Joseph Lstiburek is a principal of Building Science Corporation in Chestnut Hill, Massachusetts. John Carmody is an architect at the Underground Space Center at the University of Minnesota.

Moisture problems occur in buildings throughout North America, in almost every climate. The most common symptoms are mold, mildew, and condensation, and these can impair the health of the occupants, cause discomfort, and decrease the life of the structure.

Understanding Relative Humidity

Air contains varying amounts of moisture in the gaseous or vapor form. The actual amount of moisture contained in air is referred to as its absolute humidity. More precisely, the absolute humidity is the ratio of the mass of water vapor to the mass of dry air in a given sample of air.

Air is a mixture of several gases, including nitrogen, oxygen, and water vapor. The total air pressure exerted by a volume of air in a given container on that container is the sum of the individual (partial) pressures of these gases. The vapor pressure is the partial pressure of the water vapor.

The warmer air is, the more moisture it can hold. Relative humidity is the ratio of the amount of moisture in the air to the maximum amount of moisture the air can hold at a given temperature. Air is said to be saturated (at 100% relative humidity) when it contains the maximum amount of moisture possible at a specific temperature. Air holding half the maximum amount of moisture at a given temperature has a relative humidity of 50%. Relative humidity near surfaces is the single most important factor influencing moisture problems in buildings.

Mold and Mildew

Mold and mildew (two words for the same thing) are simple plants, of the group known as fungi, that grow on the surfaces of objects when the relative humidity is high. Mold discolors surfaces, causes odor problems, and causes deterioration of building materials. Mold can also produce allergic reactions, hypersensitivity, and infectious diseases. Certain fungi found in indoor air produce mycotoxins, which can be carcinogenic (induces cancer), teratogenic (induces birth defects), immunosuppressive (reduces immune system performance), or oxigenic (poisons tissues).

Most fungi have microscopic wind-borne spores. These spores are buoyant and can enter buildings as part of natural (wind- and temperature-driven) or controlled (fan-forced) air flow. Although their concentration varies seasonally, mold spores are almost always present in the outside air.

Fungi generally grow when the temperature is between 50deg.F and 100deg.F, with optimum growth occurring between 75deg.F and 95deg.F. However, some types of fungi can grow at temperatures as low as 35deg.F and as high as 120deg.F. Many building materials (wood products, cotton fabrics, wool fabrics, hemp fabrics, organic dust and lint, soaps, oils, paints, adhesives, certain plastics, and vinyls) provide nutrients for fungi.

Mold needs moisture to produce enzymes and to perform metabolic activities to digest carbohydrates, fats, and proteins. The optimum relative humidity for fungal growth is 70%. Since relative humidities are dependent on both temperature and vapor pressure, control strategies usually focus on either or both of these factors.

Mold Growth in Heating
and Mixed Climates

In heating climates, mold grows on interior surfaces during the winter. Typically, the interior surfaces of exterior walls are cool (due to heat loss), while moisture levels within the conditioned space are high. Mold growth can be controlled in two ways: (1) by preventing the interior surfaces of exterior wall and other building assemblies from becoming too cold and (2) by limiting interior moisture levels. Adding insulation to a wall or ceiling raises the temperature of the interior surface. Controlled ventilation and control of moisture sources limit interior moisture levels.

In buildings with similar insulation levels, interior humidity levels must be kept lower in colder climates. For example, a 25% interior relative humidity at 70deg.F would probably be appropriate for Minneapolis; in a similar building in Cincinnati, interior relative humidities up to 35% at 70deg.F should be fine. During the heating season in milder climates, interior moisture levels should generally be kept at 35% to 45% relative humidity at 70deg.F.

When there is excessive ventilation or excessive air change by infiltration and exfiltration during the heating season, uncomfortably low relative humidities can also occur. When relative humidities drop below 20%, membranes in the human respiratory system begin to dry, and defenses against infection begin to fail. At low relative humidities people wearing contact lenses become uncomfortable, and static electricity discharges can affect equipment and people. Relative humidities should be maintained above 25%. The higher the desired interior relative humidity, the higher the thermal resistance (R-value) necessary to control relative humidities adjacent to interior surfaces.

Mold Growth in Cooling Climates

Interior mold growth also occurs in cooling climates, because interior surfaces are typically cold from air conditioning, while interior moisture levels may be too high. When exterior hot air is cooled, its relative humidity increases. If the exterior hot air is also humid, cooling it can easily raise its relative humidity above the 70% optimal for mold growth.

Cold spots are often created when cold (air conditioned) air is blown against interior gypsum board surfaces due to poor design, location, or performance of supply air diffusers. Although this cold air is typically dehumidified by the air conditioner, there are often high levels of airborne moisture within the room, which contact the cooled surface.

If exterior humid air comes in contact with the cavity side of cooled interior gypsum board, its relative humidity can rise above 70% and mold growth can occur in the cavity. Impermeable wall coverings such as vinyl wallpaper can make the problem worse by trapping moisture between the interior finish and the gypsum board.

One of the most practical solutions in controlling mold and other biological growth in cooling climates is to prevent hot, humid exterior air from contacting the interior cold (air conditioned) gypsum board surfaces. This is most commonly done by maintaining the conditioned space at a positive air pressure relative to the exterior and installing an exterior vapor diffusion retarder. Airtight construction helps to pressurize building assemblies.

Interior moisture levels within conditioned spaces in cooling climates should be limited to 60% relative humidity at 75deg.F. This can be accomplished by dehumidification and source control, discussed later in this article.

Carpets located on cold surfaces such as concrete slabs are particularly sensitive to dust mite growth. Like mold, dust mites grow at about 70% relative humidity. Carpets on cold surfaces should be avoided, or these surface temperatures should be raised by installing insulation between the slab and the carpet. Slab edge insulation, though it is not cost-effective for energy savings in hot climates, should be installed in new construction for health reasons alone.