Issue link: https://hi.iaq.net/i/248503
www.epa.gov/iaq/moisture The Basics of Water Behavior Water occurs at temperatures often found in buildings as a liquid, a gas (water vapor) and in an in-between state (adsorbed on solid surfaces). Water vapor migrates from one place to another in several ways: • Water vapor in the air goes where the air goes. This is, by far, the fastest and largest mechanism of water vapor transport. All air, whether inside or outside of buildings, is constantly moving from areas of higher pressure to areas of lower pressure. If dry air is pulled into the building from outdoors, it will dehumidify the indoor air. If humid air is pulled in, it will add to the humidity load that must be removed by the mechanical system. Liquid water moves from one place to another in several ways: • Water runs through pipes and vessels. Water moves from higher pressure to lower pressure in pipes and fixtures. A leak in a pressurized pipe or tank can release much more water than a similar leak on the drain side of the plumbing system. • Water runs downhill. Rainwater, surface water, spilled water, water on the drain side of plumbing fixtures and water in condensate pans are all affected by gravity. • Water vapor migrates through materials by diffusion. Liquid water may not be present and nothing may appear to be wet, but water vapor can still slowly migrate through what appears to be solid materials. Vapor molecules will slowly bump their way through the spaces between molecules of the material. The molecules are moving from an area of higher water vapor concentration to lower water vapor concentration. The more porous a material is, the easier it is for water vapor to diffuse through it. The rate water vapor diffuses through a material is measured in "perms." Higher perms mean higher water vapor flow rates. • Water wicks upwards. Water wicks up through tiny cracks and holes. To see wicking in action, stand two plates of glass on edge in ¼ inch of water. Push them together and as they get closer the water wicks up between them. The closer together the plates, the higher water wicks. This happens because water molecules are attracted to the glass and to other water molecules. What works for cracks works for pores in materials. Stand a porous material like paper, wood, concrete, a sponge or gypsum board on edge in ¼ inch of water and the water wicks up into the material. How high it goes depends on pore size and how quickly the water can dry out the sides to the air. Water wicks through materials in a process called "capillary action." When water is in tiny pores, gravity is not the most important force acting on it. Water changes from liquid to gas (evaporation) and from gas to liquid (condensation). • Water evaporates from liquid water on surfaces, becoming water vapor. Most of the water vapor that originates inside buildings is the result of evaporation from open containers, sprays or damp porous materials. Showers, fountains, pools, sinks, pots on stoves, dishwashers and wash water on floors are all sources of indoor humidity, as are the building occupants themselves. People, plants and animals release water vapor. In typical office spaces, the occupants are probably the main source of water vapor. Wet materials such as wet concrete or exposed earth in crawl spaces or basements are also sources of indoor humidity. The evaporation rate depends on many factors including the temperature of the water and the relative humidity of the air. The warmer the water, the drier the air next to the wet surface. The faster air blows across a wet surface, and the larger the exposed surface • Water runs along the bottom or sides of materials. For the same reasons that water wicks up through porous materials, water can cling to the sides and bottoms of materials. Water is attracted to many materials and to itself. Water from rain or a plumbing leak may travel many feet along the bottom of a floor joist or roof truss before collecting in a drop big enough to fall. When water first condenses on a mirror or a cooling coil, it clings to the vertical surfaces. Water does not run down until the droplets become large enough for gravity to overcome the intermolecular forces. 24