Controlling Moisture In The Home
By Dr. Frances C. Graham, Extension Housing Specialist
Mississippi State University Extension Service

Household activities -- such as cooking, laundering, bathing, and temperature -- influence the levels of moisture vapor. The temperature of the air
determines its moisture-carrying capacity.
Warm air absorbs more moisture than does cooler air. If moisture-laden air comes in contact with a cold surface, the air is cooled and its moisture-
carrying capacity is reduced. Therefore, some of the moisture may be condensed out of the air and condensation forms on the cold surface. This
phenomenon is illustrated by the condensation on an ice-filled glass in hot weather or condensation on windows in cold weather.

Basic construction techniques and procedures can prevent or minimize most household moisture problems. Insulation, weatherstripping, storm
doors, and windows used with ventilation can help homeowners maintain comfortable levels of temperature and relative humidity within the house
without annoying problems associated with excess moisture levels.

Generally, occupants of households are comfortable when the temperature and relative humidity are maintained 68 to 72 °F and 25 to 50 percent
relative humidity. Normally, mildew is a problem at a maintained relative humidity level above 60 percent, and static electricity is noticeable in cold
weather at levels of 20 percent or lower.

Moisture vapor tends to migrate from one area of the house to another. Unless prevented by vapor barriers or retarders, moisture moves through floors,
walls, and ceilings.

Waterproofing Slabs
Concrete floor slabs must have a vapor barrier to prevent ground moisture movement through the concrete. Without a vapor barrier, penetration of
moisture affects floor adhesives and causes mustiness in carpets.
The standard construction procedure is to install both gravel (which breaks the water movement toward the concrete) and a polyethylene sheet vapor
barrier to stop vapor movement above the gravel before the concrete is poured.

Crawlspace Moisture
A source of water vapor or moisture in a home is usually the crawlspace. If the soil is damp, as much as 20 gallons of water per 24 hours can
evaporate into the air in a 1,400-square-foot crawlspace. Without a vapor barrier, the moisture passes through the floor in the form of vapor, thereby
raising the relative humidity to the point that serious condensation problems occur.
As the water vapor passes through to the interior of the house, the moisture content of the floor joist is often raised to a level that supports growth of
fungi on the wood joists. Some of these organisms do not damage the wood while others are capable of total destruction.

The primary causes of excess moisture in the crawlspace include:
  • Surface runoff water is not excluded.
  • Adequate ventilation is not achieved by vents.
  • Excess moisture vapor is released by the ground.
  • Signs of excess moisture in the crawlspace area include:
  • Soil is wet.
Surface organism growth appears on the sapwood of some floor joists. This is usually "toadstool" organisms (brown and green in color and leaves a
stain on the wood surface when smeared).
During summer, the bottomside of the floor insulation has beads of moisture on the surface or is saturated with condensed moisture.
Musty odors in the crawlspace or living area of the home.
Excessive moisture in the living area. This condensation tends to migrate toward the windows.
Installing Vapor Barrier
In a house with some hardwood floors, cover two-thirds to three-fourths of the ground in the crawlspace area. (Hardwood floors need some moisture to
keep the boards from drying out or forming cracks or spaces between floorboards.) The easiest way is to cover the entire area carefully, using 6-mil
polyethylene (color unimportant) and lapping joints at least 6 inches. After the entire space is covered, turn back 18 to 24 inches around the edge of the
foundation wall, exposing the ground of the crawlspace around the perimeter of the house. This allows moisture vapor to come up around the edge of
the crawlspace. Leave the completed installation in place for 12 to 18 months. If moisture levels still seem high after this time, go back and recover
some of the exposed ground area.
If the house has carpet, vinyl, or flooring materials other than wood, the crawlspace area can be fully covered with polyethylene.

As a final step (which is optional), one or two inches of sand may be placed on top of the polyethylene. The sand weights down the polyethylene,
preventing condensation of moisture on the undersurface. Sand absorbs condensation that occurs in the cold weather and prevents the deterioration
of any of the plastic exposed to light.

Installing polyethylene on crawlspace surfaces helps control excess moisture vapor. If water is present in the crawlspace, other remedial practices
must be taken (refer to sump pump).

Plastic ground cover reduces the foundation ventilator need to one-square foot of unobstructed ventilator area for each 1,500 square feet of plastic-
covered crawlspace area, or one standard ventilator for each 750 square feet.

Foundation Vents
A standard metal foundation vent is 8 inches by 16 inches and is usually in the top 8 inches of the foundation wall (under a window). Place vents to
provide cross ventilation. Foundation vents have a metal grid of about one-inch square with screen wire to exclude mice. There is usually an operating
metal shutter on the vent.
The foundation ventilator dissipates the moisture vapor in the crawlspace area; therefore, the ventilator should remain open year-round except during
extreme cold.

