What is under your house?
The space under a house holds framing, pipes and soil. Knowing which kind you have decides what moisture does there and what you should check.
Under most raised houses there is a low, unfinished space between the ground and the floor. And it is usually damp more often than people think. That space holds the floor framing, pipes, wiring and sometimes ducts. It is also the part of the house that gets looked at least.
This page prints no prices. It explains what is down there and what to check, so you can decide whether you have a problem.
The three things you may find under a house
- A crawl space: short foundation walls with a low gap above bare soil or a ground cover.
- Piers or posts: the floor rests on columns with open space between them.
- A slab: no gap at all, so there is nothing to crawl under.
If you can open a small hatch or see vents in the foundation wall, you have a crawl space or a pier foundation. The what is a crawl space page covers how it is built, and the pier and beam house page covers the open version.
Why the space under the house gets damp
Soil gives off water vapor all year. In a space with bare dirt, that vapor rises into the air and the wood above. Humid outdoor air, roof and gutter water that runs toward the foundation, and plumbing leaks add to it.
Moisture also travels upward. Air from under the house can reach the rooms above, which is why a musty smell in the living room sometimes starts below the floor. The crawl space moisture page walks through the three sources.
What to look at first
- Whether the ground is bare dirt, plastic sheeting or concrete.
- Dark, soft or fuzzy-looking wood on joists, the subfloor or the sill plate.
- Where downspouts and the yard slope send water.
- Standing water, wet insulation or insulation that has fallen down.
- Pipes that sweat or drip.
A flashlight and a short visit twice a year is enough for a first look. Wear gloves and eye protection, keep clear of wiring, and stay out if there is standing water or you are unsure.
Fix the outside before the inside
Most wet spaces under a house start with water arriving from outside. Check grading and downspouts before you spend effort on anything under the floor. A sealed or covered space still gets wet if the yard keeps feeding it.
Vented, sealed or somewhere in between
Some spaces have open vents. Others are sealed and conditioned. Each approach has trade-offs, and your local code decides what is allowed. The vents guide and the encapsulation explainer compare them without pushing either one.
Take a position: do not add a liner or close vents around a gas appliance until a licensed professional has checked the air supply and flue.
Where to stop and call someone
Sagging floors, soft joists and cracked foundation walls are not do-it-yourself work. A licensed structural engineer can assess them, and an independent one has no repair to sell you. Do not dig beside a foundation you suspect is moving, because that can make it worse. Ask your building department about local rules for venting, ground cover and access.
Frequently asked questions
What is under a house?
Usually a crawl space, piers with open space between them, or a slab. Raised houses have a low gap with framing, pipes and soil.
Why is the space under my house damp?
Soil vapor, humid outdoor air, water from the yard or gutters, and plumbing leaks are the usual sources.
Should I go under my house?
A short look with a flashlight is useful. Wear protection, avoid wiring and stay out if there is standing water or you are unsure.
Can moisture under the house affect the rooms above?
Yes. Air and humidity can move up through gaps in the floor, which is why smells under the house show up inside.
Who should check sagging floors or cracked foundation walls?
A licensed structural engineer. Your building department can tell you which local rules apply.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.