What Is a Moisture Barrier and Where Should It Be Installed?

A moisture barrier stops water in two forms. It blocks liquid water from seeping through, and it slows water vapor from moving through walls, floors, and ceilings. This matters because trapped moisture can rot wood, rust metal, and feed mold growth inside your home.

The material is usually polyethylene sheeting, but it can also be foil-faced board or a liquid coating. Thickness and type depend on where you install it. A crawl space barrier needs to be tough enough to resist punctures, while a wall barrier may be a thin membrane.

• Blocks liquid water from entering
• Slows water vapor movement
• Protects wood and insulation
• Reduces mold and mildew risk
• Improves energy efficiency
• Keeps living spaces drier

In a crawl space, the barrier lies directly on the ground. The plastic sheeting should overlap at seams and extend up the walls a few inches. This keeps soil moisture from evaporating into the air under your floor, which can cause wood rot and musty odors.

Seal the seams with tape or by overlapping them by at least 12 inches. Weight the edges down with gravel or soil. If you have a dirt floor, this barrier is essential. Even in areas with high humidity, the barrier helps year-round.

Installing a moisture barrier is a straightforward job in some spots, but it can be tricky in others. If you have a crawl space with standing water, a slab that is already damp, or a shower that leaks, a barrier alone may not fix the problem. Water damage behind walls or under floors needs a proper inspection first.

Local water damage pros can check for hidden moisture and advise on the right barrier type. They use moisture meters to find damp spots you cannot see. After a flood or a burst pipe, a barrier may be part of the repair, but the source of the water must be stopped first.