There is sewage in the crawl space.

Do not go into it. Stop using water in the house, and switch off the heating and air conditioning if any ductwork runs under the floor. A crawl space has no drain and almost no air movement, so what is under there stays under there, and the air in it is being pulled up into the rooms above whether you can smell it or not.

Why this is worse than the same volume on a floor

Sewage in a kitchen is visible, reachable, and on a surface that can be cleaned. The same amount under a house is none of those things.

• It does not drain. A crawl space is a bowl of soil or plastic sheeting. Water that arrives sits until it is pumped or until it soaks into the ground, and neither of those removes what is in it.
• It does not dry. There is no ventilation to speak of and no sunlight, so the humidity under the floor goes to the ceiling and stays there.
• The house breathes it. Air moves upward through a building, and a meaningful share of the air in your ground floor rooms comes up out of the crawl space. That is the stack effect, and it is the reason a contaminated crawl space is a whole house problem rather than a local one.
• Everything under there is absorbent. Subfloor, joists, insulation between the joists, and any duct insulation. Fiberglass batts that get wet fall down, hold what wet them, and stay wet for months.

In the first hour

1. Stop using water in the building. If the crawl space is filling from a broken drain line or a backup, everything you flush goes under the house.
2. Switch the HVAC off at the thermostat and at the breaker if there is any duct, plenum, or air handler under the floor.
3. Open the access hatch, and open crawl space vents if there are any, so it is venting outdoors rather than upward.
4. Keep children and pets off the floor above if the smell is noticeable in the room.
5. Photograph from the hatch before anyone goes in.
6. Call a plumber and a remediation contractor. Two trades: one to stop the source, one to remove what is under there. Neither does the other’s job.

Working out where it came from

A broken or disconnected drain line under the house. The classic version is a joint that has been dripping for a long time and finally let go. The ground under it is wet in a patch, and it usually smells long before it is found.

A backup that came up through a floor drain or a low fixture and ran under the house. If waste also appeared in a shower or a floor drain inside, that is this. The pattern of affected fixtures tells you where the blockage is.

A failed septic tank or drain field whose effluent is surfacing under the building rather than in the yard. Stop all water use and call a septic contractor.

A sewer line under the slab or the crawl space that has collapsed, which is a camera job to confirm and an excavation to fix.

If you are not sure whether what is under there is sewage or ground water, assume sewage until someone qualified says otherwise. Ground water after heavy rain is a different problem with different insurance consequences, but the safety instruction while you find out is the same.

What a proper cleanup involves

Ask for these specifically, because a cheap version of this job leaves the problem in place:

• Extraction of standing water, pumped to a sanitary sewer or hauled, not spread on the yard.
• Removal of contaminated soil, usually the top layer, and of any vapor barrier that was under the water. Sheeting cannot be cleaned in place.
• Removal of wet insulation between the joists, all of it, not the visibly soiled parts.
• Removal of contaminated duct insulation, and duct replacement where the inside took water.
• Cleaning and disinfection of the structural wood, then drying it with equipment, then a moisture reading before anything is put back.
• A new vapor barrier once the ground is dry and treated.

The order matters. Anyone who wants to put a new barrier down over wet, treated soil is sealing the problem under plastic.

Afterwards

The moisture is the part that outlasts the contamination. Wood that sat in humidity for weeks is the ideal condition for the mold problem that follows, and a crawl space is where it happens unwatched.

Ask for the wood to be metered rather than judged by hand, and ask what the reading was. If the contractor has no meter, they cannot tell you it is dry.

For insurance, the same rule applies as anywhere else in this cluster: the backup itself usually needs the endorsement, and a line that has been leaking under the house for months is exactly the case a gradual damage exclusion is written for.

The short version

Stay out of it. Stop using water. Switch off any ducted system running under the floor and open the hatch so it vents outside rather than up into the house. It will not drain and it will not dry on its own. Insist on soil, barrier, and insulation removal, and on a moisture reading before anything is closed up.