Controlling moisture in buildings. (Technical Briefs).

Technical Briefs presents short but information-pached items on a variety of field-related topics in environmental health. The information this month is provided courtesy of two sister corporations that specialize in environmental health laboratory testing: Precision Analytical Laboratories, Inc., and Aerotech Laboratories, Inc., both based in Phoenix, Arizona. Readers interested in receiving additional information about Precision Analytical Laboratories and Aerotech Laboratories, including information about how to subscribe to a free "Tech Tips" listserv for environmental health professionals, can contact Aerotech Laboratories/Precision Analytical Laboratories at 2020 West Lone Cactus Drive, Phoenix, AZ 85027. Aerotech and Precision also can be reached by phone at [phone] or [phone]; by fax at [phone]; or on the Internet at <http://www.aerotechlabs.com> or <http://www.palabs.com>.

The following article contains excerpts from the National Parks Service Preservation Brief #39 on controlling moisture in buildings. For the full article, visit http://www.c.r.nps.-gov/hps/tps/briefs/brief39.htm.

Moisture comes from a variety of external sources. Most problems begin as a result of the weather, in the form of rain or snow, high ambient relative humidity or high water tables. But some of the most troublesome moisture damage in buildings may be from internal sources, such as leaking plumbing pipes, components of heating, cooling, and climate control systems, and sources related to the use or occupancy of the building. The five most common sources of unwanted moisture are

1. above-grade exterior moisture entering the building,

2. below-grade ground moisture entering the building,

3. leaking plumbing pipes and mechanical equipment,

4. interior moisture from household use and climate control systems, and

5. water used in maintenance and construction materials.

Knowing the five most common sources of moisture that cause damage to building materials is the first step in diagnosing moisture problems. It is also important to understand the basic mechanisms that affect moisture movement in buildings. Moisture transport, or movement, occurs in two states: liquid and vapor. It is directly related to pressure differentials. For example, water in a gaseous, or vapor, state, will move as warm moist air from its high-pressure area to a lower-pressure area where the air is cooler and drier. Liquid water will move as a result of differences in hydrostatic pressure or wind pressure. It is the pressure differentials that drive the rate of moisture migration in either state. Because the building materials themselves resist this moisture movement, the rate of movement will depend on two factors: the permeability of the materials when affected by vapor and the absorption rates of materials in contact with liquid.

In the past, buildings were traditionally designed to deal with the movement of air. For example, cupolas and roof lanterns allowed hot air to rise and provided a natural draft to pull air through buildings. Cavity walls were constructed in both frame and masonry buildings to allow moisture to dissipate in the air space between external and internal walls. Radiators were placed in front of windows to keep cold surfaces warm, thereby reducing condensation on these surfaces. Many of these features, however, have been altered over time. Changes in use also affect moisture movement, particularly in commercial and industrial buildings with modern mechanical systems. Therefore, the way a building handles air and moisture today may be different from that intended by the original builder or architect, and poorly conceived changes may be partially responsible for chronic moisture conditions.

Three forms of moisture transport are particularly important to understand with respect to buildings-infiltration, capillary action, and vapor diffusion.

Infiltration is created by wind; temperature gradients (hot air rising); ventilation fan action; and the stack, or chimney, effect, which draws air up into tall vertical spaces. Infiltration does not actually involve movement of liquid water, but it is the vehicle by which dampness, as a component of air, finds its way into building materials. Older buildings have a natural air exchange, generally from one to four changes per hour, which may help dilute moisture within a building. The tighter building construction is, however, the lower will be the infiltration rate and the natural circulation of air. In the process of infiltration, moisture that has entered the building and saturated materials can be drawn in and out of materials, thereby adding to the dampness in the air. Inadequate air circulation accelerates the deterioration of materials. To reduce the unwanted moisture that accompanies infiltration, it is best to implement maintenance and repair treatments that close joints and weather-strip windows whi le providing controlled air exchanges elsewhere. The worst approach is to seal the building so completely that it cannot breathe.

Capillary action occurs when moisture in porous building materials, such as masonry, is wicked up or travels vertically as it evaporates to the surface. The liquid in the material is attracted to the solid surface of the pore structure; thus, capillary action is often called "rising damp," particularly when found in conjunction with ground moisture. It should not be confused with moisture that laterally penetrates a foundation wall through cracks and settles in the basement. Not easily controlled, most rising damp comes from high water tables or a constant source under the footing. In cases of damp masonry walls with capillary action, there is usually a whitish stain or horizontal tidemark of efflorescence that fluctuates seasonally about 1 to 3 feet above grade. This tidemark is full of salt crystals that have been drawn from the ground and building materials along with the water, making the masonry even more sensitive to additional moisture absorption from the surrounding air.

Capillary migration of moisture may occur in any material with a pore structure. The best approach to dealing with rising damp in building materials is to reduce the amount of water that comes into contact with the materials. If chronically high water tables make that approach unfeasible, it may be necessary to introduce a horizontal damp-proof barrier, such as a slate course or a lead or plastic sheet, to stop the vertical rise of moisture. Moisture should not be sealed into the wall with a waterproof coating, such as cement parging or vinyl wall coverings. Sealing the wall will only increase the pressure differential, forcing the capillary action--and its destruction of materials--higher up the wall.

Vapor diffusion is the natural movement of pressurized moisture vapor through porous materials. It is most readily apparent as humidified interior air moves out through walls to a cooler exterior. In a hot and humid climate, the reverse will happen as moist hot air moves into drier air-conditioned interiors. The movement of the moisture vapor is not a serious problem until the dew point temperature is reached and the vapor changes into the liquid moisture known as condensation. Condensation can occur within a wall or on interior surfaces. It usually takes place on a surface or film, such as paint, where there is a change in permeability.

Long-term, undetected condensation or high moisture content can cause serious structural damage as well as an unhealthy environment that is heavy with mold and mildew spores. Reducing the interior-exterior pressure differential and the differences between interior and exterior temperature and relative humidity helps control unwanted vapor diffusion. Sometimes this solution can be achieved by a reduction in interior relative humidity Vapor barriers, such as heavy plastic sheeting laid over damp crawl spaces, can be remarkably helpful in stopping vapor diffusion from damp ground into buildings. Experts differ, however, about the appropriateness of vapor harriers and when and where to use them, as well as the best way to handle natural diffusion in insulated walls.