Treat the perimeter of your soffit and facia , exterior window frames where it touches the house and every other crack/crevice noticeable entry point. Like stated they are quite small and like flies,stink bugs and other insects most enter in common areas like mentioned. Cluster flies are best treated externally when activity is noticed and in the attic space on and around attic windows. They are photo sensitive/ attracted to light. It may take several seasons to get cluster flies under good control. Foggers are effective in tight space areas not easily accessable.
We generally treat homes/businesses and out buildings in Mid September in Pa. other climates may vary with the best time frame to achieve the best result. A common product that we use commercially is bifethrin a.i./ Talstar One. There are others you may get as a consumer just make sure the product is labeled for the intended specie. Wettable powder pesticides are quite effective for longer control.
Thank you for the well informed reply. Thank you to everyone that took the time to respond. Mr. Jameson I think you nailed our problem. We have a two story house. We don't have attic storage but have kneewall attic storage. My 12 year old daughter went into her kneewall space tonight to retrieve a game and found a huddle of asian beatles on the sunny side of the house that contained 2000 beatles guessing conservatively. I need to treat the soffit in the fall
Thanks to everyone for the advice.