A 150-Bat Colony Evicted, and the Attic Put Back Together

Gradually, and quietly. A maternity colony returns to a roost it has used successfully and grows year on year, and because bats need only about a finger's width to get in, the entry point is usually a gap nobody would look twice at — a soffit return, a ridge junction, lifted flashing, a seam where an addition meets the original roof. Nothing about a colony that size happens overnight. By the time the numbers are obvious from the outside, the colony has typically been established for several seasons.

Because at that colony size the guano and urine accumulation is not a surface problem — it saturates the insulation, ruins its thermal value, and soaks through to the ceiling below. Leaving it means leaving an air-quality problem in a space connected to the home's ventilation, and leaving the residual scent that actively advertises the roost to the next colony passing through. Removing it, sanitizing, and reinsulating is what turns an eviction into a repair.

Not safely, and not legally at the wrong time of year. Wisconsin bats are protected and exclusion has to be timed around the maternity season — sealing a roost while flightless pups are inside kills them in the structure, which is inhumane and leaves you with an odor and insect problem inside a ceiling. Proper eviction uses one-way devices so every animal can leave and none can return, and it happens in the window the law and the biology allow. Source: Wisconsin Department of Natural Resources bat regulations. Reviewed by Brian Zahringer, WI Commercial Applicator #103689 — August 2026.

It is a sequence rather than a visit. Inspection to find every entry point, sealing everything except the active ones, one-way devices in place long enough for the whole colony to leave, monitoring to confirm the attic is empty, then permanent sealing, then the cleanup and reinsulation. On a colony this size the exclusion phase alone runs days, and the restoration work is its own job on top.

Not to a properly sealed building, which is the entire point of doing the exclusion rather than just removing animals. Bats are strongly faithful to a roost that worked, so a house that hosted a colony this large will absolutely be tried again — the seal is what decides the outcome. That is also why the cleanup matters: residual scent is an invitation, and a cleaned, sealed attic is neither attractive nor accessible.