How are the flies getting in my parent's farmhouse?

Could they be coming from the pipe in your sink?

Do they have a full basement, crawl space or slab? Its certainly not the same insect, but underneath our home we have tens of thousands on lady bugs that hatch every year. Some of those get into our home, and we have the same- new roof, windows and siding. Since they're ladybugs, I catch them daily during this time and let them go outside. And again, since they're ladybugs, we've done nothing about it. I know many people may have, but lady bugs are awesome- they keep other pests away from our home and are extremely beneficial to our veggie and flower gardens!

But if it was flies and we had same issue, I'd sure be looking at a natural way to treat our crawlspace so that they wouldn't hatch! If they're hatching under the house they can get in through any tiny crack or space , even through floor ducting. Just a thought.

I know this is stating the obvious, but have they brought in any live plants or flowers, or fresh vegetables or fruits that could be carrying the eggs?

They sound like Cluster Flies. They live under siding or any other covered area and are a great problem. You have to find where they are entering the house. I'd guess they are migrating to the attic, and when the temp is correct, out they come. Close off the entry area with caulk or any other obstructive product. Good luck.

I had a similar problem in a well built, brand-new home BUT our invasion was simply due to new construction going on in the area. Since it was Spring, truckloads of new soil was being delivered to the new house lots being landscaped. The fly larvae was, obviously, in the newly delivered soil as the builder contracted out the landscaping work. Again, the same thing occurred in the Fall. It didn't happn in the Summer as our Summers are quite hot and newly-laid lawns (by the builder) can burn, therefore, the flies only appeared in the Spring and Fall. Good luck in solving the fly problem!

I haven't had any luck with keeping flies out of the house with 3 kitties, 2 toddlers & one loving absent minded 80yr old mother. (The 4 ft sliding patio screen rarely stays closed). 🤔

Enter in my favorite 32oz bottle of all time!! No, not a drink that makes me care less about flies, but this amazing spray! Ultra Shield to the rescue! I purchased my bottle 5 yrs ago for $33.00 + tx. It's a little pricey, but after 5 yrs of spraying the "air" both inside & outside, spraying the pups, underneath gazebo & myself (stops lacewing & mosquito bites) I still have a 1/4 of the bottle! It sprays a very light mist, and a little does ALOT. I love it! (a nice summer day before dicovering spray I "swatted" & killed 237 flies in the house over a 6 hr time frame. I decided then I was going to do battle! After spray I maybe swat 5-6 flies a WEEK in the house). Good luck!