How Ventilation Works

The word "ventilate" comes from the Latin word for "to fan," the action of causing air to move.  And that's exactly how ventilation works:  it provides the conditions that allow air to move.

And that's exactly how ventilation works: it provides the conditions that allow air to move.  For our purposes, however, we have to get a little more technical, because efficient ventilation requires a very specific type of air movement.  We're not interested in moving air just to create a breeze that cools us by speeding evaporation.  Instead, we want ventilation that provides year-round benefits.  

If you've ever walked into the stuffy confines of a room that's been completely closed for a lengthy period, you know air tends to stay in place.  You also know that just opening a door or window doesn't solve the problem immediately.  A flow of air must be established to produce the air changes needed to remove all the stale air. That's what an efficient ventilation system must do, too - provide a steady, high volume of air movement.

That means the system components must be sized and positioned to provide a constant flow of air, moving in a constant direction.

We can create air movement in one of two ways - using natural ventilation or mechanical ventilation.  Natural air movement is created by two key forces: thermal effect and wind:

Thermal effect

We've already mentioned thermal effect briefly.  It's the inherent property of warm air to rise.  A well-designed system takes advantage of that movement in two ways:

First, since warm air rises, an effective system will include exhaust vents at or near the ridge.  That placement allows the hottest air to be removed from the attic most efficiently. Second, the thermal effect creates a natural circulation of air, because as warm air rises, cooler air falls.  A well-designed system assists this momentum by placing intake vents at the lowest point in the attic, typically in the soffit or near the roof's edge.  The cooler air entering these vents (cooler as compared to the attic air) speeds this circulation of air.

Wind

By itself, however, thermal effect cannot create the high volume of air movement needed for effective ventilation.  That's why the influence of wind is the key element in the design of a non-powered ventilation system.  Wind, after all, is a natural flow of air.  So when designing a ventilation system, you want to make the wind work to your advantage.  

To use the power of wind, you have to understand how wind force affects ventilation.  It isn't the velocity of the wind by itself that causes air to move through an attic. Instead, it's the wind's speed as it moves against and over a home's exterior surfaces.  A wind-driven flow of air creates areas of high and low air pressure.  High pressure forces air into the attic, while low pressure draws air out.  

The Balanced System

A properly designed ventilation system requires balance.  That balance is achieved in two ways:

• Airflow capacity must be balanced between intake and exhaust vents.  In general, the net free area (unobstructed area measured in square inches) of intake venting should be equal to or greater than the net free area of exhaust venting. 
• Intake and exhaust vents must be positioned to create a proper high-low balance.  That balance is achieved when two conditions are met:
  • Half the vent area must be high in the attic, with the other half low in the attic.  Without that balance, the area of effective ventilation is limited to the lesser of the two vent areas.  
  • The vents placed high must act as exhaust vents, while the low vents act as intake vents.  That placement assures a continuous flow of air, moving in the desired direction.

If you have any questions regarding your attic ventilation system, or would like us to evaluate your current system to determine if there may be some room for improvement, contact us by giving us a call or conveniently schedule an appointment using our website:  www.precisionroofingmi.com.  The appointment and the quote are both FREE.