Ridge Vents
Ridge vents are a special type of fixed exhaust venting. That distinction is warrented, because ridge vents offer unique advantages when compared to other fixed venting units.

Those advantages include:

--Maximum efficiency. The best ridge vents use an external baffle designed to draw heated air from an attic regardless of wind direction or force (Figure 18).

When wind direction is perpendicular to the ridge, it strikes the external baffle and jumps over the ridge. That movement creates a Bernoulli effect, causing negative pressure to develop on both sides of the vent. When that happens, air from the attic is "lifted" out, in much the same way negative pressure created above an airplane wing gives "lift" to the plane The same thing happens when the wind direction is parallel to the ridge. It moves up and over the ridge, creating a negative pressure area.

In addition, when little wind force exists, ridge vents take full advantage of the thermal effect to maintain air circulation across the underside of the roof sheathing. Warm air rises to the ridge and exhausts through the vent. That allows a continuous flow of cooler air to enter at intake vents. Only ridge vents use thermal effect efficiently and effectively, because only ridge vents provide continuous and uniform air movement along the full length of a roof.

NOTE: To provide this efficient air movement, ridge venting should be balanced with equal net-free area of intake venting. For best results, intake venting should be divided equally along both sides of a structure.

--Maximum air movement. Ridge vents with an external baffle provide a higher volume of air flow per square foot of attic area than any other fixed vent system. That conclusion is based on a series of independent tests that measured - and compared - the volume of air movement provided by ridge vents and other fixed vent systems.

Externally baffled ridge vents work better because they take advantage of two natural forces: the thermal effect or the fact that warm air rises and negative air pressure that is created as air is deflected by the baffle up and over the ridge vent to create an area of negative pressure over the vent.

--Uniform air movement. Because ridge vents run the entire length of a roof, they provide a uniform flow of air along the underside of the roof sheathing. That air movement helps eliminate "hot spots" that can develop with other types of exhaust vents -even powered vents. No other exhaust vent provides this type of air flow pattern.

--Maximum visual appeal. Most ridge vents offer a low profile design that minimizes its appearance on a roof. Shingle-over designs allow optimum blending with other roof materials.

It's important to emphasize that the advantages listed above apply only to ridge vents that use an external baffle design. A series of independent tests has concluded that only an external baffle can direct the wind up and over the vent.

That's significant, because it's that controlled flow of air that creates the area of negative pressure that causes air to be drawn from an attic.

Two other commonly used ridge vent designs, one employing a rolled mesh or corrugated plastic "blanket" that's placed over the ridge slot, the other using internal baffles or no baffle at all do not provide similar performance. Recent tests at the University of Illinois confirm that conclusion. Those tests show, for example, that rolled mesh and corrugated style ridge vents were unable to create the negative pressure required to pull air from an attic - even in 15 mph winds! In fact, as winds increased, many of these vents actually pushed air into the attic. By contrast, with the same wind velocity, air flow from four-foot sections of Air Vent ShingleVent II (R) and Multi-Pitch FilterVent(R) was measured at more than 75 cubic feet per minute per four feet of vent.

Ridge vents without an external baffle were ineffective, failing to create the negative air pressure needed to exhaust attic air. As a result, testers concluded that "an external baffle was the most significant contributor to the performance.


Source: Air Vent Inc. A Certainteed Company
Reprinted with permission, HouseNet,Inc.

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