Your home's interior can become wet for various reasons: flooding, plumbing leak, leaky roof, water spill, sewer back-up, or wet basement seepage. Cleanup should begin as soon as possible after the flooding stops to prevent structural damage, rotting of building materials and household furniture, and very importantly, mold growth.
Molds can grow on just about any surface if the relative humidity is greater than 70% or there is liquid water. Molds produce tremendous amounts of spores which can become airborne. Disturbed molds can form invisible clouds with more than 100,000 CFU/m3 (colony forming units per cubic meter of air). The airborne spores spread to other sites where they can grow if provided favorable conditions. They can also be inhaled. Some individuals can have allergic reactions to the inhaled mold spores. Individuals who have health conditions which have weakened their natural immune defenses may acquire serious illness from inhaled mold spores. For these health reasons and to minimize future mold growth in your home, it is important for you to clean and dry your home as soon as possible after a flood.
Wet building materials and surfaces dry as water evaporates from the material or surface. Evaporation is a phase change, from liquid to vapor, which requires an input of energy. The energy can come from added heat (ex. hand-held hair dryer), a warm surface (ex. body heat is removed by evaporating sweat), or the air (ex. air cools as it passes through an evaporative cooling pad).
The air-drying rate depends on the amount of water vapor in the air. Things dry faster if the surrounding air is dry. Conversely, things dry more slowly if the air is humid. Natural (unheated) air can dry moist materials if the air is drier than the materials.
Air is commonly heated to speed the drying process (ex. clothes dryer, hair dryer). The heated air, which has a greater moisture-holding capacity, picks up moisture as it passes through the damp material. The moist air is then exhausted to the outside. The air gives up some of its heat, as the liquid water is evaporated, and picks up some water vapor as it passes through the damp material. The moisture-laden air is exhausted. Drying continues until the material is dried.
Ventilation (air exchange) using outside air can be used to dry your home by removing humid indoor air and replacing it with dry outside air. The dry air absorbs moisture and is replaced when it becomes humid. The cycle is repeated until the building materials and furnishings are sufficiently dry. Air exchange can be accomplished either naturally by opening windows and doors or mechanically by turning on bathroom, kitchen, window or other exhaust fans.
|
|
Ventilation drying can be enhanced by using the furnace or other heaters to heat the air, which increases the air's water vapor holding capacity. Figure 1 illustrates how much the maximum moisture holding capacity of air increases as air temperature increases.
Ventilation drying can also be enhanced by increasing the air exchange rate. Excessive air exchange, however, may not allow the drying air sufficient time to absorb moisture before it is exhausted. This can waste energy, especially if the drying air is heated.
Table 1 indicates when outside air can be effectively used to dry a room or house. Outside air can be used when the outside and indoor air conditions intersect at a position with a *. At other conditions, one of several other dehumidification methods should be used to dry your home (i.e., dehumidifier, air-conditioner, or desiccant dehumidifier).
Heat may be necessary to maintain the indoor air temperature when using outside air for drying. The results in Table 1 assume that the indoor temperature is maintained at the levels listed. When the outside air temperature drops below freezing care must be taken to prevent water pipes from freezing where cold outside air is brought into the house.
Example using Table 1: If the outside air is 65°F and 50% relative humidity while the inside air is 70°F and 50% relative humidity, a room dehumidifier is recommended for further drying. If the indoor air is at 70°F and 70% relative humidity while outside air is 65°F and 50% relative humidity, outside air ventilation is recommended. Cold air has a very low moisture-holding capacity. Figure 1 shows how much the moisture holding capacity of cold air increases when it is brought into a house and heated. This increase in moisture-holding capacity enhances drying. As the outside temperature decreases, outside air ventilation drying always becomes the recommended drying method. The last line in Table 1 lists the outside air temperature below which ventilation drying is always recommended, irrespective of outside relative humidity. Care must be taken to prevent water pipes from freezing when below freezing outside air is brought into the house.
