Chapter 31 Quiz: Bathrooms — Waterproofing, Tile, Ventilation, and What Goes Wrong
Multiple Choice
1. What is the minimum recommended CFM for an exhaust fan in a standard bathroom (80–100 square feet) with both a shower and a toilet?
A) 50 CFM B) 80 CFM C) 150 CFM D) 300 CFM
Answer: C — 150 CFM (or in the 150–200 CFM range). Using the fixture-based calculation: 50 CFM for the toilet + 50 CFM for the shower = 100 CFM minimum, but sizing for actual air volume (100 sq ft × 1 CFM/sq ft = 100 CFM) plus headroom for real-world performance losses, 150 CFM is the appropriate minimum. Many builders install 50 CFM units — these are significantly inadequate.
2. A bathroom exhaust fan terminates into the soffit. Why is this problematic?
A) It creates too much airflow and depressurizes the house B) Exhaust air exiting near a soffit vent can be drawn back into the attic ventilation system C) It is too loud due to wind interference D) It violates minimum duct length requirements
Answer: B. Soffit venting is sometimes done but requires careful placement far from attic soffit vents. Humid exhaust air drawn back into the attic defeats the purpose of exhaust ventilation and introduces moisture into the attic, causing the same damage as direct attic venting.
3. The statement "tile is not waterproof" means:
A) Tile absorbs water on its surface B) Standard ceramic and porcelain tile, along with permeable grout, allow water to migrate through to the substrate over time C) Tile cannot be used in wet areas without a sealant coating on the face D) Tile requires a minimum thickness to be considered water-resistant
Answer: B. Tile and grout in a standard installation are not the waterproofing layer. Water migrates through grout joints and through the tile adhesive layer to the substrate. A separate waterproofing membrane behind the tile is what prevents moisture from reaching the framing and subfloor.
4. In a tile shower, inside corners (floor-to-wall, wall-to-wall) should be filled with:
A) Non-sanded grout, because sanded grout scratches polished tile B) Sanded grout, because wider joints need aggregate support C) Flexible silicone or latex-silicone caulk in a color matching the grout D) Epoxy grout, because corners are the most vulnerable areas
Answer: C. Inside corners experience movement as different building materials expand and contract at different rates. Rigid grout at these locations cracks predictably. Flexible caulk at changes in plane maintains a watertight seal through this movement.
5. A "pressure-balancing" shower valve protects against:
A) Water hammer in the supply lines B) Scalding when the cold water pressure drops due to another fixture being used C) Leaks at the valve body D) Excess hot water temperature at the water heater
Answer: B. Pressure-balancing valves maintain the hot/cold ratio even when pressure changes occur on the supply side (like when a toilet flushes), preventing temperature spikes. They have been required by most codes since the 1990s.
6. What is the correct substrate for tile in a shower wall application?
A) Standard paper-faced drywall (Type X) B) Moisture-resistant "greenboard" drywall C) Cement board (Hardiebacker, Durock) or a foam tile backer board with integral waterproofing D) OSB plywood with a moisture barrier membrane
Answer: C. Cement board (or comparable products) is the correct substrate for shower wall tile. Greenboard and standard drywall both have gypsum cores that will eventually deteriorate from moisture exposure even if initially water-resistant.
7. The toilet rough-in dimension refers to:
A) The minimum floor space required in front of the toilet B) The distance from the floor drain to the wall stub-out C) The distance from the finished wall to the center of the toilet floor drain D) The height of the toilet seat from the finished floor
Answer: C. The rough-in dimension (most commonly 12", though 10" and 14" exist) is essential to know before buying a replacement toilet. A toilet designed for a 12" rough-in will not fit correctly in a 10" rough-in space.
8. Epoxy grout's main advantage over cement-based sanded grout in a shower application is:
A) It is easier to apply and more forgiving of technique errors B) It is nearly impervious to moisture and staining once cured, requiring far less maintenance C) It is significantly less expensive D) It is the only grout approved for floor applications
Answer: B. Epoxy grout is highly resistant to moisture, staining, and chemical attack once cured. Its main disadvantage is that it is more difficult and time-sensitive to work with. For high-use showers, particularly shower floors, it is the superior long-term choice.
