Closing the bathroom door while the exhaust fan runs: why a NIST researcher uses the method to pull moisture outside

After a hot shower, many people instinctively leave the bathroom door open, assuming that the extra space will help the room dry faster. But one indoor air quality researcher at the US National Institute of Standards and Technology (NIST) does the opposite. Lisa Ng closes the bathroom door after showering, switches on the exhaust fan and lets it continue running on a timer. Research published in Building Services Engineering Research & Technology has examined how moisture moves from bathrooms into other parts of a home and how extractor fans can reduce that migration.

According to NIST, Ng, who leads the division responsible for indoor air quality research, uses timers that allow her bathroom exhaust fans to run for 15, 30, 45 or 60 minutes. After showering, she closes the bathroom door and leaves the fan running to remove moisture, a practice NIST says can help limit the risk of mildew and mould growth.

Why the bathroom door matters


A shower can quickly fill a small bathroom with water vapour. Warm water releases moisture into the air, which can condense on cooler surfaces such as mirrors, walls, ceilings and windows. If damp conditions persist, they can encourage mould and mildew growth.

An exhaust fan provides a route for this humid air to leave the house, provided it is properly ducted outdoors. When the fan operates, it removes air from the bathroom and draws replacement air through gaps beneath the door or other openings, helping maintain airflow towards the exhaust outlet. Studies of the migration of moisture in houses have indicated that moisture vapour emitted in bathrooms and kitchens tends to move to other areas of the house. This study shows why controlling airflow is important for preventing bathroom moisture from migrating through the house.

The aim is to remove moisture, not just move it

Opening the bathroom door can make the room feel less humid, but the moisture may simply be dispersed into adjoining rooms rather than removed from the home. Keeping the door closed while the exhaust fan runs helps contain the humid air and allows the ventilation system to expel it outdoors.

This is why NIST researcher Lisa Ng keeps her bathroom exhaust fan running after a shower, sometimes using a timer for up to an hour. NIST also notes that bathroom exhaust fans can remove particles and chemicals released during cleaning and personal-care activities. The approach is effective only when the fan is properly installed and vented outdoors. Its airflow capacity must also be suitable for the bathroom. A blocked or poorly ducted fan may move air without effectively removing moisture from the home.

A small habit based on airflow

Ng's approach focuses on removing moisture before it can spread beyond the bathroom. Keeping the door closed while the exhaust fan runs helps contain the humid air and directs it towards the ventilation system, which expels it outdoors. This gives the fan time to clear the moisture produced by a bath or shower rather than allowing it to disperse through adjoining rooms. For homes with a properly installed and functioning fan, running it after bathing can therefore be a simple way to control moisture levels and reduce the risk of excess humidity spreading through the house.

The effectiveness of this method also depends on how the ventilation system is installed and maintained. An exhaust fan should discharge air outside the building rather than into an attic, ceiling cavity or another enclosed space. Dust and debris can also restrict airflow over time, reducing the fan's ability to remove humid air. Regularly checking that the fan is operating properly and that its external vent is unobstructed can help ensure that moisture is actually being removed rather than simply moved around.