Attic Ventilation Airflow Basics

 Attic Ventilation:

• Reduces energy costs
• Reduces or prevents ice dams and leaks in winter
• Prolongs the life of shingle and roof components
• Prevents condensation and mold in the attic

 Understanding how the air and wind flows in and around your house and attic is important when deciding on how to properly ventilate your roof and attic.

 We know from the laws of physics that cold air is denser and heavier than hot air. Cold air sinks and hot air rises or is pushed above the cold air.
Hot air inside the attic will rise and exhaust itself through the roof vents to the outside of the house. If the hot air is flowing out of the attic, then that air must be replaced by an equal amount of air. Otherwise the roof would collapse. If the attic floor is fairly well sealed, then the replacement air will come from outside of the house. The difference in air temperature from inside the attic and outside the attic, and the size of the ventilation openings, is the driving force in the rate of air movement.
 

Passive Airflow - Gable Vent

A house with gable vents alone will still exhaust hot attic air. Although not very efficiently.

• The hot air will flow out the top of the gable vents as the colder air from outside flows into the attic through the lower part of the gable vent.
• The cold air entering the attic will sink down along the attic wall and fill the space being vacated by the rising hot air.

Passive Airflow - Ridge vent

An attic space that is only furnished with a ridge vent will still exhaust air without other intake vents. The warmest air in an attic is mostly concentrated in the center of the attic space. The air near the outer walls will be cooler as the heat radiates out of the structure. Especially if the house has large roof overhangs to keep the walls shaded.

• In this situation, the heat will rise and exhaust out the center of the ridge vent, and cooler air will replace it by flowing into the ends of the ridge vent.

Passive Airflow - Ridge and Gable Vents

If ridge vents and gable vents are installed the airflow pattern will look something like this.

• Cooler air will enter at the lowest point of the gable vent and sink below the warm attic air. 
• Warm air will be exhausted at the highest point of the gable vent and at the ridge vent.

Passive Airflow - Ridge and Soffit Vents

A basic ridge vent with soffit vent installation.

• Here the cool air enters the soffit vents and warm air is exhausted through the ridge vent.
• In many cases this is usually sufficient intake and exhaust for ventilating an attic space.

Passive Airflow - Ridge, Gable and Soffit Vents

If both ridge vents and gable vents are installed with soffit vents the cooler air will still flow into the attic space through the soffit vents and be exhausted at both the ridge and gable vents.

• This works especially well in hot climates to exhaust excessive heat in the attic, and in very snowy climates where heavy snowfall can blanket and block ridge vents for extended periods of time.

View a short video on passive attic ventilation with ridge, gable and soffit vents.