Cluster flies are hell. They breed in damp meadows, preferably with livestock. You could try to reduce the dampness of your meadows by planting birch trees. Next, they congregate on sunny walls in the spring and fall. You could try to discourage this. Lastly, if they are getting inside the house, it is not air tight. We had some luck with spraying the cracks around windows with a bug killer, designed to stick, but ultimately, you want to prevent cracks.

Cluster flies are a huge seasonal problem in our area. Even with spray foam insulation a handful of those nasty critters find their way inside when cool weather approaches. If your infestation is really bad have an exterminator treat the soil around your foundation, since the flies breed in the ground.

I have a new timber frame building that is obviously not air tight. They have moved in in a very big way. It looks like I cannot do much now until the spring when I can spray. Thanks for responding!

On the plus side, they die as soon as it freezes.

I guess it's not quite cold enough in the Kootenays. The few I had last winter seemed to keep moving in. Now they have recently returned with family and friends!