Sump Pumps
After proper grading and draining, if there is still water standing in the crawlspace area, use a "sump pump" to remove accumulations of water. The
pumps are electrically powered with automatic controls and are permanently installed in a reservoir or drain or at the lowest area where water
When a sump pump is installed in the crawlspace at the lowest point, a hole 20 x 20 x 20 inches is excavated to accommodate a piece of 12x12-inch
flue liner. Put three inches of gravel in the bottom of the hole; set the flue liner in the center of the hole; pour gravel in the space around the liner. The
sump pump is placed in the flue liner with guy wires from the pump to the floor joist.

Moisture Vapor On Outside Wall, Floors, And Ceilings
Moisture vapor migrates from one area of a house to another. This is caused by different temperatures on each side of a wall, floor, or ceiling, which in
turn produce vapor pressure. The vapor, unless prevented by vapor retarders or barriers, can move through floors, walls, and ceilings.
The difference in pressure causes warm-moisture-laden air to migrate toward this cooler side of the wall, floor, or ceiling.

Vapor barriers prevent moisture vapor from moving from one area to another. Barriers are needed on insulation to prevent moisture vapor from
penetrating. If moisture vapor is allowed to penetrate the insulation, it becomes wet and ceases to be an insulator.

Aluminum foil and bituminous-treated craft paper are used as vapor barriers on one side of blanket-type insulation. Aluminum foil is totally resistant to
vapor passage while craft paper is highly resistant but not totally resistant. Polyethylene plastic also is used as a vapor barrier material.

Window Condensation
Condensation in cold weather is reduced or eliminated by installing storm windows on existing windows. The air space separating the storm unit from
the regular window becomes an insulator. The space allows the temperature of the storm window unit to approach the temperature of the outside air
while the temperature of the regular window can approach the temperature within the house or at least stay above a temperature that will cause
Occasionally, after a storm window unit has been installed, moisture condenses on the storm window. If this happens, air is leaking around the
permanent window, allowing warm, moist air from inside the house to seep into the air space between the glass panes. In this case, try to seal the
leaking spaces around the regular window.

If the regular window continues to have condensation, this means the storm unit does not have a tight fit and is permitting an excessive amount of cold
air to reach the regular window. Caulking around the storm unit usually corrects this problem.

Attic Moisture
The usual source of attic moisture is migration of moisture vapor from the living area below. Many homes have loose-fill insulation blown into an attic
with no vapor barrier, permitting moisture from inside the house to rise into the attic.
This problem is compounded by inadequate attic ventilation. As moisture migrates, it condenses on the roof sheathing, rafters, and gable walls.
Sometimes frost forms on the bottom surface of plywood sheathing during the coldest weather. This sheathing sometimes turns dark due to organism
growths (e.g., mildew).

For proper ventilation, attics require 1 square foot of unobstructed ventilation area for each 150 square feet of attic area. The most efficient ventilation
technique is 50 percent of the ventilation in the soffit board and 50 percent in a ridge vent. The natural convection currents of permitting heated air to
rise draws cooler air into the attic through the soffit vent and exhausts the air through the ridge vent.

Soffit vents provide an air inlet and complete flow path when used with gable or roof vents.

In cases where dampness appears on ceilings on the inside of the house at the edges near the outside walls, there is a possibility that attic insulation
is improperly installed. If cold wind blows under the insulation, the ceiling is chilled. If wall insulation settles, cold spots occur at the top of walls. Either
way, insulation must be repositioned to prevent vapor from condensing on the cold area of the ceiling.

Household Moisture
Too high a level of moisture inside a home causes the fungi mold to produce mildew. Mildew is more prevalent during damp weather and in poorly
ventilated rooms.
Mildew usually begins to grow where the relative humidity is 60 percent and the temperature is 60 °F or higher. Mildew appears on the surfaces of
bathroom tile, leather goods and clothing stored in closets, on or behind drapes and rugs, and in other warm, poorly aired and poorly lighted areas.

Musty odors are often associated with mildew. Some deodorizers in pressurized spray cans assist in reducing musty odors in furniture and carpet

Mildew returns when the level of dampness approaches 60 percent relative humidity. Unless the source of the humidity is corrected, you are in a never-
ending cycle of cleaning mildewed surfaces with a chlorine solution.

Dry The Air
Air conditioners do a good job of removing moisture. A dehumidifier removes moisture from houses that do not have air conditioning. Dehumidifiers
usually are rated by the amount of water (in pints) they remove in 24 hours.
The water from dehumidifiers can be piped directly to a drain with a garden hose or collected in a pan that must be emptied periodically to prevent
overflowing. New models have an automatic control that warns when the pan needs emptying or shuts off the unit.

Limited maintenance includes cleaning the coil with a brush or rag and wiping the dust from the cooling fan.

Heat -- If the mildew appears generally throughout the house, turn on the furnace for a short time, open the windows, or turn on a circulating fan and
direct it against the surfaces on which mildew is forming.

A musty odor sometimes is present because damp air migrates to places such as closets or room corners with no ventilation. To dry the air in closets
and other small areas, burn an electric light bulb continuously. The heat is sufficient to prevent mildew if the space is not too large. Precaution: Be sure
bulb is a sufficient distance from clothing to avoid the danger of fire.

