How much attic ventilation do I need?

• For non-powered attic vents (ridge vents, roof louvers, wind turbines, etc.) Building codes give minimum attic ventilation requirements. Most attics require more. In fact, research suggests you need 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. This number represents the cubic feet of air moved per minute. The higher the CFM, the more powerful the ventilator. According to the Home Ventilating Institute, 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 (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. Whole-House Fans are also rated in CFM. To determine the right size for your house, calculate the interior square footage of living area (do not include 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.

How do I install a balanced ridge vent system?
A balanced system of intake and exhaust airflow through the attic is the best way to create effective, year-round ventilation. Thus, in addition to installing ridge vents, which act as exhaust vents, be sure your attic has intake vents in the soffit/eaves.

To ensure a balanced ventilation system, match the amount of net free area the ridge vent provides with as much, or more, intake area. To do so, measure the length of ridge vent you will be installing, then double that length for Air Vent continuous soffit vents because the net free area (NFA) of the soffit vent is 9" and the NFA of the ridge vent is 18".

How does the Air Vent 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.

What if a ridge vent doesn't have an external baffle?
With an unbaffled ridge vent, wind and other elements can blow directly in through the ridge vent. Air entering the vent can create pressure in the attic, which prevents air and moisture from being pulled out. Also, stronger winds can actually pass through one side of the vent and out the other, which also prevents air and moisture from escaping the attic. Tests show that externally baffled vents outperform unbaffled ridge vents.

What does Air Vent's internal weather filter do?
Air Vent's internal weather filter provides a more complete barrier from the elements. Wind-driven rain, snow, dust and insects are trapped by the filter. And because Air Vent ridge vents provide continuous airflow along the entire roof, the internal weather filter is cleaned by the flow of air coming out of the attic. Only Air Vent has an internal weather filter.

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 water begins to penetrate sheathing, insulation, wall cavities, and sheetrock or plaster:

• 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 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. What will solve it is adequate ventilation combined with proper levels of insulation.

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, the 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.

What is air exchange?
Air exchange is a system of air intake and exhaust that occurs with effective air circulation. When stale, overheated air in your home or attic is vented out and fresh air is pulled in to replace it, that's air exchange. In an attic, for example, cool air enters intake vents, which are located along the underside of the eaves. It's warmed, then rises out of the roof or gable attic vents to expel heat.

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 in 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
• Up in the attic, the insulation may become damp and compressed
• Mold and mildew can develop in the attic
• Wood can rot
• Deterioration of roof shingles accelerates

The advantage of the combination thermostat/humidistat is it provides double protection throughout the year.

What is the benefit of two-speed controls on power ventilators?
Adjustable, factory-set, two-speed controls can save homeowners energy and money by automatically switching from high to low speed depending on daily attic temperature fluctuations. On a typical summer day, the two-speed power attic ventilator runs on low during early morning and late afternoon, automatically adjusting to peak heat conditions midday by switching to high speed.