Floodlights strung through an occupied attic to illuminate all bat roosting sites may cause bats to leave and seek a new location. Large attics may require four or more 100-watt bulbs. A 150-watt spotlight is more effective. Fluorescent bulbs may also be used to illuminate dark areas. In some situations it is difficult and costly to adequately light a roosting location.
Illumination is cleaner than other methods and safer for both humans and bats.
In open spaces where illumination might not work, the area may be made more undesirable for bats if it is possible to open doors and windows or otherwise create drafts. Three dog whistles attached to a large aquarium pump also had a repellent effect on bats. It is believed that the high-frequency sound waves somehow either interfere with the bats' ability to navigate or otherwise disturbs them. Claims have been made that some variable frequency ultrasonic devices used for rodent control are highly effective against bats. The FWS has initiated research on auditory repellents.