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There
are two sets of circumstances favorable to winter condensation of moisture in
an attic:
In
cold climates, a combination of high, inside relative humidity (60% or above)
and low outside temperature (30 degrees F or below) may cause condensation on
the underside of the roof sheathing.
Condensation develops from a combination of high relative humidity and
temperature differentials. Condensation cannot form when the relative humidity
is low, or the temperature is high.
Even
in moderate climates with high relative humidity, the day-night temperature
cycle, combined with high humidity, may cause condensation on the underside of
the roof sheathing.
Effective attic
ventilation is often more critical in newer than in older homes. Incongruous as it may seem; progress in home
construction has created conditions that increase the possibility of winter
moisture condensation. Modern homes are
better insulated, thus easier to heat and cool. They are “tighter,” thus cleaner and less drafty. They are better planned and more
compact. They incorporate more
labor-saving appliances. All of these
factors mean more comfortable living, but they have combined to increase the
quantities of water vapor within smaller spaces and have made it more difficult
for the vapor to escape.
The result is a series of problems such as wet (and consequently
less effective) insulation, wood decay, and peeling paint. These conditions may go unnoticed until
considerable damage has been done.
If little or no insulation is present, there is little
possibility that a ventilation problem will exist because without adequate
insulation, the heat that is lost to the attic will allow the air to control
the rising relative humidity. Homes with little or no insulation are likely to
have 2 to 10 times more air-changes per hour than modern, relatively tight homes.
Since homeowners have become aware of the importance of insulating and
tightening up their homes to conserve energy, condensation and ventilation
problems have become widespread. Saving energy is recommended, but it is
important to understand what happens to the moisture in the air when the
relative humidity goes up and down.
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During
the summer, a poorly ventilated attic can reach or exceed a temperature of 150
degrees F. Even with insulation
covering the attic floor, the rooms below may have excessive heat gains and,
therefore, be less comfortable and increase air conditioning costs. Such a situation could also shorten the life
of the air conditioning system as well as some roofing materials. The air
conditioning system may suffer significant inefficiencies due to the heat,
especially if the ductwork is located in the attic. Cool air may also be lost
through the ductwork and the unit may have to work longer.
- High
attic temperatures may cause deterioration of many fire-retardant plywood roof
sheathings, joist and truss members to split and deform, and truss plates to
deteriorate and loosen.
- Humidity primarily comes from within the house (i.e.
from tubs and showers, unvented clothes dryers, humidifiers, cooking, basement
and crawl spaces, etc.). It also comes
from less obvious sources, such as plants, standing water in a sink and even a
large number of people who may stay in the house for a prolonged period of
time. The very act of breathing by a
family of four can expel approximately 1/2 pint of water per hour into the
atmosphere of a home. Mopping a kitchen
floor of about 150 square feet can release approximately 4 ½ pints of water;
washing the dinner dishes can release about 1/2 pint. A wind-blown rain can cause water to enter and evaporate into the
attic area through roof leaks or poorly designed or installed ventilators.
- Condensation
in an attic is due to saturated air. The first place that the air will usually
saturate is on the north side, at the lowest area in the attic, just above the
insulation. The reason for this is two-fold:
1.
The
north side will be colder than the south side.
2.
The
biggest temperature change takes place just above the insulation. There is also
a smaller volume of air at this point than there is closer to the center of the
attic or roof system. Mold will form at this north side (lowest area first); it
progresses up the north side, and when it gets up about halfway, it starts at
the lowest area of the south side. If the conditions are serious enough, the
mold will continue to rise on both sides until all of the sheathing is black
with mold.
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Attic
Main Summer
Heat Build-Up Other
Key Considerations
Determining
Condensation Problems and Concerns
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