We have a fly problem!!!?

My cabin is in northern Minnesota.When we are not there we keep the thermostat at 45 degrees.when we arrive we turn up thermo to 68. and the flys show up. The cabin is 3 yrs.old and is on a concrete slab.Metal roof and outside walls.well insulated and tight. With outside temps at 20 to 30 below zero I assume the flys have been in the house structure. Where are they coming in at and how do I get rid of them.When we arrive after a couple weeks we are constantly vacuuming dead flys Where are they coming from? I have sprayed when we are leaving and when we get back after a couple weeks the floor upstairs and down are covered with dead flys.HELP