How Do Flies Get Into A Sealed House?

It is not an uncommon thing to see flies in our home when windows have been open and doors used often. In the spring, when letting in a breeze or just airing out the house from a long winter, the occasion fruit flies or adult flies common in our homes will show up. Removing food sources and cleaning up any organic matter that could become breeding grounds is all it takes to prevent this kind of fly infestation.

What’s much stranger is when various fly species show up inside when the window frames are closed tightly, the doors are sealed, and all apparent vents and entryways are in good shape. I grew up in an old house, and every winter, we sealed for drafts, or we would freeze. With no way for air to enter, I also found it odd how common types of flies, which I thought had a short life cycle, pop up in a sealed house in winter.

I’ll explain it all below! 

Key Points:

  • Flies can enter a sealed home through vents, garbage cans, and food items, as well as crawl spaces.
  • Flies can breed quickly and can enter homes through produce bags or other food products and through gaps in attics or basements.
  • Flies can be prevented by sealing entryways, removing fly-attracting sources, and using fly repellents.

How Does a Fly Enter a Home When It is Sealed Tightly?

While it is almost impossible to prevent a few flies from sneaking inside our houses regardless of how it’s closed up when you start to notice several and they seem to persist, you might want to investigate possible causes. Flies can enter the home through normal channels, garbage cans, stagnant water, food items, and even dead animals in crawl spaces. Vents of air conditioning units also let flies in. 

Adult house flies breed fast, and one or two female house flies can lay between 150 to 500 eggs in just one week. If there is good food availability, these eggs will hatch and grow through the life stages in a couple of weeks. Anything from animal feces to food can attract flies that are in your home, and eggs on food waste will have no problem surviving and growing into an infestation.

In cold temperatures, the average house fly can hibernate and prolong their life, allowing them to enter homes in produce bags or other human food products we bring inside. Once inside, the flies wake up, find some food scraps, and breed. It only takes only 8 to 10 days for fly larvae to become adult flies. If your compost pile is near your house for easy access during the winter, house flies can crawl into your closed home through any gap they find. 

Ways That Flies Sneak In

With so many types of flies, one species or another is bound to slip inside. Drain flies in dirty drains and stagnant water, house flies coming back in with garbage cans, or even animal droppings from litter boxes and pet pads. Fruit flies can grab onto produce from markets or survive in the compost bucket in the kitchen for a few days. By understanding all the ways flies get inside your home, you can take steps to keep your house clean and pest-free. 

Entry MethodHow to IdentifyWhat to Do
PetsNotice flies when pets go in and outRemove fly-attracting sources near doorways and window frames
Vents Indoor House flies near dryer vents, kitchen vents, and other compromised ventsCover vents with mesh or screen that can keep house flies out and clean any build-up regularly to avoid ventilation issues
Open Doors and WindowsComfortable conditions indoors bring flies through doors when environmental conditions outdoors are terribleSpray fly sprays around doors for entry to keep them from accumulating in bad conditions
Grocery BagsScattered flies appear after shopping for vegetables and fresh foods Take produce out of bags on the porch and carry it inside without the bags, then clean it immediately 
Potted PlantsFlies appearing after watering plants mean adult fungus gnats are living in your soilUse hydrogen peroxide solution or other soil additives to treat infestation and let the soil dry out 
Basement and Attic GapsHouse flies are common in the attic or basement during cooler monthsSeal all gaps and cracks, then spray fly repellents in areas with low traffic 


When flies grab onto something that is going in and out of our homes at a time when most of the entryways are sealed, they can be said to be hitch-hiking. While it is unlikely that enough flies will come into your home this way to represent an infestation, once in your home, they can hide and present a larger future problem.

Every time an adult female fly sneaks inside and finds a food source for the eggs to be laid on, an infestation becomes possible. Keeping doggie doors and the areas where pets wander in your yard clean and free of filth sources can reduce the number of flies that hitch-hike into your home.


When cool temperatures set in, some types of flies can hibernate and prolong their lives by staying still until warmth and potential food sources arrive. Even if with closed doors, these flies can drop into potted plants, product bags, or compost buckets and either appear dead or remain unseen.

Once these flies warm up, they can find food lay eggs, mate, and do everything needed to grow their population. Eventually, a formation of maggots can form in garbage bins and become common pests even in a sealed home. 


Even if you killed those intrusive flies with a fly swatter, there is a chance they laid eggs, and when they hatch, you may have more flies to deal with than what you killed originally. Any bits of food can become a maggot breeding ground and lead to flies popping up in what seemed like a completely sealed house. Keeping the house clean and getting rid of daily food waste and other organic waste can remove places where flies can lay eggs and hatching maggots can feast. 

