Why is ventilation important?
Proper ventilation reduces moisture build-up in your home
• The average family of four generates approximately 2 to 4 gallons of water vapor
each day through activities such as breathing, perspiration, showering, cooking and
• When moisture vapor remains in a colder/dryer attic, it can potentially condense
damaging your roof deck and insulation.
• In cold weather climates, ice dams can form along your eave edge increasing
the chance of a roof leak and damage to your gutters.
• Excess moisture may also lead to moisture build-up in your insulation, which
can lessen the insulating value over time, and even lead to mold build-up in
Proper ventilated spaces reduce the Stack Effect by pushing warm, moist air out of the
attic space. In addition to moisture build-up, proper ventilation also reduces heat
build-up in your attic.
• Improper ventilation can lead to premature deterioration of your shingles and roof
• Proper ventilation means your attic stays cooler, reducing load on your air
conditioning units. The less your AC unit works, the longer it will last, likely
resulting in lower cooling bills.
How much ventilation do I need?
• A balanced ventilation system is best. A balanced system is where intake
ventilation at the eave areas is equaled at or near the ridge area.
• Minimum ventilation requirements are 1 sq. ft. unobstructed or net free area of
ventilation for every 300 sq. ft. of attic space.
• The preferred ration is 1 sq. ft. of net free area of ventilation to every 150 feet of
attic space. In order to qualify for a FHA loan, ventilation must meet the 1/150
rule. Many calculators to help you plan the right amount of ventilation for your
home and the ventilation system you select are available.
• There are different styles of ventilation systems available; many are 100% green,
utilizing no electrical power.
Can I over ventilate?
If the system is not balanced at the top or ridge area, or if there were gable end vents
with ridge vents, without adequate soffit ventilation you may actually pull moisture
into the home during a heavy rain or snow storm.
The exception to the balance rule is at the soffit in passive systems. Since the air that
enters at the soffit acts to push out moisture and warm air, having extra soffit
ventilation will not create an off-balance system.
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