Vapor Barriers & Moisture Control

Vapor barriers, or – more accurately – vapor diffusion retarders, should be part of a moisture control strategy for a home in most U.S. climates. A vapor barrier or vapor diffusion retarder is a material that reduces the rate at which water vapor can move through a material. The older term “vapor barrier” is still used even though “vapor diffusion retarder” is more accurate.

Vapor barriers prevent water from entering the walls, floors and ceilings of a structure, preventing rot, mold and mildew.

Moisture in the walls is not just a problem because of the rot, mold and mildew issues.

In cold climates, having wet insulation can actually decrease the efficiency of the insulation, making the house colder and potentially raising heating bills.

The water can also freeze and then melt rapidly, creating water stains on the walls and ceilings. Chronic moisture can also cause the wood in a structure to warp, causing structural instability.

The level of moisture in the home is crucial to achieving a healthy home environment.

The signs that conditions of excessive moisture exist may be clearly visible or they may be hidden in places such as behind walls, under furniture and carpets, and in crawlspaces or attics.

In assessing moisture problems in the home environment, it is important to know such potential hiding places and visually inspect all areas that are reasonably accessible.

Preventing and eliminating moisture control problems involves removing, diverting, blocking, or otherwise controlling the source of moisture while also providing adequate ventilation.

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Combination air barriers/vapor diffusion retarders

An air barrier/vapor diffusion retarder attempts to accomplish water vapor diffusion and air movement control with one material. This type of material is most appropriate for southern climates where keeping humid outdoor air from entering the building cavities is critical during the cooling season.

Air barriers/vapor diffusion retarders are generally placed around the perimeter of the building just under the exterior finish, or they may actually be the exterior finish. The key to making them work effectively is to permanently and carefully seal all of the seams and penetrations, including around windows, doors, electrical outlets, plumbing stacks, and vent fans.

Missed gaps of any size not only increase energy use, but also increase the risk of moisture damage to the house, especially during the cooling season. An air barrier/vapor diffusion retarder should also be carefully inspected after installation before other work covers it. If small holes are found, you can repair them with caulk or polyethylene or foil tape. Areas with larger holes or tears should be removed and replaced. Patches should always be large enough to cover the damage and overlap any adjacent wood framing.

Frequently Asked Questions

How do vapor barriers help with moisture control?

Vapor barriers prevent water vapor from penetrating structural components, reducing the risk of moisture-related issues such as insulation inefficiency, wood warping, and mold growth. Proper installation ensures a healthier home environment.

Do all homes need vapor barriers?

Most U.S. homes benefit from vapor barriers, but their necessity depends on climate. In colder regions, they help keep insulation dry and efficient. In humid southern climates, they block moisture-laden air from entering the building’s structure.

Where should vapor barriers be installed in a home?

Vapor barriers are typically installed in walls, ceilings, floors, crawlspaces, and attics. They can be placed under exterior finishes or used in combination with air barriers to enhance energy efficiency and moisture control.

What materials are commonly used for vapor barriers?

Common materials for vapor barriers include polyethylene plastic sheets, builder’s foil, foam board insulation, and specialized exterior sheathings. The key is ensuring all seams and penetrations are sealed properly to maximize their effectiveness.

Can vapor barriers help reduce energy costs?

Yes, vapor barriers can improve insulation performance by preventing moisture buildup, which can decrease insulation efficiency. This helps maintain indoor temperatures, reducing heating and cooling costs.