Bathroom ventilation, ducted outside — or it is theatre

Size the fan to the room and the bathing load

Capacity should match room volume and how wet the use is — a powder room and a steam-capable primary shower are different loads. More CFM is not always better if the duct cannot deliver it or the noise makes people switch it off.

Watch for: A huge fan on a crushed flexible duct that moves little air and much noise.

Plan a real duct path to outdoors

Short, smooth, correctly sized duct with minimal elbows outperforms long corrugated flex full of dips that pond condensation. Terminate through a wall or roof with a proper cap — not into a soffit that can re-ingest moist air, and not into the attic volume.

Watch for: A contractor who points at an attic and says the house will "breathe it out." That is how sheathing grows mould.

Choose controls people will use

Humidity sensors, timers and separate switching from lights all help the fan run after showers. A fan that only runs while someone stands under the light often stops before the room dries.

Watch for: A single switch for light and fan with no overrun. Convenient on day one; damp on year two.

Install the housing airtight to the ceiling plane

Leakage around the fan body pulls from attics or cavities and dumps dust into the room while reducing effective exhaust. Seal the housing to drywall or use rated airtight assemblies. Keep insulation from blocking the unit per instructions.

Watch for: A fan dropped into an oversized hole with daylight visible from the attic around the box.

Prove airflow at the grille and at the termination

Tissue tests are crude but better than nothing; better still is confirming the exterior damper opens and air actually leaves. Listen for rattling dampers and check that the cap is not painted shut or nest-blocked.

Watch for: A silent exterior damper that never moves because the duct is disconnected in a dark bay.