Moisture control is fundamental to the proper functioning of any building. Controlling moisture is important to protect occupants from adverse health effects and to protect the building, its mechanical systems and its contents from physical or chemical damage. Yet, moisture problems are so common in buildings, many people consider them inevitable.
Excessive moisture accumulation plagues buildings throughout the United States, from tropical Hawaii
to arctic Alaska and from the hot, humid Gulf Coast to the hot, dry Sonoran Desert. Between 1994 and 1998, the U.S. Environmental Protection Agency (EPA) Building Assessment Survey and Evaluation (BASE) study collected information about the indoor air quality of 100 randomly selected public andprivate office buildings in the 10 U.S. climaticregions. The BASE study found that 85 percent of the buildings had been damaged by water at some time and 45 percent had leaks at the time the data were collected.2
Moisture causes problems for building owners, maintenance personnel and occupants. Many common moisture problems can be traced to poor decisions in design, construction or maintenance. The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) notes that, more often than not, the more serious problems are caused by decisions made by members of any of a number of different professions.3 However, such problems can be avoided with techniques that are based on a solid understanding of how water behaves in buildings.
Moisture control consists of:
- Preventing water intrusion and condensation in areas of a building that must remain dry.
- Limiting the areas of a building that are routinely wet because of their use (e.g., bathrooms, spas, kitchens and janitorial closets) and drying them out when they do get wet.
To be successful, moisture control does not require everything be kept completely dry. Moisture control is adequate as long as vulnerable materials are kept dry enough to avoid problems. That means the building must be designed, constructed and operated so that vulnerable materials do not get wet. It also means that when materials do get wet, the building needs to be managed in such a way that the damp materials dry out quickly.
Moisture Damage in Buildings
Moisture can damage building materials and components in the following ways:
• Prolonged damp conditions can lead to the colonization of building materials and HVAC systems by molds, bacteria, wood-decaying molds and insect pests (e.g., termites and carpenter ants).
• Chemical reactions with building materials and components can cause, for example, structuralfasteners, wiring, metal roofing and conditioning coils to corrode and flooring or roofing adhesives tofail.
• Water-soluble building materials (e.g., gypsum board) can return to solution.
• Wooden materials can warp, swell or rot.
• Brick or concrete can be damaged during freeze-thaw cycles and by sub-surface salt deposition.
• Paints and varnishes can be damaged.
• The insulating value (R-value) of thermal insulation can be reduced.
The following photos show some of the damage that can result from moisture problems in buildings.
|Mold growth on painted concrete masonry. The cool masonry wall separates a classroom from an ice rink. Humid air in the classroom provides moisture that condenses on the painted surface of the masonry. That moistureallows mold to grow on the paint film.|
|Corrosion of galvanized fluted steel floor deck. The floor is at gradelevel. The source of the water is rainwater seepage.|
|Rainwater leaks in a rooftop parapet wall result in damaged plaster and peeling paint. Rainwater is drawn into this brick assembly by capillary action, and the moisture is aided in its downward migration by gravity. The peeling paint contains lead and results in an environmental hazard as well as physical damage to the plaster.
See more photo examples in the Moisture Control Guidance for Building Design, Construction and Maintenance Report at epa.gov.
Moisture Problems are Expensive
Health problems and building damage due to moisture can be extremely expensive. Berkeley Lab estimates that the annual asthma-related medical costs attributable to exposures to dampness and mold total approximately $3.5 billion in the U.S.8 But many more adverse health outcomes due to damp buildings have been reported, each with associated costs of its own. And damage to the building itself is alsocostly. Building owners and tenants bear a significant proportion of these costs, including:
- Absenteeism due to illnesses such as asthma.
- Reduced productivity due to moisture-related health and comfort problems.
- Increased insurance risk, repair and replacement costs associated with corroded structural fasteners, wiring and damaged moisture-sensitive materials.
- Repair and replacement costs associated with damaged furniture, products and supplies.
- Loss of use of building spaces after damage and during repairs.
- Increased insurance and litigation costs related to moisture damage claims.
How Water Causes Problems in Buildings
Mention water damage and the first image that comesto mind for most people is liquid water in the form
of rain, plumbing leaks or floods. Many water leaksare easy to detect. When it rains, water may driparound skylights, or a crawl space may fill with water. If a toilet supply line breaks, the floor will likely be flooded.
On the other hand, many water-related problems are less obvious and can be difficult to detect ordiagnose. For example, the adhesive that securesflooring to a concrete slab may not cure properly if the slab is damp, resulting in loose flooring and microbialgrowth in the adhesive. Or, humid indoor air may condense on the cool backside of vinyl wallpaper that covers an exterior wall, providing ideal conditions for mold to grow. These problems are less obvious than a leak because water is not running across the floor,and the real damage is being done out of sight underflooring or behind wallpaper.
Moisture problems are preventable. They do not happen until water moves from a source into some part of a building that should be dry. The actual damage begins after enough moisture accumulates to exceed the safe moisture content limit of moisture- sensitive materials.
To diagnose or prevent a moisture problem, keep in mind four key elements of moisture behavior in buildings:
- Typical symptoms of moisture problems. They include corrosion of metals, the growth of surface mold or wood-decaying molds, insect infestations, spalling exterior brick or concrete, peeling paint,failing floor adhesives, stained finishes and healthsymptoms.
- Sources of moisture. Among them are rainwater, surface water, ground water, plumbing water, indoor and outdoor sources of humidity and sewer water.
- Transport mechanisms. They include liquid water leaking through holes, wicking through porous materials, or running along the top or bottom of building assemblies and water vapor carried by warm, humid air leaking through assemblies and by diffusion through vapor-permeable materials.
- Common failures of moisture control elements and systems. Moisture controls include site drainage, gutter systems, above- and below-grade drainageplanes, effective flashing, condensate drainageand humidity controls. Failures can occur during any phase of a building’s life and may include poor site selection or design, poor material or equipment selection, improper installation or sequence of building materials and equipment,insufficient coordination between trades during construction and insufficient or impropermaintenance of materials or equipment.
Moisture Control Principles for Design
To control moisture for long building life and good indoor air quality, follow these three principles:
- Control liquid water.
- Prevent excessive indoor humidity and water vapormigration by air flow and diffusion in order to limitcondensation and moisture absorption into cool materials and surfaces.
- Select moisture-resistant materials for unavoidably wet locations.
Armed with an elementary understanding of these principles, readers will be prepared to control moisture and prevent the vast majority of moisture problems that are common in buildings.
Read more in the Moisture Control Guidance for Building Design, Construction and Maintenance Report at epa.gov.