What humidity should a crawl space be, and what to do when it is higher

The number

Wood starts to grow mold when the relative humidity around it sits above roughly 60% for days at a time, and at 70% and up the timber itself begins to take on enough moisture to soften and, eventually, rot. So the target for the air under a house is below 60% relative humidity, year round, and a well-sealed crawl space with a dehumidifier will hold 50 to 55% without effort.

A cheap hygrometer from a hardware store, left on a joist for a week in July, tells you where you stand. Read it in the evening, not the morning, and take the highest reading, not the average. The joists do not care about the average.

Why Louisville crawl spaces are worse than most

Two things work against a crawl space here. The first is the ground: Louisville sits on heavy clay over limestone, and clay holds water against a foundation for days after every storm. Ground moisture rises out of bare dirt as vapor around the clock, and a crawl space with an open dirt floor is standing on a wet sponge.

The second is the summer air. From May to September the outdoor dew point in the Ohio Valley runs in the upper 60s and low 70s. Open foundation vents pull that air under the house, where the ductwork and the ground are cooler, and the moisture condenses on the coldest surfaces first: metal ducts, water lines, then the wood. That is why a “ventilated” crawl space is often wetter than a sealed one. The vents were designed for a drier climate.

What high humidity does, in order

• Smell. The musty smell upstairs is crawl space air. A large share of the air in the living space rises from below through gaps around pipes, ducts and the subfloor.
• Insulation. Fiberglass batts soak up moisture, sag, and fall out of the joist bays. Wet insulation is worse than none; it holds water against the wood.
• Mold. White, green or black growth on the joists and subfloor. It starts at the coldest, dampest corner and spreads.
• Wood. Softened joists, sagging floors above, sticking doors. This is the expensive stage.

The fix, in the right order

Homeowners often buy a dehumidifier first because it is the easy purchase. In a vented crawl space over bare dirt it runs constantly, fills a bucket a day and barely moves the number, because it is trying to dry the outdoors and the ground at once. The order that works:

1. Water. If water collects under the house after rain, that is a drainage problem, not a humidity problem. A perimeter drain and a sealed sump come first.
2. Ground. A heavy liner across the whole floor and up the walls, seams taped, stops the ground vapor. This is the single biggest change.
3. Air. Seal the vents and the gaps so summer air stops coming in.
4. Dehumidifier. Now a properly sized unit, plumbed to drain and set to about 50%, holds the space dry with very little running time.

That sequence is a crawl space encapsulation. The dehumidifier is the last step of it, not a substitute for it.

What about the winter?

Cold outdoor air holds less moisture, so winter humidity under the house drops on its own. The problem season is late spring through early fall. A sealed crawl space stays steady all year; a vented one swings from too wet in July to dry and cold in January, and the joists move with it. Steady is what the wood wants.

A note on “conditioned” crawl spaces

Some builders condition a sealed crawl space by pushing a little supply air from the furnace into it instead of using a dehumidifier. It works when the system is designed for it. In an older Louisville house that was never built that way, a dedicated dehumidifier with its own drain is the simpler and more reliable choice, and it does not depend on the furnace running.