A bat exclusion device is a one-way valve that lets bats leave a building but prevents them from getting back in. It’s the standard method for removing a bat colony from a structure without harming the animals, and in many states, it’s the only legal approach. The devices work by covering the entry points bats use, allowing them to crawl or fly out through a tube or netting flap that they can’t navigate in reverse.
How One-Way Exclusion Devices Work
Bats leave their roost at dusk to feed, then return before dawn. Exclusion devices exploit this nightly departure. When a bat crawls toward its usual exit, it passes through the device and flies away normally. When it returns, the device’s design blocks re-entry: a smooth tube it can’t grip, or a loose flap of netting it can’t push past from the outside.
The two main types are exclusion tubes and exclusion netting, and each suits different situations on a building.
Exclusion Tubes
Tubes work best on rough-surfaced buildings like brick, stone, or log construction, and at corner joints or horizontal surfaces like soffits. The standard tube is 2 inches in diameter and about 10 inches long, made from PVC pipe or flexible plastic tubing. The smooth interior is key: bats can’t cling to it, so once they’ve crawled through and exited, they have no way to climb back in.
The tube should project horizontally or angle slightly downward from the opening, never upward. Only about a quarter inch of the tube should extend inside the wall, so bats can easily find and enter it from within the roost. The outer end gets fitted with a collapsible plastic sleeve that collapses on itself after a bat passes through, sealing off re-entry. You seal any gaps around the tube’s base with caulk so the only way out is through the tube itself.
Empty caulking tubes (with both caps cut off) are a popular DIY option because they’re flexible enough to squeeze into a crevice or have one end cut into flaps that can be stapled or screwed over an opening. If you go this route, clean the tube thoroughly first. Dried caulk creates a rough interior surface that bats could grip to climb back in, defeating the purpose entirely.
Exclusion Netting
Netting works better on smooth exterior walls and attic louvers where tubes don’t sit well. Use lightweight, flexible plastic netting with a mesh size of one-sixth inch or smaller. Secure the netting along the top and both sides of the bat’s entry point, but leave the bottom edge completely unattached. The netting should hang 18 to 24 inches below the lower edge of the opening.
This loose bottom acts as the one-way valve. Bats crawl down behind the netting and drop out from underneath. When they return, the netting lies flat against the wall and they can’t find their way back up behind it to the opening. It’s a simple concept, but the dimensions matter. If the netting doesn’t extend far enough below the hole, bats may be able to crawl back up to the entry point from the outside.
Finding Bat Entry Points
Before installing any exclusion device, you need to identify every opening bats are using to enter the building. Bats can squeeze through gaps as narrow as three-eighths of an inch, roughly the diameter of a dime. That means any gap measuring about a quarter inch by three-quarters of an inch, or any hole five-eighths of an inch wide, is a potential entry for smaller species like little brown bats.
The telltale signs are straightforward. Look for dark, greasy rub marks around openings where bats brush against surfaces as they pass through. Droppings (guano) accumulate below entry and exit holes, and you’ll often see dark staining and spotting on the siding, windows, or ground beneath a roost. Common entry locations include gaps where siding meets edge boards, spaces behind shutters, loose-fitting doors, gaps around windows, and openings where plumbing, wiring, or air conditioning conduits pass through walls.
The most reliable method is to watch the building at dusk. Stand back far enough to see the roofline and upper walls, and note exactly where bats emerge. Do this on two or three evenings to catch all active exits.
The Full Exclusion Process
Exclusion isn’t just about the device itself. It’s a sequence. First, seal every gap and potential entry point on the building except the ones bats are actively using. Hardware cloth with quarter-inch mesh will block bats (and mice) from any sealed opening. Then install one-way devices over the active entry points only.
Leave the devices in place for a minimum of five to seven days. This window ensures the entire colony has had enough nights to exit. Some bats may not leave every night, especially during poor weather, so a full week accounts for stragglers. After that period, remove the devices and permanently seal those final openings with caulk, mesh, or both.
If you seal the active holes before giving the full colony time to exit, bats trapped inside will die in the walls, creating a much worse problem than the one you started with.
Seasonal Restrictions
Exclusion is only legal during certain months because of maternity season, the period when female bats give birth and nurse pups that can’t yet fly. Excluding the mothers during this window traps flightless young inside the structure, killing them.
The exact dates vary by state and climate. In Florida, for example, maternity season runs from April 16 through August 14, and using exclusion devices during that window requires a special permit. The legal exclusion window there is August 15 through April 15. In cooler northern states, maternity season typically starts later in spring and ends earlier in summer, but the principle is the same: check your state wildlife agency’s rules before starting.
Many bat species are under increasing conservation scrutiny. The little brown bat, one of the most common species found in buildings across North America, is currently under federal status review by the U.S. Fish and Wildlife Service due to population declines. State-level protections may add further restrictions beyond the maternity calendar, so verifying local regulations is a necessary first step.
Cleanup and Health Risks After Exclusion
Once the colony is gone and all entry points are sealed, you’re left with whatever guano and urine has accumulated in the attic, walls, or soffit areas. This isn’t just a mess. Bat droppings harbor fungi that cause two infections worth knowing about: histoplasmosis and cryptococcosis. Both spread when contaminated dust is disturbed and inhaled.
Histoplasmosis is the more common concern. Most people who inhale the fungal spores never develop symptoms, but when they do, it presents as a respiratory illness with fever, chest pain, and dry cough. Cryptococcosis is rarer but more serious, potentially causing meningitis, severe headache, fever, and blurred vision. People who are very young, elderly, or have weakened immune systems face the highest risk from either infection.
For small cleanups, the protective gear list includes a respirator fitted with HEPA filter cartridges (not a simple dust mask), nitrile or vinyl gloves, eye protection, disposable coveralls, and shoe coverings. Mist the guano with water before disturbing it to keep dust down. For large accumulations, especially in enclosed attic spaces, professional remediation is a safer choice. Rabies is also a concern with any direct bat contact, so never handle a bat, alive or dead, with bare hands.
Common Mistakes That Lead to Failure
The most frequent error is missing entry points. If you install exclusion devices on two openings but miss a third gap under the fascia board, the colony simply reroutes. A thorough inspection of the entire building envelope, not just the obvious spots, is what separates a successful exclusion from a frustrating cycle of bats reappearing.
Other common problems include using netting with mesh that’s too large (anything bigger than one-sixth inch lets bats push through), positioning tubes so they angle upward (bats can crawl back up), and removing devices too early before the entire colony has exited. Timing the project outside maternity season and giving the devices a full week are the two non-negotiable parts of the process.