EC 1437 • $2.50 Reprinted March 2008 Home moisture problems oisture problems can occur in many holding capacity of the cool air is less Inside: places in homes and for many rea- than that of warm air, the excess moisture M sons: when high levels of moisture condenses, usually on the first cold surface. Indoor and outdoor enter building cavities or get inside, when The temperature at which the air can hold causes of moisture excessive moisture is produced indoors, or no additional moisture is called the “dew when indoor air comes into contact with point.” • cold surfaces such as single-pane windows Indoor humidity is the result of the or uninsulated walls. Excess moisture often amount of moisture produced inside, the Drainage problems originates outside the structure, as with relative humidity of the air outdoors, and and how to fix them foundation drainage problems, or it may the building air-exchange rate. Depending be the result of activities by the occupants on the building air-exchange rate, the same • indoors. Cold surfaces usually are the result amount of moisture may cause no moisture Symptoms of of air leakage or inadequate insulation in problems in one house, while resulting in wood decay building cavities, or in rooms where less serious mold and mildew in another. In heat is provided in the winter. coastal areas, where the outside relative • Solving home moisture problems usually humidity may be quite high year round, starts with a little detective work—looking outside ventilation air may produce little Ventilation for moisture sources outside the house and drying effect. and dehumidification determining how to control them, mini- Lack of humidity can be a problem in mizing production of humidity inside the houses with very high air-exchange rates. house, and installing ventilation where Houses “dry out” when cold winter air needed. Localized moisture problems often enters the house through infiltration and can be solved by warming up interior is warmed up by the heating system. Since surfaces. In some cases, moisture problems the total amount of moisture cold air can may be not be easily or cheaply solved, and hold is small even at 100 percent RH, use of a dehumidifier or whole-house venti- when it is warmed up indoors, its relative lation system may be part of the solution. humidity is lower and it feels dry to us. This is why houses with serious air leakage What causes moisture problems? problems in areas east of the Cascades often require humidifiers to maintain a comfort- The amount of water vapor that air can able level of indoor humidity. After weath- hold depends on its temperature. Since erization or in new construction, houses warm air can hold far more moisture than should not require humidifiers to maintain cold air, we talk about “relative humid- 30 to 50 percent RH, which minimizes ity” (RH). During the winter, condensation indoor air pollution problems and which occurs when warm indoor air is cooled most people find comfortable. down as it comes into contact with a cold surface, such as a single-pane window or an uninsulated wall. Since the moisture- David M. Brook, former Extension agent, Multnomah County, Oregon State University. Indoor relative can be a sign of excess moisture in the air, humidity levels indicating a need to stop air leaks cool- between 30 and 50 per- ing the cavity or to warm the surface with cent RH are comfort- insulation. able for most people, Damp feeling The sensation of dampness and minimize health is common in areas with high humidity. and structural moisture Surface discoloration, staining, texture problems. High humid- changes These usually indicate some mois- ities lead to mold, ture damage, no matter what the material. mildew, and high con- These changes may appear as black or dark centrations of biological streaks or lines bordering a discoloration. microorganisms such The area may or may not feel wet. as bacteria and dust Mold and mildew often appear as a mites, which can affect discoloration, which may be white, orange, the health of occupants. green, brown, or black. Surface conditions Condensation discol- that may indicate decay often are noticed ors ceilings and walls. as a musty odor. They can be found under High humidity in the carpets, behind cupboards, on framing building’s structural Figure 1.