Chemicals -- Use silica gel, activated alumina, or calcium chloride to absorb moisture.

Silica gel and activated alumina are not harmful to fabrics. They can be put in small paper containers and hung with clothes or put in drawers or chests.
They also can be put in pans and placed on the floor or on shelves in closets. Be sure the container or closet is closed tightly so that moisture from the
outside will not seep into the area. These two chemicals can be heated to 300 °F for several hours and reused. Silica gel is blue when dry and turns
pink as it absorbs water.

Calcium chloride liquefies as it absorbs water. The liquid is corrosive and may burn clothing.

Ventilate Areas
Air movement, when the relative humidity is low, will dry the surfaces. Air movement might be with a fan or by opening the closet doors to permit natural
Hang clothes loosely so air circulates around them.

Before hanging them in the closet, dry all clothing wet from rain or perspiration.

Ventilate cooking, laundering, and bathing areas since one or more gallons of water can be added to the house in one day.

To Get Rid Of Musty Odors
Use chlorinated lime in cellars with dirt floors. Sprinkle this chemical over the floor. Let it stay until all mustiness disappears, then sweep it up.
On concrete floors and tiled walls, scrub with a dilute solution of sodium hypochlorite or chlorine bleach. Use 1/2 to 1 cup of bleach to a gallon of water.
Rinse with clear water and wipe as dry as possible.

Precaution: Work quickly and carefully on plastic and asphalt tile to avoid spotting the surface.

Quaternary ammonium compounds also may be used on floors and walls. Use one ounce of a 10-percent solution to each gallon of water.

Aerosol sprays also are available for freshening the air, but the surfaces must be dry so more mold will not form.

To Remove Mildew From Surfaces
If dark grey or black spots appear on walls, behind curtains, under rugs, or other damp places, it is probably mildew. You can usually remove this by
scrubbing thoroughly with this solution:
8 to 12 ounces liquid household bleach
2 ounces liquid detergent
1 gallon of water
Washing mildew from the surface is only a temporary solution. If conditions are not changed, the mildew will probably return.
Controlling External Water Problems In The Home
Control of surface water and moisture vapor in and around a home can prevent conditions that are favorable for mildew growth inside the house and
wood-decaying fungus in the structure of the house.
Lot Drainage
Every home should have grading and landscaping plans that help control all surface water on the lot.
Water must not be allowed to stand against the foundation on the outside of the home. If it does, the surface water can find its way down the foundation
wall and under the footing; it then resurfaces as water in the crawlspace area.

To prevent water from standing against the house, provide a minimum slope of 6 inches 10 feet away from the house on all sides. Accomplish this by
grading soil against the foundation wall or by lowering the elevation of the yard or both.

If the grade is higher on one side of the house, form a swale or a grass drain to intercept surface water as it flows toward the house. This drain should
have the slope of 24 inches per 100 feet so water drains away. The swale or grass drain should look a natural part of the landscape and be designed
for convenient use of a lawn mower.

Roof Drainage
To disperse water away from a house's foundation during rainstorms, install gutters and downspouts with outlets on splash blocks or storm-well
A gutter system should have a correctly aligned downspout. Install leaf guards in the gutter system to prevent clogging in the drainage. Frequently
inspect your gutters and remove accumulated leaves.

Install masonry splash blocks (a precast concrete product) at the base of the downspouts to receive the gutter water. They prevent erosion and rapidly
conduct and release the water at least two feet from the foundation wall. Drainage away from the house prevents water from accumulating near the
foundation wall. Some splash blocks are plastic but lack the sturdiness and durability of masonry or concrete.

Water from the downspout also can be released into a clay tile or flexible pipe and conducted underground to a suitable release outlet. PVC may be
used to conduct the water for some distance underground to a release point. (Rigid and flexible pipe are satisfactory underground and require a
minimum of maintenance.) Large downspouts with a minimum of sharp turns from the gutter to the release near the groundline have fewer clogging

A footing drain also can be installed to carry excess water away from the house. Dig a trench 12 to 18 inches wide (and 12 to 18 inches deep) directly
under the eaves. Lay in a couple of inches of gravel. On top of the gravel, lay 4-inch perforated drain pipes that are slightly sloped toward a solid drain
pipe to carry the rain water well away from the house to a drywell, storm sewer, or stream. Fill the trench to ground level with more gravel. Water from
the roof settles into the trench and is picked up by the perforated pipe and removed from the vicinity of the house before it builds up against the
foundation walls and forces its way through.

Foundation Waterproofing
Well-graded lots, correctly installed landscaping, and waterproofed foundations help prevent objectionable water problems around and under a
structure. A masonry wall also can be effectively waterproofed with a bituminous foundation coating material. Mop the masonry foundation wall (below
grade) with one or two coats of a coating material. This material is one method to help prevent excess water under the home. Use with the other
systems previously discussed.
Controlling Moisture in the Home