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moisture control





moisture insulation
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, it is important to understand how moisture moves through a home.


How moisture moves through a home

Moisture or water vapor moves in and out of a home in three ways:
  • With air currents

  • By diffusion through materials

  • By heat transfer
moisture migration
Of these three, air movement accounts for more than 98% of all water vapor movement in building cavities. Air naturally moves from a high pressure area to a lower one by the easiest path possible – generally through any available hole or crack in the building envelope. Moisture transfer by air currents is very fast (in the range of several hundred cubic feet of air per minute). Thus, you need to carefully and permanently air seal any unintended paths to control air movement.

The other two driving forces – 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.

The laws of physics govern how moist air reacts within various temperature conditions. The study of moist air properties is technically referred to as "psychrometrics." A psychrometric chart is used by professionals to determine at what temperature and moisture concentration water vapor begins to condense. This is called the "dew point." By understanding how to find the dew point, you will better understand how to avoid moisture problems in your house.

Relative humidity (RH) refers to the amount of moisture contained in a quantity of air compared to the maximum amount of moisture the air could hold at the same temperature. As air warms, its ability to hold water vapor increases; this capacity decreases as air cools. For example, according to the psychometric chart, air at 68F (20C) with 0.216 ounces of water (H2O) per pound of air (14.8g H2O/kg air) has a 100% RH. The same air at 59F (15C) reaches 100% RH with only 0.156 ounces of water per pound of air (10.7g H2O/kg air). The colder air holds about 28% of the moisture that the warmer air does. The moisture that the air can no longer hold condenses on the first cold surface it encounters (the dew point.) If this surface is within an exterior wall cavity, wet insulation and framing will be the result.

In addition to air movement, you also can control temperature and moisture content. Since insulation reduces heat transfer or flow, it also moderates the effect of temperature across the building envelope cavity. 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 retarders work together to reduce the opportunity for condensation in a house's ceilings, walls, and floors.

To effectively control moisture in your home, you need to first consider your climate when exploring your moisture control options.


Moisture control strategies

These typically include the following areas of a home:
  • Attics
  • Foundation
    • Basement
    • Crawl space
    • Slab-on-grade floors
  • Walls

Moisture control in basements

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):

water-managed basement wall and foundation
  1. Keep all untreated wood materials away from earth contact

  2. Provide drainage, such as gutters, to conduct rainwater away from the house

  3. 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

  4. Add a sill gasket to provide air sealing

  5. 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

  6. Damp-proof all below-grade portions of the foundation wall and footing to prevent the wall from absorbing ground moisture by capillary action

  7. 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

  8. 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

  9. Underneath the basement's slab floor, install a capillary break and vapor retarder, consisting of a layer of 6- to 10-mil polyethylene over at least 4 inches of gravel


Consult a qualified builder, basement designer, basement waterproofing contractor like Basement Systems, and/or insulation contractor in your area for specific basement moisture control measures concerning your climate, type of insulation, and construction style.


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:
  1. Keep all untreated wood materials away from the earth.

  2. Provide rain drainage, such as gutters, to conduct rainwater away from the house.

  3. 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.

  4. Add a sill gasket to provide air sealing.

  5. Install a protective membrane, such as an EPDM-type membrane, to serve as a capillary break that reduces wicking of water from the masonry wall. This membrane, in addition to metal flashing, can serve as a termite shield.

  6. Damp-proof the below-grade portion of the foundation wall to prevent the wall from absorbing ground moisture by capillary action.

  7. Install drainage plane material or gravel against the foundation wall to relieve hydrostatic pressure and channel water to the foundation drain.

  8. 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.

  9. 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:

floating slab-on-grade construction
  1. Keep all untreated wood materials away from the earth.

  2. Install well-designed guttering and downspouts that are connected to a drainage system, which diverts rainwater completely away from the house.

  3. Add a sill gasket membrane between the slab and bottom plate to provide air sealing.

  4. 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.

  5. 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.

  6. 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.


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.

vapor barrier placement
All climates require these moisture control steps:
  • Install a polyethylene ground cover on the earth floor of houses with crawl spaces 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.

Causes of rain leaks through exterior walls include improper installation of the following:
  • Siding materials

  • Poor-quality flashing

  • Weatherstripping or caulking around joints in the building exterior (such as windows, doors, and bottom plates)

  • Wind-driven rain can also penetrate the exterior finish

To enhance protection against rain penetration, create a drainage plane within the wall system of the home.


Related category

   • HUMIDITY AND MOISTURE CONTROL