CONDENSATION Elimination of condensation on or within walls and floors is as important as reducing the heat loss through the wall or floor. In addition to the moisture damage caused to buildings by condensation, conditions which will cause condensation may make the structure uncomfortable for living. Condensation can be of two types: visible and hidden. Visible condensation occurs when water vapor condenses on a surface and the moisture droplets can be seen. Hidden condensation occurs when water vapor migrates through the wall materials and condenses within the wall section causing wet materials or insulation which may not be detected until severe wetting and damage occurs. Section Break (Next Page)
Normal air is a mixture of "dry air" and water vapor. There is a maximum amount of water vapor which can be retained in a given volume of air at any given temperature. This maximum amount or "saturated" condition is defined as 100-percent relative humidity. A condition of 50-percent relative humidity has 1/2 of the vapor necessary to saturate it at that particular temperature. The higher the temperature, the more water vapor it takes to reach saturation. The temperature which corresponds to 100-percent saturation is called the "dew-point" temperature. When a mixture of air and water vapor is cooled, the temperature drops until saturation is reached at the dew-point temper- ature. (NOTE: The " dew-point" and "air" temperature as measured by the thermometer are the same at 100-percent relative humidity or "saturation".) If the temperature is further reduced, vapor leaves the mixture as conden- sation. If the surface temperature of any area is below the dew-point of the air and water vapor mixture surround it, the air in contact with that area will be cooled to the dew-point and visible condensation will form. This is what happens to car and house windows and many other objects in the winter. PREDICTING CONDENSATION The dew-point temperature can be determined from a psychrometric chart if the temperature and relative humidity of the air are known. Figure E1.1 is a psychrometric chart illustrating these relationships for predicting the dew-point temperature. For a building, the inside wall surface must not cool to the dew-point temperature for the existing air-vapor mixture or visible condensation will form on the wall. This inside surface temperature is dependent upon the air temperature inside and outside the building the heat transfer coefficient (U-value) of the wall. Figures E1.2a, 2b, 2c, 2d show the relationships for condensation and (and frost) on the inside surface for a given U-value, inside temperature, relative humidity and outside temperature. How To Use Figure E1.2a, 2b, 2c, and 2d: 1. Select figure for inside temperature being used. 2. To find the outdoor temperature for condensation to occur on inside surface: begin at left margin with inside relative humidity and move horizontally right-ward to the proper U-value curve. Move downward to outside temperature. EXAMPLE: for inside temperature = 60 o F, RH = 75%, and U =.60, condensation occurs at outside temperature of 40 o F. 3. To find the outside temperature for frost to occur on inside surface: begin at left margin with inside relative humidity and move horizontally right-ward to the proper U-value curve. Follow U-value curve down and to the left to intersect the dashed frost line. Move down-ward to outside temperature. EXAMPLE: above conditions result in frost forming at outside temperature of -7 o F. 4. To find U-value required to prevent condensation or frost on inside surface: find intersection of horizontal relative humidity line and vertical outside temperature line, read required U-value. Section Break (Next Page)
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HIDDEN CONDENSATION AND VAPOR BARRIERS Most wall and insulation materials are porous enough that water vapor will migrate right through them. If the dew-point temperature is reached within the wall section, hidden condensation occurs and a wet wall or insulation results which reduces the insulation effectiveness and deteriorates these materials. A vapor barrier such as 4- to 6- mil plastic film or heavy aluminum foil must be used under the finish covering on the warm side of the wall to block this movement of vapor. Paint, in general, is not an adequate vapor barrier. Certain expanded foam or vinyl-faced insulation boards are inherently resistant to vapor flow and thus do not require the supplemental vapor barrier. The main requirement with these boards, is to seal the joints to prevent seepage of vapor and moisture through the cracks. The rate of vapor movement through a material is measured as " perms". A perm is equal to 1 grain (7000 grains = 1 pound) of moisture flow through the material per square foot) (hour) (inch of mercury vapor-pressure difference). Residences and confinement animal structures need to have a vapor barrier on the "warm" side of the insulation which transmits less than one perm. The outside surface should be more permeable (that is, have a greater permeance rating) so any moisture will escape outward and not get trapped between two barriers. This means that plastic film, or a similar material, should not be "wrapped" around the outside of the framing and insulation of a building. E1.26