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Ventilation FAQ
How much attic ventilation do I need?
How does the an external baffle work?
What if a ridge vent doesn't have an external baffle?
How can I prevent ice dams?
Why are my shingles buckling?
What is a cold roof system and how does it help prevent ice dams?
What is air exchange?
What are the benefits of installing a power vent with a thermostat/humidistat?
How much attic ventilation do I need?
For non-powered attic vents (ridge vents, roof louvers, wind turbines, etc.) Building codes just
give minimum attic ventilation requirements, however most attics require more. It is recommended
that you have 1 square foot of ventilation space, or net free area, for every 150 square feet of
attic floor space. Net free area is the total unobstructed area through which air can enter or
exhaust a non-powered vent.
For new home construction with a vapor retarder, the minimum is 1 square foot of ventilation
or net free area for every 300 square feet of attic floor space. If your vents are split
between ridge vents and intake vents, the minimum requirement is also 1 square foot of
ventilation or net free area for every 300 square feet of attic floor space.
For power attic vents, Power Attic Ventilators are rated in CFM (the cubic feet of air
moved per minute). The higher the CFM, the more powerful the ventilator.
To determine the correct CFM rating needed, multiply attic square footage by 0.7.
For example: 1500 square footage attic x 0.7 = 1050 CFM. This means you would look for a power
ventilator rated 1050 CFM or higher.
When using power vents, be sure to provide sufficient attic intake ventilation to match the exhaust
capacity of the fan. To determine this, divide the CFM by 300 for the number of square feet of intake
needed. Because most vents are rated in inches instead of feet, simply multiply the number of square
feet by 144 to calculate the net free area of intake needed.
For whole-house fans, also use the CFM rating. To determine the right size for your house, calculate
the interior square footage of your living area (not including garage, attic or basement) and multiply
the total square footage by three. This will give you the CFM number to look for when choosing your
whole-house fan.
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How does the an external baffle work?
Air Vent ridge vents feature an advanced designed, external baffle that is one of the keys to
the ridge vent's outstanding performance. The baffle deflects wind up and over the vent,
creating an area of low pressure above the vent openings that causes lift. This is called
the Bernoulli Effect. It's the same effect that causes airplane wings to lift. This low
pressure works to pull air out of the attic. The baffle also deflects wind over the vent
to help prevent wind-blown rain and snow from entering the attic.
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What if a ridge vent doesn't have an external baffle?
Curling is when the corners of shingles turn up and it is generally caused by excess moisture
attacking the underside of the shingle. Inadequate ventilation causes the entrapment of heat
and moisture in the attic. This will eventually penetrate the roof deck and cause the shingles
to curl. Improving air circulation within the attic space can stop the progress and rectify the
situation if the curling is not too severe.
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How can I prevent ice dams?
Ice dams are barriers to water runoff on the roof. They usually form at the roof edge, just above
the gutter, in cold, snowy climates. They form when snow melting on warmer areas of the roof, usually
near the ridge, runs down and refreezes at colder roof overhangs. Warm spots on the roof are caused
by the heat that escapes from the living space into the attic.
Once this cycle of melting and refreezing begins, a barrier is formed, trapping the snowmelt and
allowing it to flow up and under shingles. As this water begins to penetrate sheathing, insulation,
wall cavities, sheetrock or paint can begin peeling on both inside and outside walls, roof coverings,
fascia and gutters can be damaged. Structural damage can result from the weight of the ice dam.
Homeowners usually blame damage from ice dams on their gutters, since that's where the problem
appears to be. But newer, wider, deeper gutters won't solve the problem. Nor will additional layers
of insulation alone. Adequate ventilation combined with proper levels of insulation is needed.
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Why are my shingles buckling?
Buckling shingles can become humped or wavy, usually running in a straight line up, or across
the roof. Some possible causes:
- Deck or felt movement often caused by varying moisture levels in the material under the
shingles. The shift will cause the nails to move in accordance with the deck forcing the shingles
closer or farther apart depending on the situation.
- Improper nailing - nails will "pop up" creating the buckling effect.
- Shingles nailed too tightly together.
- Thin decking (3/8) over 24 centers can be the cause due to clips, meant to hold the decking
in place, coming lose or coming off.
Depending on the cause of the buckling, improving attic ventilation to eliminate excess humidity,
removing fasteners and refastening or replacing distorted shingles can be some possible solutions.
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What is a cold roof system and how does it help prevent ice dams?
The key to solving ice dams is to create a cold roof, with the temperature of the inside roof sheathing
near that of the outside air temperature. To create such a condition, large volumes of outside air must
enter at soffit intakes, sweep along ridge rafters, then exit at vents near the ridge. To prevent trapping
warm air in the attic, an equal balance must be established between intake and exhaust air volumes.
Since such a ventilation system is bringing cold air into an attic, insulation must minimize heat loss
at the attic floor. As an added precaution, use waterproofing shingle underlayment. It provides a
waterproof-barrier beneath roof shingles that pooled water from melting ice dams and driven rain cannot
penetrate.
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What is air exchange?
Set one end of a 12 level on the roof surface make it level. Take a tape measure from the
other end down to the surface of the roof. This will give you the slope of the roof. For example, a
roof rising 4-inches for each horizontal foot has a 4/12 slope. Slope is also sometimes referred to
as pitch.
The greater the slope, the more material will be required to complete the installation of the
roof, hence the slope factor. The slope factor multiplied by the area of the roof will give you
the number or sq. ft. to be covered.
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What are the benefits of installing a power vent with a thermostat/humidistat?
With combination thermostat/humidistat controls, power attic ventilators monitor heat and humidity
levels summer through winter for year-round attic protection from the damaging effects of moisture
and heat buildup.
Moisture naturally generates in homes from washers, dryers, dishwashers and showers. The water vapor
travels upward, passing through ceilings and insulation until it reaches cooler, dryer conditions in
the attic. When the vapor hits cooler rafters, trusses and roof sheathing, it rapidly condenses into
water droplets or frost.
The trouble starts as water begins to drip into the attic or wick into the sheathing. Water stains may
form on ceilings just under the attic and the paint may peel. Insulation may become damp and compressed,
mold and mildew can develop, and wood can rot. Deterioration of roof shingles also accelerates.
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