Animals in the Walls: Scratching Behind Drywall

What is scratching in the wall?

It can be a bat or a mouse that got in somewhere else. Washtenaw County’s health department describes the route: bats and mice can also come in at the basement or crawl space, and from there may sometimes climb up inside a wall to the attic or into the house, which is why the county says not to ignore a gap or crack anywhere.

The EPA notes that mice nest in warm places such as walls, and the National Park Service lists sounds of gnawing or movement in walls and ceiling voids among the signs of rodents.

The guide to identifying attic animals covers the checks that tell the animals apart, from droppings to the hours each animal is active.

How are animals in a wall dealt with?

From the outside in. The animal in the wall is using an opening somewhere above or below it, so the work starts by finding that opening: along the roofline and soffits, around vents and the chimney, where the roof meets a wall, and at gaps near the foundation.

• Mice and rats. The National Park Service’s rodent exclusion manual says mice squeeze through openings of a quarter inch and rats through half an inch, so every gap that size or larger is closed. Larger holes get wire mesh or galvanized sheet metal before they are caulked, and foundation cracks get concrete or mortar.
• Squirrels. Holes squirrels chew through siding usually have open space behind them, so the manual covers them with thin galvanized sheet metal and paints it; it also notes squirrels can gnaw through aluminum. Ask the operator how they will get the squirrel out before the hole is closed.
• Bats. The U.S. Forest Service’s method is to allow bats to leave the roost, stop them getting back in, and close the openings while the bats are gone. USFWS says exclusion should never happen during the maternity season, typically May 15 to August 15. The bat page has the timing.

Order matters. Closing the entry while an animal is still in the wall can shut it in. The CDC warns that bat-proofing between May and August can trap young bats inside, and the National Park Service’s manual says that where rodent burrows run under a foundation with no other exit, the animals must be removed before exclusion devices go in. Ask the operator what order they will work in for the animal you have.

This site does not arrange for drywall, plaster or a finished ceiling to be opened to get at an animal. Work happens at the entry points, in the attic and in other spaces that can be reached without cutting into the house.

What if the scratching stopped and a smell started?

Then the situation may have changed: the animal may have died somewhere it cannot be reached, which is a different problem from a live one. This site does not arrange for finished walls to be opened.

What can still be done is worth doing. An operator can check the places that can be reached without cutting, such as the attic, a crawl space or the area around the entry point, in case the animal is there. The opening it used still needs to be found and sealed so the same thing does not happen again. This page gives no figure for how long an odor lasts.