Bugs get into your house through gaps and cracks that are surprisingly small. A space just 1/16 of an inch wide, thinner than a coin, is enough for insects and spiders to squeeze through. Most homes have dozens of these openings around doors, windows, pipes, vents, and foundation walls, and each one is a potential entrance for pests looking for food, moisture, or shelter.
The Most Common Entry Points
Every home has structural gaps, and bugs are remarkably good at finding them. The most frequent entry points include cracks around windows and doors, gaps where pipes and wires penetrate walls, spaces under exterior doors, torn or ill-fitting window screens, and openings around chimneys. Foundation cracks are another major route, especially for ants, cockroaches, and spiders that live close to the ground.
Where utility lines enter your home, there’s almost always a gap around the penetration point. Electricians, plumbers, and cable installers focus on getting the line functional, not on sealing it tightly against pests. These openings tend to hide behind appliances, inside cabinets, or in corners nobody checks regularly. Insects use the lines themselves as guide routes, traveling along pipes and conduits in and out of your walls without detection.
Vents are another overlooked pathway. Dryer vents, bathroom exhaust fans, attic ventilation systems, and ridge vents can all be exploited. The problem compounds because these vents often lead directly into wall cavities, crawlspaces, or attics, where pests can establish themselves and go unnoticed for weeks or months.
Some bugs bypass the walls entirely. American cockroaches crawl up through sewers and drains. House centipedes enter through drains and sump pumps. Sliding glass doors on slab foundations are particularly vulnerable, creating ground-level gaps that let in sowbugs, earwigs, millipedes, centipedes, and springtails from the damp soil right outside.
What Draws Bugs Inside
Gaps alone don’t explain why bugs come in. Something has to attract them. The three biggest draws are moisture, food, and warmth.
Moisture is the dominant factor for many species. Certain arthropods like sowbugs, pillbugs, millipedes, and centipedes physically cannot survive in dry environments because they lack the waxy coating that protects most insects from drying out. They’re confined to damp areas: under leaf piles, mulch, wood chips, and in cracked, moist soil. When the soil near your foundation stays shaded and damp (which it often does, thanks to landscaping and fertilizer), these creatures are already living inches from your walls. Heavy rain or drought can push them through the nearest crack and into your basement or garage.
Other pests, like springtails and certain small beetles, are drawn to indoor dampness because it supports mold growth. Mold is their food source. Wall voids, basement corners, and crawlspaces that stay humid for long periods become feeding grounds. The bugs aren’t random invaders. They’re following the moisture to the food.
Ants are a different story. Pavement ants enter through foundation cracks specifically in search of food. A few scouts find crumbs or grease residue in your kitchen, lay a chemical trail back outside, and soon a stream of workers follows. Cockroaches are similarly motivated by food scraps, pet food, and organic waste, though they also seek warmth during cooler months.
Millipedes present an interesting case: they often enter accidentally. When leaf litter or mulch accumulates against doors and windows, millipedes living in that material wander indoors without any real benefit to themselves. They typically die within a day or two from the dry indoor air.
Hitchhikers and Stowaways
Not every bug crawls in through a crack. Some are carried in. Bed bugs are the classic hitchhiker, traveling on luggage, used furniture, clothing, and boxes. Once inside, they spread room to room through wiring conduits and pipe chases in the walls. Pantry moths and beetles arrive in dry goods like flour, rice, and birdseed purchased from the store. Spiders and their egg sacs ride in on firewood, potted plants, and cardboard boxes stored in garages or sheds.
Seasonal Patterns That Increase Entry
Bug pressure isn’t constant throughout the year. In spring and summer, ant colonies expand and send foragers farther from the nest, increasing the odds they’ll find a gap in your foundation. Summer heat drives moisture-dependent creatures toward cooler, damper indoor spaces. In fall, insects like stink bugs, lady beetles, and cluster flies seek sheltered spots to overwinter, often packing into attic spaces and wall voids by the hundreds through gaps around rooflines and soffits.
Heavy rain events cause sudden surges. When the ground becomes saturated, millipedes, centipedes, and earwigs flee upward, and the nearest dry space is often your basement. Drought has a similar effect, pushing bugs toward any available moisture source, including your plumbing fixtures and damp crawlspace.
How to Seal Them Out
The most effective long-term approach is exclusion: physically blocking the gaps bugs use. This works better than spraying pesticides because it addresses the root problem.
For small cracks and gaps (under two inches), silicone caulk is the standard material. It stays flexible, resists weather, and adheres well to most surfaces. For gaps larger than two inches, stuff copper mesh into the opening first, then seal over it with caulk. Copper mesh won’t rust the way steel wool does, and insects can’t chew through it. Expanding foam works for larger voids but should be the type designed for pest exclusion, as standard foam can be penetrated by determined rodents.
Focus your sealing efforts on these priority areas:
- Exterior doors: Install door sweeps or rubber seals along the bottom and sides to create a snug barrier against the threshold.
- Windows: Replace torn screens and caulk any gaps between the frame and the wall.
- Utility penetrations: Check every spot where a pipe, wire, or cable enters the house, including behind the washing machine, under sinks, and around the water heater.
- Vents: Cover dryer vents, attic vents, and chimney flues with metal mesh that allows airflow but blocks entry.
- Foundation cracks: Inspect the exterior foundation wall at ground level and seal any visible cracks with caulk.
Reducing What Attracts Them
Sealing gaps is half the equation. The other half is making your home less appealing. Pull mulch, leaf litter, and wood chips at least 12 inches away from your foundation walls. That shaded, moist soil right against the house is prime habitat for the very creatures that end up inside. Store firewood well away from the house and elevate it off the ground.
Indoors, fix leaky pipes and faucets promptly. Use a dehumidifier in basements and crawlspaces to keep humidity below 60%. Clean up food crumbs and grease, store dry goods in sealed containers, and don’t leave pet food sitting out overnight. These steps won’t make your home bug-proof on their own, but combined with good sealing, they remove the two things most bugs are chasing: water and food.