Flies in a V cold December in new renovation

I have a query about flies that we have in our house.Its a newly renovated farm house out in the coutryside.No real immediate farming activity around the house.We have a bunch of mature tree's near the house alright which might be causing it.Its start of December and we are in the middle of a big freeze with bitter cold temperatures outside so i thought that would see the end of ouir fly problem but to my amazment we still have lots of new dead flies every day on our windows sils.The flies only seem to be on the upstairs of our house.We have it well insulated it and i was envolved i managing the renovation so i went around each evening filling all gaps with expandable foam to make sure were well insulated.Unfortunately we have not got around to sealing fully all the outside of the house aroud pipes so this might be an entry point but i cant understand what they are living on as a food source......
In start of October for two weeks they were really really badflies everywhere but thankfully now its not so bad.
Any help would be great to try and remove our fly problem...