Dirt crawl spaces
Bare soil under a house is a steady source of moisture. Understanding that tells you what a ground cover can and cannot do.
A dirt crawl space is one with bare soil for a floor. And that soil releases water vapor into the space every day of the year. That vapor reaches the wood above, the insulation and sometimes the rooms upstairs. It is the most common reason these spaces smell musty.
This page prints no prices. It explains how a dirt floor behaves and what the options are, so you can read a proposal with some confidence.
Why bare soil is a moisture source
Even soil that looks dry holds water. Vapor moves out of it and rises, and it condenses on cool surfaces such as ducts, pipes and the underside of the floor. Humid summer air that enters through vents adds to the load.
That is why damp insulation, a musty smell and dark wood so often turn up in the same house. The crawl space moisture page walks through the three sources.
What a ground cover does
A heavy plastic sheet laid over the soil cuts most of the vapor that rises from it. That is its whole job. It does not stop liquid water from outside, and it does not dry out wood that is already wet.
- Seams overlap and are sealed, or vapor finds the gaps.
- The sheet runs up the walls and is fastened there.
- Gaps around piers and pipes are sealed too.
A loose sheet with open edges helps a little. A sealed one helps much more. The encapsulation explainer compares the two.
Vents, dirt and humid air
Open vents let outdoor air in, and in humid weather that air is wetter than the space. Some homes do better with the vents sealed, others with them left alone. Local code often decides. Read crawl space vents: open or closed before changing them.
Water comes first, cover second
If water collects on the soil, a cover will not fix that. Sort out the yard first: grading, gutters and downspouts. The standing water in a crawl space page covers that step. Do not spend effort on a liner until the space stays dry after rain.
Is a concrete floor better than dirt?
A poured floor blocks more vapor and gives you a clean surface, but concrete is porous and it is not a finished fix. The concrete crawl space page explains what a slab does and does not do. Take a position: for most damp dirt spaces, a sealed ground cover is the sensible first step, once the water source is handled.
Where to stop and call someone
Soft or sagging floors, cracked foundation walls and gas appliances in the space call for a licensed professional. A licensed structural engineer can assess framing and walls, and an independent one has no repair to sell you. Ask your building department about local rules for ground cover, venting and combustion air, because requirements are set locally.
Frequently asked questions
What is a dirt crawl space?
A crawl space with bare soil as its floor, without concrete or a ground cover.
Is a dirt crawl space bad?
It is not always a problem, but bare soil gives off moisture, so it often leads to damp air, wet insulation and musty smells.
Does a plastic ground cover fix a dirt crawl space?
It cuts vapor from the soil. It does not stop water from outside, so drainage comes first.
Should I close the vents in a dirt crawl space?
It depends on your climate, your house and local code. Check with your building department before changing them.
Who should look at a dirt crawl space with sagging floors?
A licensed structural engineer can assess the framing. Your building department can explain local rules.
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.