In describing how ventilation works, we discussed intake and exhaust vents in general terms, perhaps giving the impression that a single type existed to serve each function. In fact, however, you can choose from a wide range of intake and exhaust components, allowing you to tailor ventilation systems to the specific characteristics of every home.
In general, ventilation components can be divided into two broad categories: intake vents and exhaust vents. Within each category there are various styles. Furthermore, ventilation components are either fixed (also called static) or powered.
Fixed ventilation components are exactly that: units that don't require moving parts or power assistance for proper functioning. But don't let that description lead you to believe fixed ventilation is a low-tech alternative to high-efficiency systems. Just the opposite is true. Fixed ventilation components form the core of all attic ventilation systems, from the simplest to the most sophisticated. In fact, in most cases, your initial goal should be to try to design a ventilation system that uses only fixed vents (or ridge vents - a special, high-efficiency type of fixed ventilation).
Obviously, since fixed ventilation can be used to create an entire system, units are available for both intake and exhaust functions.
Intake vents are available in many designs. In choosing the right unit for a particular job, you have to consider the structure of the home, the area where the units will be located and the net free area provided by each unit. The most common types of intake venting are:
- Undereave vents which are mounted in soffit.
- Continuous soffit vents which are also mounted in the soffit.
- Vented drip edge which is used on homes without an eave area.
- The Edge™ Vent shingle-over intake which is a roof-top installed vent.
- Mini-louvers which are typically used with other types of intake venting; they're too small by themselves to provide sufficient net free area of intake. In most applications, they're installed in an exterior wall to help eliminate moisture that collects in the wall cavity. To be effective, mini-louvers must be installed below the source of humidity (such as a bathroom or laundry area). That placement allows a flow of air to collect the humidity and carry it into the attic.
Exhaust vents are designed to permit an efficient, unobstructed outflow of attic air. In addition, because they're installed high in the attic where there's greater exposure to the weather, these units must be designed to prevent (or at least minimize) rain and snow infiltration.
Exhaust vents must be used with intake vents to provide a balanced system and thus an adequate flow of air through an attic. It's also worth repeating another point made previously: for maximum efficiency, the net free area of intake vents should be equal to or greater than the net free area of exhaust fans.
As with intake vents, exhaust vents are available in different designs. Two commonly used fixed exhaust vents are:
- Roof louvers which are installed as close to the roof ridge as possible to allow maximum release of moisture and overheated air. Because they're installed near the ridge, they provide a continuous airflow along most of the underside of the roof sheathing. The airflow pattern isn't uniform, however, so for maximum effectiveness, vents should be spaced equally along the roof.
- Gable louvers which are typically installed in the gables.
Ridge vents are a special type of fixed exhaust venting. That distinction is warranted, because ridge vents offer unique advantages when compared to other fixed venting units. These 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.
- Maximum air movement. Externally baffled ridge vents work better because they take advantage of two natural forces: thermal effect or the fact that warm air rises and low air pressure that is created at the vent openings as wind is deflected by the baffle.
- 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 airflow pattern.
- Maximum visual appeal. Most ridge vents offer a low-profile design that minimizes their 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 low pressure that causes air to be pulled from an attic.
Technically, wind turbines aren't a fixed vent system because they use a moving part to help exhaust air from an attic. Although not as effective as ridge vents, wind turbines provide a low-cost alternative in areas where consistent wind speeds of at least 5 mph are typical. Without that minimal wind speed, wind turbines act essentially as roof louvers. When the wind is blowing, however, wind turbines can be effective air movers.
For the most part, a power fan is a motor-driven version of a wind turbine. Power fans are used to move large volumes of air - a good option for hard-to-vent hip roofs that have limited horizontal ridge length available for ridge venting. When evaluating the feasibility of using power fans, it's important to evaluate one factor which is considered to be a major disadvantage namely, that power fans cannot vent away moisture during the winter unless they are equipped with humidistat controls.
When it comes to adequate attic ventilation, there is a bit of science involved and it may seem a bit confusing to many homeowners. If you have questions about attic ventilation, or would like us to take a look at the ventilation system currently in your home, give us a call at (517) 393-9386 or visit our website: www.precisionroofingmi.com and schedule an appointment. The appointment and the quote are both FREE.
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