath needs a 50 to 80 CFM fan. A 100 to 150 square foot primary bath needs 100 to 150 CFM. For duct runs over 8 feet, add 25 percent to the CFM to cover resistance.
  2. Vent the fan outside. A vented fan ducts through the roof or the side wall. A recirculating fan pushes air through a charcoal filter and back into the room, which does not control moisture. A ducted fan is the only right call for a wet bath.
  3. Add a timer or a humidity sensor. A fan that runs 20 minutes after a shower removes 80 percent of the moisture load. A $40 humidity-sensor switch or a $30 countdown timer covers the case when someone forgets to flip the switch.
  4. Insist on the waterproofing layer. A modern San Diego shower should have a PVC liner, a Schluter Kerdi membrane, or a RedGard liquid membrane behind the tile. Each one is a real waterproofing system. Cement board alone is not waterproof.
  5. Check the slope on the shower floor. A shower pan should slope 1/4 inch per foot to the drain. A pan that holds water at the edges is a sign the slope was missed, and the standing water feeds mold.
  6. Run the fan during the shower and for 20 minutes after. A bath that gets 20 minutes of fan time after every shower stays dry at the grout lines, the curb, and the corners. The single biggest mold prevention habit is also the cheapest.
Safety note

A vented exhaust fan is the single highest-ROI upgrade in a San Diego bath remodel. A $200 to $400 fan with a $30 timer switch prevents the kind of moisture damage that turns a $30,000 remodel into a $50,000 rebuild inside of 8 years.

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