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Moisture Management

Moisture can effect both a home's durability and indoor air quality. 'Wet walls' can lead to mold and even rotting studs. The extensive use of vapor barriers and air sealing can help prevent moisture problems.

Air Sealing

Proper air sealing will lower a home's energy use and increase comfort levels by reducing the amount of air infiltration into a home. Air sealing will also improve a home's durability by minimizing the amount of moisture penetration into the walls and living spaces.

Southface's Air Sealing Checklist

• Seal the bottom plate of exterior walls with caulk or gasket. Seal inside edges with caulk after walls are up.
• Seal band joists with caulk, spray foam, or gasketing between the top plate and band joist, and between the band joist and subfloor.
• For bathtubs on outside walls, insulate the exterior wall and air seal behind the tub with sheet goods or plastic before the tub is installed. After the drain is installed, seal the tub drain penetration with sheet goods and caulk or spray foam. Is sheet goods correct?
• For dropped ceilings or soffits, duct and flue chases, and open partition walls, use sheet goods and sealant to stop air leakage from the attic into the soffit and then insulate. Another alternative is to install framing and drywall for the soffits after the taped ceiling drywall is installed.
• Caulk the backsides of window flanges to the sheathing during installation.
• Seal between door thresholds and subflooring with caulk.
• Seal window and exterior door rough openings with backer rod and caulk, or use non-expanding latex-based spray foams that will not pinch jambs or void window warranties.
• Seal all electrical wire, plumbing, and HVAC penetrations between any conditioned and unconditioned spaces with caulk or spray foam.
• Seal wiring and knockouts in electrical boxes with caulk. Also seal outdoor mounted boxes to the exterior sheathing.
• Repair any damaged sheathing pieces.
• Seal all exterior penetrations, such as porch light fixtures and phone, security, cable and electric service holes with caulk or spray foam.
• Seal the weather-resistive barrier paper. Be sure to properly overlap sheets.
• If you are not using housewrap or another weather-resistive barrier paper, seal all sheathing seams with housewrap tape or caulk.

Be sure to seal the platform intersections, such as the joists to sill plate and deck to wall plate, since these are frequently overlooked.

Water Sealing

A vapor retarder, or a vapor barrier, is a layer in the building envelope that restricts the diffusion of water vapor. Water vapor will go from an area of high vapor pressure (i.e., high humidity) to low vapor pressure (i.e., low humidity). Typically, indoor air in cold climates is at a higher vapor pressure than outdoor air, which is dryer and colder. The opposite is true in hot/humid climates, where the lower vapor pressure is indoors (and accentuated by the use of air conditioners and dehumidifiers).

A smart vapor retarder exhibits low permeance under dry conditions and much higher permeance under damp conditions. In cold weather, the product is designed to function much like a conventional vapor retarder, blocking vapor flow from inside the house into the wall cavity. However, during hot weather, a smart vapor retarder will permit a damp wall cavity to dry towards the indoors. It can also be used in mixed-humid climates where conventional vapor retarder placement is problematic. Smart vapor retarders are not recommended for hot-humid climates.

When installing vapor retarders, remember to also install them next to the attic insulation. Most homes are now built with vapor retarders in the exterior walls, but the attic is often neglected. Attics are critical because they often experience high levels of humidity.

For additional information on vapor retarders, visit PATH's Durability by Design.

Content updated on 3/2/2006

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