Outside air is not recommended for some combinations of outside weather and indoor air conditions in Table 1. Dehumidifiers, air-conditioners, or desiccant dehumidifiers can be used when these conditions exist.
-----------Indoor Relative Humidity (%)-----------
Outside 50 70
Outside Relative ----------------------- -----------------------
Temperature Humidity Indoor Temperature (F) Indoor Temperature (F)
(F) (%) 60 65 70 75 60 65 70 75
---------------------------------------------------------------------------
75 70
50 * *
30 * * * * * *
---------------------------------------------------------------------------
70 70 *
50 * * *
30 * * * * * * *
---------------------------------------------------------------------------
65 70 * *
50 * * * *
30 * * * * * * * *
---------------------------------------------------------------------------
60 70 * * * *
50 * * * * * *
30 * * * * * * * *
---------------------------------------------------------------------------
55 70 * * * *
50 * * * * * * *
30 * * * * * * * *
---------------------------------------------------------------------------
50 70 * * * * * *
50 * * * * * * * *
30 * * * * * * * *
---------------------------------------------------------------------------
45 70 * * * * * * *
50 * * * * * * * *
30 * * * * * * * *
---------------------------------------------------------------------------
40 70 * * * * * * * *
50 * * * * * * * *
30 * * * * * * * *
---------------------------------------------------------------------------
Outside temperature (F) 34 39 45 51 45 50 56 61
Table 1 is based on a drying rate of four gallons per day and one air change per hour. Ventilation drying using outside air is recommended when the difference between the outside and indoor air's moisture holding capacity is 0.0015 lb H2O/lb DA or greater. This corresponds approximately to a dew-point temperature difference of 5°F. When the outside air dew-point temperature is at least 5°F lower than the indoor air dew-point temperature, ventilation air drying is recommended.
Houses or rooms can be dried using common room dehumidifiers. They come in various capacities that can remove between one and six gallons of water per day. They are not recommended for use in spaces where the air temperature is below 65°F (18°C); at temperatures below 65°F common room dehumidifiers have problems with frost forming on the cooling coil.
Common room dehumidifiers have a refrigerating unit (i.e., motor, compressor, condenser, and cooling coil), an air-circulating fan, and a water collecting or disposal device. The fan draws humid room air through the cooling coil. The refrigeration unit cools the cooling coil to a temperature below the air dew-point temperature. As the humid air passes over the cooling coil, it is cooled, and some of the water vapor condenses on the cooling coil. The water drains off the cooling coil and is either collected in a container for disposal or passed through a tube to a drain. The cooled air is passed through the condenser where it is reheated before it is returned to the room. By circulating the room air through the dehumidifier, water vapor is removed from the air and the relative humidity of the room is reduced.
In hot weather, when air-conditioning is desirable, whole house and room air-conditioners can both cool and dehumidify air. Air-conditioners have refrigerating units like dehumidifiers. As humid air passes over the cooling coil, water vapor is condensed and removed from the air. Air-conditioners, however, do not pass the air through the condenser to reheat it. Air-conditioner condensers dump their heat outside. Air-conditioners can be used to speed drying in warm weather when the room or house air temperature is above 75°F and air-conditioning is desired.
Desiccant dehumidifiers can operate at cooler temperatures than common room dehumidifiers. They are more expensive because the equipment needed to implement desiccant drying is more complex than common dehumidifiers.
Desiccants are materials that readily attract and adsorb moisture. In a desiccant dehumidifier, humid air is passed through a desiccant. The desiccant adsorbs water vapor, drying the air. When the desiccant is filled with moisture, it is heated to drive off the moisture. The moisture is discarded via an air stream exhausted to the outside.
Cleanup and drying is very important after a flood in your home to prevent structural damage and prevent mold growth. It may take several months to thoroughly dry a home. Under favorable weather conditions, outside air can be used to effectively dry your home. Under less favorable conditions, dehumidifiers, air-conditioners, and desiccant dehumidifiers are more effective.
