It is the dead of winter. The outside temperature is 19 (F). Never the less, I frequently find flies between my interior windows and the exterior storm windows.
I know that they don’t come through any gaps in the interior window because those are sealed and tightly locked. Furthermore, the storm windows, while not sealed like the interior windows, are pretty snug - I can’t find any gaps.
Where do these flies come from?
Maggots. Gross yea, but start cleaning and figure out what’s rotting. I’m guessing they’re not coming from outside, but rather are born and raised in the house somewhere. Start with the kitchen, go through every cabinet, drawer, crack and small spot where something might have fallen and find the source.
Ok, Maybe. But I don’t see how they can get through into that space from the interior. The walls are insulated with a spray-in insulation that is treated with boric acid. Furthermore the interior windows have been smoke-tested and are air-tight.
But here’s the big question I pose in refutation: If the files are breeding inside, why aren’t they elsewhere in the house? They are only* in the windows.
Were the storm windows open during the fall? It’s possible flies might have taken refuge in cracks in the space between the two windows, and have emerged on warm winter days.