Reasons For Flies Inside a Home

There are countless reasons flies will come into a home, including trash and strong-smelling food. Built-up food in kitchens and garages, anywhere where household waste is held between trash days, are all places flies will try to get to and thrive. Removing caked-on food and keeping garbage areas clean, especially during cooler months, can help keep errant fly populations at bay. 

Outdoor flies often sneak indoors when the temperatures drop to try and get some warmth. When we open outdoors for pets or to carry things inside, the instant change in air temperature is a huge magnet for any cold flies in the area. If you will be opening doors and windows in winter, make sure screens are intact and try to spray a repellent like cinnamon to block any nearby pests. 

Fungus gnats are a common occurrence in sealed houses with houseplants. During the winter, plants use less water, and unless you water less, you could invite fungus flies. These bugs eat roots and live in the potting soil, and will come out to mate. If you don’t take care of your plants, these flies will become a mainstay and constant annoyance. 

Fruit flies can build up in baskets with old fruit or in bins that have food scraps. If they can breed and are protected from the elements, a group of these pests can hang around your kitchen for a few days. Once the food waste is removed and the rotten fruit disposed of, the remaining flies should disappear pretty quickly. 

Pests like rodents and birds that sometimes break into and nest in homes can also invite flies into the interiors of our houses. When these pests bring food or leave waste, flies can use these spots as a breeding ground and allow flies to come into rooms from odd places. When cracks in the interior walls or gaps in the floorboards seem to be the source of flies, chances are there is a deeper pest problem that needs to be addressed.

Steps to Eliminating Flies? 

Fly on White Counter

Getting rid of flies is easy once you know why they are coming in. Flies are not difficult to kill, and getting rid of the sources of fly infestations generally only requires a thorough cleaning. In some ways, flies are a good indicator that an area needs cleaning and can even indicate worse problems like rodent infestation in walls before they become very expensive issues. Once flies are found, you can get rid of them with these methods. 

Seal Fly Entry Points

Reducing the places flies can enter either by calling or being carried inside, can stop large infestations from entering your home. Before the weather gets bad, check around your home for gaps and cracks that insects can enter. Anything you seal now will keep out flies or worse and can make it easier to discover how other flies are getting in.

Vent covers and window screens also wear out over time, so checking these at least once a year can help you catch these breach points early. Once your perimeter is secured, you can focus on removing what brings flies around. 

Remove Waste and Fly Attractants 

Pet feces around the yard, as well as trash and food waste, can contribute to flies. If you clean up waste like this or at least store it further from doors and windows, you can decrease pests hanging around. It can be hard to keep on top of some waste during the winter, and in order to prevent pests, you will need a method of cleaning that works in any weather. Keep trash in clean bins with tight-fitting lids and bags, and remove pest waste immediately. 

Reduce Damp Areas Indoors

Water attracts pests, and flies love a reliable source that they can count on. If you notice any standing water, leak stains, or other indicators of water problems, you will want to fix them to prevent pests. Leaking pipes can be tightened, taped, or replaced, and stained drywall can be replaced if needed.

All stagnant water and grime, and stains need to be removed, or flies will still accumulate. Clogged drains also invite cluster flies that can quickly populate a damp room and lead to an unpleasant situation. 

Spray Repellents 

Essential oils sprayed near openings where flies hang out are a great way to mask food smells that would typically bring these pests.  Smells like cinnamon and peppermint can keep flies from exploring your home and give you time to open doors and carry bags inside without being followed by pests.

Areas that are hard to reach and keep spotless can also be sprayed with repellents that can keep Housefly breeding grounds from forming. If maggots and life are already present in any hard-to-reach places, apply a pyrethrin-based spray that will kill all insect pests immediately. 

Fly Traps

If a few flies get inside and swatting isn’t your forte, you can set up a few traps to help catch the pesky buggers. Kitchens, garages, and patios are common places flies will hang out, and a trap or two can go a long way in passively handling your problem. Some traps catch flies with liquid or sticky solutions and can be replaced frequently and easily. These are usually cheap and effective methods. 

Zappers that hang and can kill pests are great as they run 24/7 and don’t need to be replaced nearly as often as other traps. As these zappers fill up with flies and other insects, you can empty the tray to keep it from bringing a pest problem of its own. Zappers are good for garages and places where beneficial insects are not common, but insect pests are.