Year-Round Benefits of Proper Attic Ventilation

What’s the purpose of attic ventilation? It seems like a simple question, easy enough to answer. Unfortunately, all too often, that’s not the case. Most homeowners – and even some experienced builders and contractors – believe the purpose of attic ventilation is to remove heat that builds up in the summer. That’s accurate, of course. But what that answer leaves out is just as important as what it includes. If you understand the principles of attic ventilation, you know an effective venting system provides year round benefits.

• During warmer months, ventilation helps keep attics cool.

• During colder months, ventilation reduces moisture and prevent ice dams.

We can make that answer more specific – and more meaningful – by translating those functional descriptions into a list of benefits: Several purposes of an attic ventilation system are to provide added comfort, to help protect against damage to materials and structure, and to help reduce energy consumption – during all four seasons of the year. Your goal should be to provide those benefits whenever you design and install an attic ventilation system. The rest of this booklet will show you how. Ventilation During Warm Weather Dealing with the effects of heat. Why, on a hot day, are the upper rooms of a home always warmer? Part of the answer, of course, is simple physics: hot (lighter) air rises while cooler (denser) air falls. But in most homes – the vast majority of homes without adequate attic ventilation – a far more important factor comes into play: the downward migration of heat. Consider what happens in such a home on a typical summer day. Radiant heat from the sun hits the roof. The roof temperature increases and heat travels (technically, it conducts through the roof sheathing) into the attic. As heat builds up in the attic, it radiates to the attic floor, then into adjacent living areas, raising temperatures there. You appreciate the effects of that process when you look at the temperatures involved. These are typical temperatures for a home with no attic ventilation, on a sunny day, with an outdoor temperature of 90°F (32°C):

• Temperature at roof sheath: as high as 170°F (77°C).
• Temperature at attic floor: up to 140°F (60°C).
• Temperature in rooms directly beneath attic will be uncomfortable.

Of course, the longer these hot, sunny conditions last, the more uncomfortable it becomes in the home. That’s because an unventilated – or inadequately ventilated – attic seldom loses enough heat overnight to compensate for the heat gained during the day. Ironically, the effect is magnified in modern homes with heavier insulation. Eventually, this accumulation of heat begins to have more practical – and costly – consequences.The most obvious are the actions taken by homeowners to cool themselves. To reduce the effect of the heat – not only the daytime heat gain but also the excess heat being stored in the attic – they turn on fans, window air conditioners or central air conditioning systems. As the hot weather continues, these appliances run longer and longer – a fact well documented by utility companies across the country. Homeowners pay for all this added energy consumption in higher utility bills. A less obvious – but equally costly – consequence can be found on the roof itself. Homeowners can’t see it happening, but over time excess attic heat can cause some shingles to distort and deteriorate. The result is premature failure of roofing materials – and perhaps a leaky roof. Once that happens, the cost of a new roof is the least homeowners can expect to pay. More than likely, they also may face added costs for structural and interior repairs related to water infiltration.

How Ventilation Works

“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. 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.

Using natural 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.

A properly designed ventilation system requires balance. That balance is achieved in two ways:
1) Airflow capacity must be balanced between intake and exhaust vents. In general, the net free area of intake venting should be equal to or greater than the net free area of exhaust venting. To determine how much net free area a particular home requires.

2) Intake and exhaust vents must be positioned to create a proper high-low balance. That balance is achieved when two conditions are met:

a) Half the vent area must be high in the attic, withm the other half low in the attic.

b) 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. Without that balance, the area of effective ventilation is limited to the lesser of the two vent areas. For example, if 75 percent of the venting is high and 25 percent low, ventilation is limited to the air moving through the lower vents.

In planning the location of intake and exhaust vents, two other factors must be considered:

1) Intake and exhaust vents must be positioned so they assure continuous airflow along the underside of the roof sheathing. As we learned in the introduction, this is where ventilation is most effective. During summer, airflow along the sheathing removes heat before it can radiate to the attic floor. During winter, airflow along the sheathing removes moisture before it can condense into water droplets or frost.

2) Intake vents must be located so there is little possibility of rain or snow infiltration. Conventional intake products require installation in the soffit (area underneath the eave of the house) for maximum weather protection. However, there is now a product that allows roof-top installation and maximum weather protection, The Edge™ Vent.

Note: To assure optimum performance of intake vents, you must make certain the area above the intake opening isn’t blocked by dirt, building debris or attic insulation.