The most common reason for flies swarming all over your house is an infestation inside or nearby your home.
House fly infestation in attic.
Rotting garbage and pet feces can attract flies and cause an infestation.
The one highlighted above is made for fast flying insects.
If this is the case for you your best option to get a good sized bug zapper.
The source is likely inside your house garage attic or garden.
If you suddenly see a swarm of flies that means dozens of eggs have already hatched and developed into flies.
Unfortunately there is little that can be done for flies already inside the attic and walls.
Another thing that can really help with a house fly infestation is to do some serious cleaning indoors and outdoors around your property.
They do not fly around much in attics.
A few sticky traps or launching airborne pesticides may not be your preferred choice.
The most effective way to get rid of attic flies is to make sure that they stay outdoors.
Place traps around attic windows to catch flies.
They enter houses through cracks in the roof or in the walls cracks in door and window jambs and open doors and windows.
Space sprays and fogs into the attic have little if any affect as the flies are usually under insulation or deep in cracks and crevices.
Solutions include disposing of the attractants and using repellents to keep the flies away in the first place.
If you have a large infestation in your attic then you probably need a large solution.