Properly controlling moisture in your home can improve the effectiveness of your air sealing and insulation efforts—which will in turn help control moisture. The best strategies for controlling moisture in your home depend on your climate and how your home is constructed. Proper ventilation should also be part of a moisture control strategy.

What contributes to moisture in your home

Before you decide on a moisture control strategy, it helps to understand that moisture or water vapor moves in and out of a home in three ways:

  • With air currents—accounting for more than 98% of all water vapor movement in buildings
  • By diffusion through materials
  • By heat transfer

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Moisture transfer by air currents happens quickly.

Air naturally moves from high-pressure areas to lower pressure areas by the easiest path available—through any available hole or crack. To effectively control moisture, you must carefully and permanently air seal any unintended paths for air movement in and out of the house.

Moisture movement by diffusion through materials and heat transfer are much slower processes. Most common building materials slow moisture diffusion to a large degree, although they never stop it completely. Insulation also helps reduce heat transfer or flow.

What causes condensation?

The ability of air to hold water vapor increases as it warms and decreases as it cools. Once air has reached its dew point—the temperature and moisture concentration at which water vapor condenses—moisture in the air condenses on the first cold surface it encounters. If this surface is within an exterior wall cavity, the result is wet insulation and framing.

How you can help control moisture

Because moisture is transferred predominantly by air currents, air sealing your home is essential. You can also control temperature and moisture content. Installing insulation reduces heat transfer, so it also moderates the effect of temperatures across your home. In most U.S. climates, properly installed vapor diffusion retarders can be used to reduce the amount of moisture transfer. Except in deliberately ventilated spaces such as attics, insulation and vapor diffusion retarders work together to reduce the opportunity for condensation in a house’s ceilings, walls, and floors.

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Moisture can cause problems in attics, various types of foundations, and walls, and the solutions to those problems vary by climate. See Building America’s Climate-Specific Publications for construction details specific to your climate.

Foundation Moisture Control

The potential for moisture problems exists anywhere building components are below grade, whether you have a basement, crawlspace, or slab-on-grade foundation. Most basement water leakage results from water flowing through holes, cracks, and other discontinuities into the home’s basement walls or water wicking into the cracks and pores of porous building materials, such as masonry blocks, concrete, or wood. These tiny cracks and pores can absorb water in any direction—even upward. To create an energy-efficient and comfortable living space in your basement, you will need to insulate as well as properly control moisture.

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Preventing foundation moisture

The best approaches for preventing foundation moisture problems will depend on your local climate, type of insulation, and style of construction. If you need to correct moisture problems in your existing home, consult a qualified builder, basement designer, and/or insulation contractor in your area for specific basement moisture control measures tailored to your climate, type of insulation, and construction style.

In a new home

If you’re building a new home, pay particular attention to how water will be managed around the foundation. The following guidelines will apply in most circumstances:

  • Keep all untreated wood materials away from earth contact.
  • Install well-designed guttering and downspouts connected to a drainage system that diverts rainwater completely away from the house.
  • Slope the earth away from all sides of the house for at least 5 feet at a minimum 5% grade (3 inches in 5 feet). Establish drainage swales to direct rainwater around and away from the house.
  • Add a gasket under the sill plate to provide air sealing.
  • Install a protective membrane, such as rubberized roofing or ice-dam protection materials, between the foundation and the sill plate to serve as a capillary break and reduce wicking of water up from the masonry foundation wall. This membrane can also serve as a termite shield on top of foam board insulation.
  • Damp-proof all below-grade portions of the foundation wall and footing to prevent the wall from absorbing ground moisture by capillary action.
  • Place a continuous drainage plane over the damp-proofing or exterior insulation to channel water to the foundation drain and relieve hydrostatic pressure. Drainage plane materials include special drainage mats, high-density fiberglass insulation products, and washed gravel. All drainage planes should be protected with a filter fabric to prevent dirt from clogging the intentional gaps in the drainage material.
  • Install a foundation drain directly below the drainage plane and beside (not on top of) the footing. This prevents water from flowing against the seam between the footing and the foundation wall. Surround a perforated 4-inch plastic drainpipe with gravel and wrap both with filter fabric.
  • Underneath the basement or on-grade slab floor, install a capillary break and vapor diffusion retarder, consisting of a layer of 6- to 10-mil polyethylene over at least 4 inches of gravel.
  • If your new or existing home has a crawlspace, you can also install a 6-mil polyethylene vapor diffusion barrier across the crawlspace floor to prevent soil moisture from migrating into the crawlspace. Overlap all seams by 12 inches and tape them, and seal the polyethylene 6 inches up the crawlspace walls. As an option, pour two inches (51 mm) of concrete over the vapor barrier to protect the polyethylene from damage.

Moisture control in walls

It is a myth that installing vapor barriers is the most important step for controlling moisture in walls. Vapor barriers only retard moisture due to diffusion, while most moisture enters walls either through fluid capillary action or as water vapor through air leaks. Most climates require taking specific moisture control steps.

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Steps to control moisture in walls

To control moisture within a wall you must seal any pathways that allow for air movement. Depending on the building code and local climate zone, you may also have to install a vapor retarder along with insulation. Additionally, you must control heat transfer. The purpose of insulation is to retard the flow of heat from one place to another and to maintain temperatures such that condensation does not occur inside a wall’s surface.

Do I need a vapor retarder?

The IRC (primary national model building code) does not require or prohibit the use of vapor retarders in climate zones 1, 2, 3, and 4. NAIMA recommends using either a Class II or III vapor retarder in these warmer climate zones and avoiding the use of Class I (very low perm) vapor retarders. Kraft-faced batts can be installed in all climate zones. A map of the different climate zones can be found here.

In the warmer climate zones, installing vapor retarder with a very low perm rating on the interior of a wall assembly can lead to moisture problems. Even vinyl wall paper, which has a low perm rating, can induce moisture problems in warm, humid climates where hot, moist conditions tend to drive moisture into the wall from the outside of the building.

In very warm, humid climates, if a vapor retarder is used, NAIMA recommends installing it to the exterior side of the wall.

Dealing with rain leaks

Rain—especially wind-driven rain—can also cause moisture problems in walls. Rain leaks through exterior walls are usually a result of:

  • Improperly installed siding materials
  • Low-quality flashing
  • Poorly installed weatherstripping or caulking (around windows, doors, and bottom plates)

To protect against rain penetration, improperly installed materials should be corrected. You should also create a drainage plane within the wall system of your home.

What to Do With Wet Insulation

Whether or not insulation or other building materials should be replaced after getting wet depends on several factors, and opinions vary widely. The American Red Cross and the Federal Emergency Management Agency (FEMA) suggest that fiberglass batts can be removed, dried and replaced if they are wet from clean water. They also suggest that cellulose (loose or blown-in treated paper) insulation can lose its antifungal and fire retardant abilities when wet and, therefore, should be replaced. The U.S. Environmental Protection Agency (EPA), however, recommends that all wet insulation be discarded and replaced.

Although fiberglass can be reused in many instances, it’s recommended that you consult FEMA, EPA, the American Red Cross, or other reputable sources for specific guidance. If in doubt, err on the side of caution and replace all wet insulation.