9. Which sone rating represents a noticeably quiet bathroom exhaust fan?
A) 4.0 sones B) 2.5 sones C) 0.5 sones D) All sone ratings are subjectively equal
Answer: C — 0.5 sones. Sone scale is roughly perceptual-linear: 1.0 sone is considered "quiet," and 0.3–0.7 sone fans are essentially inaudible in normal use. A 4.0 sone fan is the type you're aware of when running. Many older builder-grade fans are 3.5–5.0 sones.
10. In a full bathroom renovation, when should the exhaust fan duct be routed to the exterior?
A) After tile installation, when the final layout is confirmed B) Before drywall, while walls and ceiling are open during rough-in C) After the vanity and toilet are installed but before painting D) The duct can be routed at any stage; it doesn't affect sequence
Answer: B. Duct routing is far easier and produces better results when done with walls and ceiling open during rough-in. Routing a duct after walls are closed requires opening them — or making compromises (like improper attic termination) that create long-term problems.
Short Answer
11. Explain why opening bathroom windows in summer often makes humidity problems worse rather than better.
Model answer: Warm summer outdoor air contains significantly more water vapor than the cooler air inside the bathroom or the conditioned space of the house. When warm humid outdoor air enters a cooler bathroom, it cools and its relative humidity rises — sometimes high enough to cause condensation on cold surfaces like tile walls. The correct approach in summer is to keep the bathroom sealed, run the exhaust fan to remove moisture-laden air, and allow the HVAC or a dehumidifier to maintain humidity below 50%.
12. A homeowner finds that tile in their shower sounds hollow in three locations. Explain what this indicates and what the correct next steps are.
Model answer: Hollow-sounding tile (detected by tapping with a knuckle) indicates that the tile has lost its bond to the substrate beneath it — either the thinset never achieved full contact, or moisture infiltration has degraded the bond over time. The correct investigation is to determine whether the substrate behind the hollow tiles is wet or dry. If dry, the tile was simply poorly bonded at installation and can be re-bonded. If wet, there is an active moisture infiltration issue that must be corrected (waterproofing membrane failed, grout cracked) before re-bonding. Hollow tiles should not be left indefinitely — they will eventually crack under foot traffic (floor) or separate from the wall entirely.
13. Describe the correct sequence for a tile shower renovation in a room where the existing shower is being completely demolished.
Model answer: 1) Demolition of existing tile and substrate. 2) Inspect framing for moisture damage or rot — repair as needed. 3) Install cement board on walls and foam tile backer panels on floor. 4) Apply waterproofing membrane (liquid-applied or sheet membrane) over all wet area surfaces, paying special attention to corners and penetrations. Allow to cure. 5) Optional flood test (fill the shower pan and verify no leaks). 6) Install tile using polymer-modified thinset; begin with floor, then walls. 7) Grout after thinset cures (typically 24 hours minimum). 8) Apply caulk at all inside corners and transitions. 9) Seal grout. 10) Install fixtures.
14. A homeowner is choosing between a 50 CFM fan ($40) and a 110 CFM Panasonic fan ($120). Make the case for the higher-cost option.
Model answer: The 110 CFM Panasonic fan provides more than twice the airflow, which means it clears post-shower humidity in under 7 minutes versus 20+ minutes for the 50 CFM unit. In a heavily used bathroom, the 50 CFM fan never catches up between morning showers, meaning the room is persistently humid and moisture accumulates in walls and ceiling. Over 5–10 years, this moisture damage to drywall, framing, and finishes costs far more than the $80 price difference. Additionally, Panasonic's WhisperLine fans are rated at 0.3 sones — essentially silent — versus 3–4 sones for most budget fans. The Panasonic also tends to last 25,000–40,000 hours versus 5,000–10,000 for budget fans, meaning it will likely outlast two or three cheap fan replacements.