—Gutters, a between subfloors, in crawl spaces, and cavities, such as attics, slope away from the house, in attics. Mold and mildew can get a start walls, and foundation area, can lead to and drainage tile in the whenever the relative humidity of air near a wood decay and eventually to structural foundation keep the left surface is above 70 percent RH. They grow failure. If in wintertime relative humidity is side of this house dry. No fastest at temperatures above 40°F. above 50 percent, you should takes steps to gutters, a slope toward the Deformed wood surfaces control sources of excess moisture in your Wood swells house, and no drainage home, as described in this publication. Low when it becomes wet, and warps, cups, and tile expose the right side to indoor humidity may prevent moisture cracks when it dries. moisture damage. problems in localized areas, such as closets Wood decay Wood rot and decay indicate or building cavities, but it doesn’t ensure advanced moisture damage. Unlike surface that moisture problems won’t occur else- mold and mildew, wood decay fungi pen- where in the house. etrate the wood and make it soft and weak. When you understand the sources of the Look for any type of rot or mushroom-like moisture in your home and the way mois- growths. (See page 11 for more information ture moves, you should be able to control it on detecting wood decay.) effectively. Sweating pipes, water leaks, and dripping Water vapor may condense and drip off Symptoms of excess cold pipes, or the pipes may be leaking. indoor moisture Peeling, blistering, cracking paint Moisture Many signs of moisture problems and may be moving from outside or inside the excess moisture are readily apparent, but home to damage paint. others are difficult to detect. Here are Crusty, powdery, chipping concrete and some common clues of indoor moisture masonry A buildup of salt or other powdery problems. substance indicates that water has evapo- Odors Odors increase in intensity with rated after moisture has moved through it. high relative humidity. Musty smells may Freeze-and-thaw cycles speed the process signal mold, mildew, or rot. Everyday of deterioration, causing chipping and household odors that seem to linger may be crumbling. a signal of too much moisture in the air. High indoor humidity Indoor humidity Frost and ice on cold surfaces Frost or ice levels are best when maintained between on windows, attic framing, or any surface 30 and 50 percent RH. They can be mea- is an indication of trouble. Condensation sured easily with a hygrometer which is 2 a low-cost relative humidity indicator, sometimes coupled with a thermometer How moisture gets into houses and available at hardware stores and home • Bulk moisture—in the form of rain, snow or ground water, centers. Remember, low indoor humidity leaking into basements, crawl spaces, roofs, and walls. does not ensure the absence of moisture • Capillary action—movement of water through a porous problems elsewhere in the house. material, such as vertical movement through a cement block wall, like a sponge or paper towel absorbing moisture. Outdoor sources of moisture • Air movement—water vapor transported through air leakage. Poor foundation drainage around the • Vapor diffusion—moisture permeating through solid surfaces, house often is the major source of exterior such as cement, gypsum board, or wood. moisture getting into the house (Figure 1). Proper drainage is a critical first line of defense against moisture problems. Precipi- The bottom edge of the siding should be tation, surface water, ground water table, painted and sealed to prevent moisture and outdoor water use can change season- from rising by capillarity. If necessary, ally, creating problems that may show only install wedges in the siding to provide a at certain times in the year. capillary break. Foundation drainage Plugged downspouts Construction details Flat ledges, inade- and blocked foundation footing drains quate drip edges, and other construction are common sources of outdoor moisture details also can allow exterior moisture to getting into the house. If indoor moisture enter the house. Old roof shingles and miss- problems suddenly develop, these areas are ing flashing around chimney and plumbing prime suspects. Ground that slopes toward vent stacks may allow moisture into attics the house also may contribute to wet base- or walls for many years before the problem ments or crawl spaces. is discovered. Slabs If no moisture barrier was installed Blocked exterior air circulation Foliage underneath the slab when it was poured, close to the dwelling or items such as fire- water in the ground may permeate through wood that are stored next to the house can the slab and then evaporate into the air block air circulation and cause local areas inside the house. This often is the source of high humidity. Roof and soffit vents can of the problem in a garage that has been become clogged by dust, leaves, or tree converted to living space. flowers. Below-grade walls Moisture may move up through cement block foundations, damp- Indoor sources of moisture ening the walls above and raising indoor Many sources of excess moisture can humidity levels even though the basement lead to high indoor humidity and cause or crawl space is dry. If ground water is a a wide variety of problems (see Figure 2, suspect, use the capillary test on page 4 to page 5, and “Household moisture sources,” determine whether moisture is wicking up last page).