Droppings along the wall behind the furnace. A chewed box of old clothes in the storage corner. A faint musky smell when you go down to do laundry. Mice in basement areas and crawl spaces often go unnoticed for weeks, because these are the parts of the house people visit least, and that is exactly why mice like them.
This guide covers why mice settle in basements and crawl spaces, the signs to look for, where they get in, and a step-by-step plan to clear them out and keep them out.
The Short Answer
Seal the gaps along the foundation, around pipes, vents, and drains, and at the crawl space entry. Fix moisture problems, swap cardboard for sealed plastic bins, and trap any mice already inside. Clean up droppings the CDC way, never by sweeping. A scent repellent can add a layer near sealed entry points and storage, but it does not replace sealing.
Why Mice Choose Basements and Crawl Spaces
According to the National Pest Management Association (NPMA), house mice prefer dark, secluded areas, and messy or unorganized basements offer places to hide and breed. In crawl spaces, the NPMA notes that stored boxes, clothing, and packing materials give mice nesting material, and that moisture draws pests in.
Basements also sit right at ground level, next to the foundation, where many pipes and lines enter the house. The University of California Integrated Pest Management Program (UC IPM) notes that house mice often enter homes in autumn as nighttime temperatures drop, so late fall and winter are when you are most likely to notice them.
Signs of Mice in Basement Spaces
- Droppings about 1/8 to 1/4 inch long, rod-shaped with pointed ends, along walls and behind appliances
- Gnaw marks, and chewed boxes or packaging
- Greasy rub marks along walls where mice travel the same routes
- Shredded insulation or nests in stored items
- A musky or urine odor, which UC IPM and the NPMA link to larger infestations
- Scratching or strange noises, which the NPMA flags especially in quiet areas like basements and attics
Not sure what you found? Our guide on what mouse droppings look like can help.
Where Mice Get Into Basements
According to the CDC, mice can fit through a hole the width of a pencil, about 1/4 inch. The CDC lists these spots to inspect, and most of them are in or next to a basement:
- The gap between the foundation and the ground, and crawl space vents
- Holes where electrical, plumbing, cable, and gas lines enter
- Pipes near washing machines, water heaters, and furnaces
- Floor drains, floor air vents, and dryer vents
- Where the floor meets the wall
The NPMA adds loose mortar and worn weather stripping around basement foundations and windows. For a whole-house view, see how mice get into your house.
How to Keep Mice Out of Basement Spaces: Step by Step
Step 1: Inspect With a Flashlight
Walk the outside of the foundation first, then the inside. Check every spot on the list above, plus window wells and the crawl space entry. Note droppings and rub marks, since they usually point toward the route mice are using.
Step 2: Seal the Gaps
The CDC recommends filling small holes with steel wool held in place with caulk or spray foam, and using hardware cloth, lath screen, metal sheeting, or cement for larger ones. UC IPM recommends sealing foundation cracks and openings around pipes, vents, and utility cables with metal or concrete, and warns that rubber, vinyl, foam, and wood are gnawable and do not work on their own. Replace loose mortar and worn weather stripping, as the NPMA suggests, and consider covers for window wells.
Step 3: Close Off the Crawl Space
The NPMA recommends properly sealing the crawl space entry and regularly inspecting the space for signs of pests. Screen crawl space vents with hardware cloth so air still moves but mice cannot get through.
Step 4: Fix Moisture
The NPMA recommends keeping basements and crawl spaces well ventilated and dry, running a dehumidifier, fixing leaking pipes and clogged drains, and making sure rainwater drains away from the house.
Step 5: Change How You Store Things
The NPMA recommends plastic bins instead of cardboard boxes in basements, and sealed plastic containers for anything stored in a crawl space. Declutter, keep items off the floor where you can, and keep pet food and bulk pantry items in sealed containers.
Step 6: Trap the Mice Already Inside
The CDC recommends snap traps, advises against glue traps and live traps, and lists basements and crawl spaces among the places to set them. UC IPM suggests placing traps close to walls and about 10 feet apart, since house mice seldom go more than 30 feet from their nest. Keep traps out of reach of children and pets, and keep going until you stop catching mice.
Step 7: Clean Up Safely
The CDC advises against sweeping or vacuuming droppings or nests. Wear rubber or plastic gloves, soak the material with a bleach solution (1.5 cups of household bleach per gallon of water) or a disinfectant for 5 minutes, and wipe it up with paper towels.
A Word on Mothballs in Crawl Spaces
Mothballs come up often for crawl spaces. They are a registered pesticide, and EPA notes that registered mothball labels specify the use site as "air-tight containers and storage closets," and that using a pesticide in a manner inconsistent with its labeling is generally unlawful. Tossing them into a crawl space to repel mice is not what those labels describe.
When to Call a Professional
Call a licensed pest control professional if you find droppings on more than one floor, if activity continues after sealing and trapping, if the crawl space is hard or unsafe to get into, or if you find a heavy infestation. If you hear activity upstairs too, read our guide on mice in the attic.
Cheat Sheet
- Basements and crawl spaces are dark, quiet, and close to entry points, which is why mice like them.
- Seal gaps of 1/4 inch or more around pipes, drains, vents, and the foundation.
- Use steel wool plus caulk, or metal, hardware cloth, and concrete. Skip foam and wood alone.
- Screen crawl space vents and seal the crawl space entry.
- Run a dehumidifier and fix leaks.
- Store items in sealed plastic bins, not cardboard.
- Snap traps along walls. Never sweep or vacuum droppings.
Where BioGuard Fits
We sell BioGuard Natural Rodent Repellent, a pack of 12 scent balls infused with peppermint and cinnamon essential oils. In a basement, it fits near sealed entry points, in storage areas, and near the crawl space entry. It is a repellent, not a removal product, and it works best after sealing and cleanup. Replace it every 4 to 6 weeks, keep it away from children, pets, and food, and expect results to vary.
Frequently Asked Questions
Why do I have mice in my basement?
Basements are dark, quiet, and full of pipes and lines that enter through the foundation. Clutter, cardboard, and moisture make them even more appealing.
How do I keep mice out of my crawl space?
Seal the crawl space entry, screen the vents, store anything kept there in sealed plastic containers, and inspect it regularly, as the NPMA recommends.
Does a dehumidifier help with mice?
It helps with one part of the problem. The NPMA recommends keeping basements dry because moisture attracts pests, but a dry basement with open gaps will still let mice in.
Can mice get from the basement to the rest of the house?
Yes. Gaps around pipes, wires, and vents that run up through the house can carry mice to other floors, so seal those openings in the basement ceiling and walls too.
Sources
- CDC, How to Seal Up to Prevent Rodents
- CDC, How to Trap Up to Control Rodents
- CDC, How to Clean Up After Rodents
- UC IPM, Pest Notes: House Mouse
- NPMA, Pests in the Basement
- NPMA, Bugs and Mice in Crawl Space
- NPMA, 10 Easy Tips to Prevent Mice and Rodents Inside the Home
- NPMA, Rodent Prevention Tips for Fall
- U.S. EPA, Pesticide Labeling Questions and Answers (mothball use sites)