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Moisture Control Strategies To Protect Against Mold
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 may first want to understand how moisture moves through a home.
Moisture control strategies typically include the following areas of a home:
- Attics
- Foundation
- Basement
- Crawl space
- Slab-on-grade floors
- Walls
Basement Moisture Control and Mold Retardation
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. The best approaches for preventing these problems will depend on your local climate, type of insulation, and style of construction. However, the following general rules apply to most basement designs for creating a water-managed foundation system (see corresponding illustration):
- Keep all untreated wood materials away from earth contact.
- Provide drainage, such as gutters, to conduct rainwater 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.
- Add a sill gasket to provide air sealing.
- Install a protective membrane, such as caulked metal flashing or EPDM-type membrane, to serve as a capillary break that reduces 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 the footing, 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's 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.
Crawl Space Moisture Control
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. However, the following general guidelines for creating a water-managed foundation system apply to most crawl space designs:
- Keep all untreated wood materials away from the earth.
- Provide rain drainage, such as gutters, to conduct rainwater away from the house.
- Slope the earth away from the house for at least 5 feet at a minimum 5% grade (3 inches in 5 feet). Establish drainage swales to direct rainwater around the house.
Add a sill gasket to provide air sealing.
- Install a protective membrane, such as an EPDM-type membrane, to serve as acapillary break that reduces wicking of water from the masonry wall. This membrane, in addition to metal flashing, can serve as a termite shield.
- Damp-proof the below-grade portion of the foundation wall to prevent the wall from absorbing ground moisture by capillary action.
- Install drainage plane material or gravel against the foundation wall to relieve hydrostatic pressure and channel water to the foundation drain.
- Provide a foundation drainage system at the bottom of the footing, not on top, when the foundation floor (interior grade) is below the exterior grade.
- Surround a perforated 4-inch drain pipe with gravel, and cover them with filter fabric.
- Install 6-mil polethylene vapor diffusion barrier across the crawl space floor to prevent soil moisture from migrating into the crawl space. Overlap and tape all seams by 12 inches. Seal the polyethylene 6 inches up the crawl space walls. As an option, pour two inches (51mm) of concrete over this to protect the polyethylene from damage.
Slab-on-Grade Foundation Moisture and Air Leakage Control
To maximize your home's energy efficiency and to protect the foundation, you should use the following moisture and air leakage control techniques when installing slab-on-grade floors:
- Keep all untreated wood materials away from the earth.
- Install well-designed guttering and downspouts that are connected to a drainage system, which diverts rainwater completely away from the house.
- Add a sill gasket membrane between the slab and bottom plate to provide air sealing.
- Install a protective membrane (such as rubberized roofing material or ice-dam protection membranes) to serve as a capillary break that reduces wicking of water up from the foundation. This membrane can also serve as a termite shield.
- Install a foundation drain directly beside the bottom of the footing. The foundation drain assembly includes a filter fabric, gravel, and a perforated plastic drain pipe typically 4 inches in diameter. Locate the drain beside the footing, not on top, to avoid water flowing against the seam between the footing and the foundation wall, and to prevent wicking from a web footing through the stem wall.
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.
All climates require these moisture control steps:
- Install a polyethylene ground cover on the earth floor of houses with crawlspaces and slope the ground away from the foundations of all houses.
- Install a continuous vapor barrier, if your climate needs one (see map on this page) that has a perm rating of less than one.
- Place a termite shield, sill gaskets, or other vapor-impermeable membrane on the top of the foundation wall. This action will prevent moisture from wicking into the framed wall from the concrete foundation wall by capillary action.
- Install a capillary break and moisture barrier under the slab floor, consisting of a layer of 10-mil polyethylene vapor diffusion retarder placed over at least 4 inches of gravel.
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