Moisture and Mold Problems

Moisture Problems in the Home

Quick Links: Options  Damp Basement   Moisture Problems   Moisture on Windows

To help understand the principles of moisture control, you need to understand the basics of how moisture can move through your 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

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.

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.  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.

If water damage is concentrated in a small area, and is near a roof, bathroom or kitchen, it is most likely caused by a water leak. It is important to get these leaks fixed quickly to prevent further mold growth and to be able to repair the damage that has occurred.

However, if the area affected is larger, encompassing a whole wall or room, then you might have an excess humidity problem in your home. The moisture could be coming from a basement or crawlspace, result from poor ventilation, or be the result of improperly sized or maintained heating and cooling equipment.

Peeling Paint. Peeling paint is directly related to moisture problems. On interiors, it is best to reduce moisture by checking for exterior and interior leaks and then dealing with moisture from basements and crawlspaces as outlined in the Moisture and Mold section.

For exteriors, chronic peeling paint may indicate a water leak from inside or outside, or moisture penetration from the interior to the outside wall sheathing.  Moisture is absorbed by the material the paint is applied to and causes the paint to lose its adhesive qualities.

In the absence of a water leak, moisture penetration from inside to outside indicates a problem in the wall system, especially where there is a naturally moist environment on the inside.  Peeling paint on the outside wall of a bathroom might indicate a problem with the wall tile or substrate, missing grouting around tub or shower, or lack of an operating bath exhaust fan.

What are my Options?

Moisture problems originating in a 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:

   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. Caulk the sill 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 exterior foundation wall and footing to prevent the wall from absorbing ground moisture by capillary action.
   7. Paint the interior of the foundation wall with a masonry waterproofing paint
   8. 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.
   9. 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.
  10. Paint the basement floor with an epoxy-based waterproofing paint to prevent moisture diffusion through the floor.

Moisture problems originating in a crawlspace. 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. Caulk the sill 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. Paint the interior of the foundation wall with a masonry waterproofing paint.
   8. Install drainage plane material or gravel against the foundation wall to relieve hydrostatic pressure and channel water to the foundation drain.
   9. 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.
  10. Install 6-mil polyethylene 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.

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

Sufficient ventilation is important to limit the amount of moisture build-up in your home, so installing exhaust fans in kitchens and bathrooms as well as checking the vent on your clothes dryer may help to reduce humidity levels. Lastly, have a heating, ventilating and air conditioning contractor check your home’s heating and cooling equipment to make sure that it is working properly to reduce humidity levels in your home.

Damp Basement.  Basements, especially those whose walls are completely below grade, are naturally damp in the summer months.  The walls of basements are cooler than outside conditions most of the time, and sometimes the dew point of the air rises above the temperature of the walls, and condensation occurs.  Since the temperature of basement air is often cool compared to the outside, its relative humidity will naturally be higher.

Both of these conditions can result in a “damp” basement.  A basement can be considered damp if the relative humidity is 70% or above, and it should not be used for general storage.  If the measures for basement moisture control described above have been applied, and dampness is still prevalent, mechanical humidity control is the next step.  This can take the form of a stand-alone de-humidifier for the basement, or by supplying air conditioning to the basement from a central whole-house system.  Strive to keep the humidity level at or below 60%.

Moisture on Windows. Moisture on windows during the winter usually results from a combination of interior humidity levels and the window’s energy performance. Condensation or moisture naturally occurs on windows because of the temperature difference between the interior and exterior.  When the inner pane of glass falls below the dew point temperature of the interior air, condensation occurs.  In general, if you have single-paned or metal-framed windows, condensation will be more prevalent than if the window has two layers of glass and a wood or vinyl frame.

Condensation on windows may indicate that the humidity level in your home is too high.  The humidity level can be easily checked with an inexpensive humidity meter.  During cold weather, relative humidity should not exceed 50% indoors, and may need to be much lower to prevent condensation in extremely cold conditions.

Where excess humidity is a problem, reduce moisture at its source (as in the basement and crawlspace measures above); or increase ventilation throughout the home by regularly venting moisture from cooking and bathing.

Condensation may also be reduced by improving a window’s performance.  Fitting single-pane windows with interior or exterior storm windows will often solve condensation problems.  Temporary “window insulator” kits are sold at hardware stores, and consist of plastic sheeting that can be applied to the interior of a window for one season.

A better, although more expensive solution, is to replace windows with new windows with insulating glass.  A window with double pane, low-emittance (Lo-E)  glass has a much greater resistance to condensation than single-pane or even regular double-pane glass.     

It is nearly impossible to completely eradicate all condensation on windows, especially in cooler climates, but having more efficient, insulating windows helps to reduce this effect.

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