Chapter 31 Key Takeaways: Bathrooms

The Overriding Principle

Inadequate ventilation is the single most common and most destructive bathroom failure. Every other system in this chapter — waterproofing, tile, grout, caulk — is degraded faster by chronically high humidity than by any other factor. Fix ventilation first.

Ventilation

• Minimum sizing: 1 CFM per square foot; 150–200 CFM for a standard bathroom with shower and toilet; 50 CFM for very small powder rooms only.
• Sone rating: Below 1.5 for quiet; Panasonic and similar quality fans achieve 0.3 sones. Budget fans (3–4 sones) are noticeably loud and less reliable.
• Duct termination must be to the exterior — through a roof cap, wall cap, or properly placed soffit exit. Never terminate into the attic or wall cavity.
• Duct must be insulated where it passes through unconditioned attic space to prevent condensation inside the duct.
• Summer humidity rule: In humid climates, opening bathroom windows increases humidity rather than reducing it. Keep the bathroom closed and run the fan.
• Timer switches ($15–$30) ensure fans run long enough after showers without occupant intervention.

Waterproofing

• Tile alone is not waterproof. A separate waterproofing membrane is required in shower enclosures.
• Water-resistant vs. waterproof: Greenboard (water-resistant) is insufficient for shower wall substrate. Use cement board (Hardiebacker, Durock) or integrated waterproofing panels (KERDI-BOARD, Wedi).
• Waterproofing membranes: Sheet membranes (Schluter KERDI) or liquid-applied (RedGard, AquaDefense) go over the substrate, under the tile. This is what prevents water from reaching the framing.
• Shower pan is the most critical area. Traditional mud-set pans use a continuous waterproof liner. Prefabricated pans eliminate liner complexity.

Tile and Grout

• Substrate must be flat (within 1/8" in 10 feet for standard tile) before any tile installation.
• Use polymer-modified thinset for all wet area tile installation. Back-butter large tiles.
• Epoxy grout is worth the extra cost and effort for shower floors and other high-wear wet areas.
• Cement-based grout must be sealed after installation and re-sealed every 1–3 years.

Caulk vs. Grout

• Rule: Changes in plane = caulk. All other joints = grout.
• Caulk locations: All inside corners (floor-to-wall, wall-to-wall), tile-to-tub transition, around fixtures.
• Caulk product: 100% silicone or silicone-latex blend labeled for bathroom use. Color-match to grout.
• Replace caulk every 3–5 years — more often if cracking or mold appears.

Exhaust Fan Replacement: The DIY Project

Replace an exhaust fan by: turning off the circuit, removing the old unit, noting wiring connections, installing the new housing bracket (no attic access required), connecting wiring, connecting the duct (6-inch rigid preferred), installing the motor unit and grille, restoring power, testing airflow.

Cost: $50–$200 for a quality fan. 1–3 hours of work. High return on investment.

Plumbing

• Standard toilet rough-in: 12" (measure yours before buying a replacement).
• Toilet clearance: 15" minimum from center to any side wall (18" recommended), 21" minimum in front.
• Showerhead rough-in height: 80" from floor (standard).
• Pressure-balancing shower valves are required by most codes — they prevent scalding when other fixtures are used.

Renovation Sequence

Demolition → Plumbing rough-in → Electrical rough-in → Exhaust fan duct routing → Inspection → Substrate installation → Waterproofing → Tile → Plumbing trim-out → Electrical trim-out → Vanity, toilet, accessories.

Cost Reference