Modern home construction methods result in better insulated and tighter homes, which
save you the homeowner money on energy bills, but makes the need for a well ventilated
attic even more important. Attic ventilation is necessary to prevent moisture problems, to
prevent heat build-up, to reduce heating and cooling cost. The benefits of proper
ventilation for the homeowner are a cooler attic in the summer, a dryer attic in the
winter and protection of the structural integrity of the home from moisture damage.
In the summer, heat builds up in the attic space which will cool at night if the attic
is properly ventilated. However, if the attic is unventilated or ventilated incorrectly,
the heat will build up over a period of days and continue to increase the temperature
inside the attic. According the Air Vent Inc.'s "Principles of Attic
Ventilation," a 90 degree day with no attic ventilation can heat the roof sheath to
170 degrees or more. Heat radiating from the sheathing down to the attic floor can raise
its temperature to as much as 140 degrees. Without adequate ventilation, today's heavier
insulation will absorb and hold more heat built up during the day and make the attic less
likely to cool at night. Overheated ceiling insulation conducts the heat through the
ceiling down to the residents of the home. This causes the air conditioning system
to operate for longer periods of time to reduce the heat.
The well insulated attic also will hold moisture in the winter and moisture causes
considerably more problems than excessive heat. During the winter, air inside the home is
warmer and carries more water vapor than the colder, dryer air into the attic. Cooking,
laundry, showers, and humidifiers add moisture to the air inside the home. Atmospheric
pressure causes the vapor moisture to naturally migrate from high humidity air to low
humidity air and as it moves into the attic, the moist air will condense into liquid on
the cooler rafters, trusses, and roof sheathing. As the air gets colder, its ability to
absorb moisture is reduced. And, as the insulation levels are increased in the ceiling,
the attic becomes colder and more likely to retain the moisture, which can damage the
structure. In some parts of the United States, snow melting on a warm roof surface can
cause an ice dam. The melting snow, usually near the ridge, runs down to the colder roof
overhang and then refreeze into ice. Subsequently melting of the snow on the roof can
accumulate as a pocket of water that eventually backs up under the shingles to cause major
damage. The results can be soaked insulation, stained sheet rock, or peeling paint. In
some cases, the build up of the ice weight can cause structural damage.
For more information on ventilation visit the Lomanco
page or Airvent
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