Rats in the Crawl Space: Signs, Safety and Sealing

Get the rats out, seal the vents, doors and drains they used, and then clean up with proper protection, in that order. Look in from the hatch before you crawl in, because bad wiring or a heavy mess changes who should do the work.

A crawl space is a dark, low room under your home that most people rarely open. That makes it easy for rats to settle in and hard for you to spot them until you hear scratching, catch a smell or find damage.

Let's break it down. Here is how each step works, with the safety rules health departments lay out and the vent mesh sizes extension services recommend.

Look for rat runs and footprints in the dust on the crawl-space floor, along with droppings, gnawed wood and nesting debris. NC State Extension's rat guide pictures exactly that: a worn path through a crawl space with prints left in the dust.

Start at the access hatch with a strong flashlight and look before you climb in. Hold the light low and pass the beam across the soil or slab so tracks cast a shadow. Then check along the foundation walls and around pipes and ducts that run up into the floor.

Signs can show up inside the house too. Scratching or scurrying under the floor at night, or a stale, musty odor near floor registers, can point below. If the sounds seem to come from inside walls or overhead instead, the guide to scratching noises in walls and ceilings can help you pin down the source.

A crawl space with a broken vent or a loose door gives rats a sheltered place to nest. NC State Extension lists crawl spaces with missing or badly installed vents and doors among the spots rats use for harborage, next to wood piles and heaps of hay or soil.

Once inside, rats have a quiet room that you seldom visit. Nobody disturbs the nest, and nobody notices fresh gnawing until the damage adds up.

Drain lines are another path. The Department of Energy (DOE) crawl-space guide recommends a backflow valve on crawl-space drains and a check valve on the sump pump discharge line, in part to lower the odds of vermin getting in. Gaps where pipes, wires and ducts pass through the foundation count too, and the guide to rodent entry points in the house covers how to find them around the rest of your home.

It can be. The two big hazards are droppings and urine, which health departments treat as a hantavirus risk in closed spaces, and faulty wiring you may have to crawl right past.

Start with the wiring. The DOE guide says that if faulty wiring creates any chance of shock, you should put off going in until the problem is fixed and the space is safe. From the hatch, look for bare, sagging or damaged wires. If you see any, stay out and call an electrician first.

Then think about how long the space has gone unchecked. The DOE guide warns that old crawl spaces left unattended can hold large amounts of animal droppings, plus the remains of animals that found a way in and died there.

A sharp, rotten smell under the floor can mean a carcass is down there. If the odor seems to come from inside a wall rather than below you, the page on a dead rodent smell in a wall covers that situation.

Air the space out first, put on protective gear, wet the droppings and nests before you touch them, and seal everything in bags. Heavy contamination is a job for a professional.

Open the access door and any vents you can reach, then step away. Chelan-Douglas Health District's hantavirus guidance says a closed space should sit open for at least 30 minutes before you start cleaning. Come back wearing disposable coveralls, goggles without vents, gloves made of rubber, latex or vinyl, and a respirator rated N-95 or P-100.

Leave the shop vac and the broom in the garage, because sweeping or vacuuming a dirt crawl-space floor kicks dust up right at face level. The CDC and Chelan-Douglas both say to wet droppings and nesting material before removal, so soak each pile, pick it up damp and seal it in a plastic bag. Wet the next patch before you crawl onto it.

Know your limit. A few droppings near the hatch are one thing, but thick layers, nests across the floor or a dead animal far from the opening are another. Spokane Regional Health District says heavy contamination in a closed area like a crawl space calls for a professional remediation firm.

Any exposed insulation soiled with urine or droppings has to come out. The CDC says to put contaminated insulation into plastic bags for removal, so plan to replace those sections once the rats are gone.

In many homes, insulation sits between the floor joists right above your head as you crawl, so give it a close look with your light. Tunnels, flattened patches, stained spots and torn pieces pulled down into nests all point to sections that are likely soiled.

Damaged insulation is more than an odor problem. UNL Extension notes that rodent damage to insulation pushes up heating and cooling bills, something few owners see coming. Work in the same gear you use for droppings, bag the soiled pieces as you go, and hold off on new insulation until the openings are sealed.

Cover every vent with heavy galvanized metal mesh that has 1/2-inch openings, and make sure the access door shuts tight. UC IPM names two choices for vent grills: 18-gauge galvanized expanded metal or 16-gauge galvanized wire cloth, both with 1/2-inch (13 mm) mesh.

So why not go finer? NC State Extension's guide on mice says you can fit crawl-space openings with tight doors and commercial screened vents, or cover them with 1/4-inch hardware cloth. But UNL Extension cautions that screen with openings smaller than 1/2 x 1/2 inch will cut airflow a lot. That leaves you weighing tighter screening against ventilation, a fair question for a pro if mice are part of the problem.

Timing matters here. Seal the vents while rats are still inside and you can trap them under the floor, where they may die and leave you with the carcass problem the DOE guide describes. Clear the rats first, then close the vents, and check the access door, foundation cracks and pipe gaps on the same pass.

Call a pro if you find damaged wiring, heavy droppings or nests, a dead animal you cannot reach easily, or rats that return after you seal up. Those are the cases where safety and access make a do-it-yourself fix hard to do well.

A pest pro starts with an inspection of the crawl space and the outside of the foundation to find how rats are getting in, from torn vent screens to gaps around pipes and drains. Next comes trapping to remove the rats already living there, with follow-up checks until the activity stops. Once the space is quiet, the pro seals the openings with materials made for vents, doors and foundation gaps.

Cleanup and insulation removal may be part of the same job, or the pro may point you to a remediation firm for heavy contamination. When you call, ask what the inspection covers, how the company screens vents without choking off airflow, and whether the company is licensed in your state.