A dead-animal smell that stays around one wall, gets stronger on warm days and has no source you can see usually means a small animal has died inside the wall cavity or close to it. This article covers what it is likely to be, how to tell whether it can be reached without cutting anything, and how long the smell tends to last.
What usually causes a dead smell inside a wall?
Most often a mouse or a rat. They are small enough to travel inside wall cavities, between floors and along pipe and wiring runs, so when one dies there, it dies out of sight. A bat is possible in a wall next to an attic or a chimney; read the bat section below before anyone handles one. A squirrel or a raccoon is more likely to be found in the attic, the soffit or the crawl space than sealed inside a finished wall.
Not every rotten smell is an animal. A smell like rotten eggs can be natural gas, which is a call to your gas utility from outside the house, not something to trace. A drain trap that has dried out in a little-used bathroom or a basement floor drain can let sewer gas in. Small flies gathering at one outlet, baseboard or window on that wall are a stronger sign that something has died nearby.
Is there a way in from above, below or beside?
Sometimes. A wall cavity is often open at the top or the bottom. Find the stretch of wall where the smell is strongest, then look at what sits above it, below it and beside it:
- Above: an attic over an outside wall or an upstairs wall, where the top of the cavity, the soffit edge and the insulation can be checked from the attic floor.
- Below: an unfinished basement or a crawl space under that wall, where an animal may have dropped to the sill or the floor beneath.
- Beside: an existing opening, such as an access panel, a vent, the gap behind a dishwasher, range or washer, or an unfinished garage wall.
If one of those leads to the animal, it can usually be taken out without touching a finished surface. If the smell sits in the middle of a finished wall, with a finished ceiling above and a finished room below, the animal is probably sealed in.
Why not open the wall?
Opening the wall is often not the first step, for three reasons. Finding the right stud bay by smell is guesswork: the smell moves along the cavity and comes out at outlets, switch boxes and baseboards away from the animal, so a first hole is easily in the wrong place. Cutting, patching and repainting drywall or plaster is building work rather than carcass removal, and requests passed on through this site do not include cutting into finished walls, ceilings or floors. And the U.S. Centers for Disease Control and Prevention advises against vacuuming or sweeping rodent urine, droppings or nesting material, because doing so can put tiny droplets containing viruses into the air; cutting into a cavity where mice have nested is likely to stir up the same material.
Could it be a bat?
Do not handle it. The Missouri Department of Conservation says all bats are protected by the Wildlife Code of Missouri, and Missouri's endangered-species rule lists four species (the gray, Ozark big-eared, Indiana and northern long-eared bats) as state-endangered. Missouri health officials say rabies in Missouri is found mostly in bats and skunks. The Department of Health and Senior Services (DHSS) advises seeking medical advice if you wake up to find a bat in your room, or if a bat has been in a room alone with a child, or near someone who cannot communicate clearly enough to say whether they were bitten.
If anyone may have had contact with a bat, keep it if that can be done safely, and call your doctor, your local public health agency or DHSS on 573-751-6113 (after hours 800-392-0272). Live bats in a wall, and keeping bats out of a building, are not offered through this site.
How long does the odor tend to hang on?
There is no fixed answer. The odor generally lessens as decomposition runs its course, and how long that takes depends on the size of the animal and on the temperature, humidity and airflow inside the wall. A dry, heated interior wall tends to be at the quicker end. An insulated outside wall, damp masonry, or a bay with a hot-water pipe in it can take longer, and a Missouri summer can make the first stretch worse.
As a rough guide, and these are field patterns, not laboratory figures: with a mouse the worst often passes within one to two weeks, and with a rat it can take two to four, with the first week or so usually the strongest. A smell that is still strong well past that, or that keeps getting worse, points to a larger animal or more than one, and is worth tracing again from the attic or the crawl space.
What helps while you wait?
- Air the room. Open windows when the weather allows and run a fan blowing out of the room rather than into it.
- Use an absorber rather than a masking spray. Open containers of activated charcoal or baking soda near the wall take up some of the odor; a scented spray only makes it harder to tell whether the smell is fading.
- Keep pets away from that wall and from any spot where flies gather.
- If you find droppings or nesting material while checking the attic or crawl space, the CDC's advice is to wear rubber or plastic gloves, spray them with disinfectant and let it soak before wiping up, and not to vacuum or sweep them.
When does it make sense to call?
When the smell leads somewhere that can be reached: an attic floor, a crawl space, a basement sill, a garage, or the space under a deck or porch. Say where the smell is strongest, what is above and below that wall, and how you get into the attic or crawl space. If the animal turns out to be sealed inside a finished wall, waiting for the smell to lessen is often the practical course, and a visit may not change that.
This article is general information about Missouri homes, not medical, legal or building advice. Questions about a possible exposure go to your doctor, your local public health agency or DHSS.