Q: I live on a low-lying lot. My crawl space is always wet, even though I have a dehumidifier and a sump pump down there. People have suggested I use French drains; one said exterior ones, another interior. In this area, one company pretty much has a monopoly, and it’s backed up for months. Do French drains really work?

A: Yes, French drains do work — in combination with other measures. On a slope, where water is moving underground toward a house, an exterior French drain across the property, uphill of the house, can sometimes redirect the flow around the building enough to keep a crawl space or basement dry. Landscapers typically install this type of drainage system.

On a low-lying lot, however, the drainage system usually needs to completely surround the house. An exterior system has the benefit of keeping the water outside, and it’s easy and inexpensive to install when a house is being built. Retrofitting it into an existing house is difficult and expensive, however. Installers need to excavate down to the level of the crawl space or basement, even where driveways, walkways or established plants are located. And if the system ever clogs, it’s not easy to repair.

For solving the problem of a wet basement or crawl space in an existing house, an interior French drain system is much more common. Installers cut through the concrete or dig into the dirt just inside the foundation wall, creating a trench for perforated pipe and gravel that directs any water that seeps in toward a pit for a sump pump. Or, depending on how a house is laid out, they might slope different parts of the trench to more than one sump pump.

You mention that you already have a sump pump, but a pump alone often isn’t enough. “Just because they have a sump pump doesn’t mean the water can get to it,” said Bill Anderson, owner of DryZone in Ellendale, Delaware. And even if you could regrade the crawl-space floor to direct all the water to the sump-pump pit, water would still be flowing across the space to get to it. A perimeter trench captures water where it flows in.

But installing an interior French drain probably isn’t all you will need to do. Anderson said fixing a damp crawl space is often a four-step process, with adding the drain system being the first step. The second involves sealing off the soil using thick plastic that he compares to a pool liner. It extends down from the foundation walls, across the floor, and up and around any interior columns, with special tape to secure all seams. The liner “encapsulates” the space and keeps moisture from moving through the soil and into the crawl-space air.


The third step involves blocking humid outside air from getting into the crawl space. Installers close off the foundation vents, seal piping or other penetrations, and replace leaky access doors. This runs counter to the crawl-space ventilation requirements of past decades, but building scientists now recognize that enclosing the crawl space usually works better than ventilating it with outdoor air, especially where air conditioners run in humid weather. Otherwise, the humid air in the crawl space hits the cold floor, causing condensation that allows mildew to take hold. Anderson said he often recommends taking down insulation on the floor and instead installing rigid foam insulation on the foundation walls.

The final step is adding a dehumidifier, but one designed to turn on and off automatically and run only when the air gets too humid.

With these measures in place, your crawl space should stay dry, and the rest of your house should be protected from the issues — mildew smells, rotting wood — that can occur when a crawl space stays damp. But there’s one more step you might want to add, because the protections work as long as the power stays on. Especially if you live where tree limbs often knock out power in windstorms, you might want to invest in a battery backup for the sump pump. Any company you hire to install the system should be able to add that.

How much does all of this cost? Anderson said bills from his company have ranged from $3,000 to $30,000, depending chiefly on the size and layout of the house, with a majority of jobs in the $8,000 to $10,000 range. His company typically has a backlog of two to three months.