Moisture Control

Properly controlling moisture in your home will improve the effectiveness of your air sealing and insulation efforts, and vice versa. Thus, moisture control contributes to a home's overall energy efficiency.

The best strategy for controlling moisture in your home depends on your climate and how your home is constructed. Before deciding on a moisture control strategy for your home, you should understand how moisture moves through a home.

Moisture control strategies typically include the following areas of a home:


Attic ventilation is essential. Without it, moisture that moves through the ceiling will be trapped in the attic because most roofing materials prevent moisture from escaping. Basically, the idea of cold-side venting is to relieve the vapor pressure in the attic by providing a vent to the outside air, which usually has a lower vapor-pressure.

Ventilate the attic with inlet vents distributed along the eave and with the outlet vents near the ridge. You'll get the best results when the ventilation is uniformly distributed along the roof and is equally divided between the high and low. Warm air in the attic rises and escapes through the ridge vents: cooler outside air enters at the eaves. See Figure 7. In this way, ventilation is continuous and does not depend on the wind.

For proper ventilation, attics require one square foot of unobstructed ventilation area for each 150 square feet of attic area. Five vent types are common: eave (soffit), gable, turbine, roof or continuous ridge.


To effectively insulate your basement for energy efficiency and to create a comfortable space, you need to properly control moisture in your basement.

Most basement water leakage results from either bulk moisture leaks or capillary action. Bulk moisture is the flow of water through holes, cracks, and other discontinuities into the home's basement walls. Capillary action occurs when water wicks 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.

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Crawl spaces

To effectively insulate your crawl space for energy efficiency and to create a comfortable home, you need to properly control moisture in your crawl space.

A crawlspace is susceptible to moisture and deterioration problems because of contact with the earth. The best approaches for preventing these problems will depend on your local climate and the style of your home's construction.

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Slab-on-grade floors

To maximize your home's energy efficiency and to protect the foundation, you should use six moisture and air leakage control techniques when installing slab-on-grade floors.

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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.

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In most U.S. climates, you can use vapor diffusion retarders in these areas of your home to control moisture.

Proper ventilation should also be part of a moisture control strategy.

Controlling Moisture in Log Homes

Since trees absorb large amounts of water as they grow, the tree cells are also able to absorb water very readily after the wood has dried. For this reason, a log home is very hydroscopic—it can absorb water quickly. This promotes wood rot and insect infestation.

It is strongly recommended that you protect the logs from any contact with any water or moisture. One moisture control method is to use only waterproofed and insecticide-treated logs. Reapply these treatments every few years for the life of the house.

Generous roof overhangs, properly sized gutters and downspouts, and drainage plains around the house are also critical for moisture control.

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