Cluster flies become a nuisance when they enter homes seeking shelter from cold weather. They do not complete any of their lifecycle indoors, since they are dependent on earthworms and soil to evolve.
In the spring, cluster flies lay their eggs in soil near earthworms. After they hatch into larvae, or maggots, the flies find earthworms to crawl into, feed on, and complete the larva part of their lifecycle. After this, they enter the soil to pupate, after which they have completed the transition to adulthood. During the summer months, adults feed on fruit juices and flower nectar.
As summer wears on and daylight hours shorten, cluster flies seek protection from the impending cold weather. Colder temperatures cause the flies to crawl upwards, and in this way they enter cracks and openings in buildings. Gaps under eaves and siding are common places for cluster flies to enter. Generally, buildings located next to large lawns or meadows are attractive to these flies, while buildings next to large trees hardly ever have infestations.
Once inside buildings, cluster flies congregate in partitions and then move to heated rooms with the onset of winter temperatures. When inside heated rooms, they fly erratically and aimlessly around lights. When the air is chilly, they gather together in clusters, which is how they were given their name. During the winter, many flies die inside partitions; because of this, they become food for secondary pests including larder beetles and mice.

The best way to control cluster flies is to deny them access to a building. To prevent entry, do the following:

Use yellow, non-attractive insect lights outside at night
All cracks in siding and eaves should be sealed
Windows, chimneys, and trim should be tightly caulked
Windows, Louvers, air vents, etc. should be tightly screened
When doing the above, pay particular attention to the sunny side and upper areas of a building