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.
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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 = 60oF, RH = 75%, and U = .60, condensation occurs at
outside temperature of 40oF.
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 -7oF.
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.
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E1.22
E1.23
E1.24
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E1.25
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