Venting: Gables
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For even air flow, the preferred method is to install a combination of ridge and soffit vents. When doing a retrofit of an attic, adding a mix of gable vents, turbine vents, louver vents and soffit vents is typical.

Ed the Roofer provide the following remarks:

The following descriptions of passive venting strategies are based more on anecdotal evidence and a consensus of practitioners than on actual measurements (see Forgues, 1985 for a more detailed discussion of attic venting strategies):

" Gable vents, usually consisting of a set of horizontal louvers shaped to discourage the entrance of precipitation, are installed on vertical surfaces in the attic. They can provide high flow rates but depend strongly on wind direction. They are inexpensive and are often the only existing ventilation, however they do not usually provide good air flow at lower portions of the attic unless combined with soffit vents. They do not get covered with snow but may allow entry of wind driven snow in some circumstances if not properly re-designed when installed (some contractors fasten a thin layer of fiberglass over the opening and fabricate a drip tray with flashing in order to avoid snow entry). Contractors report commonly finding plastic over gable vents installed by homeowners to solve this problem. The large air flows through gable vents have also been reported to create suction at roof louvers and thus draw snow into the attic. Roof louvers, sometimes called mushroom vents, are mounted on sloping portions of roofs near the peak. Used alone, they can leave dead zones and may create localized moisture problems as moister air escapes through them.

They are more effective if used high near the ridge and in combination with low venting such as soffit vents. Because roof louvers usually provide only 40 to 80 square inches of net free vent area each, it is often impractical to install enough of them to meet code. In regions with high snow loads, roof louvers may become completely covered in snow rendering them useless during the coldest weather.

Soffit vents are installed in soffits, of course. They may be continuous or may consist of a series of individual vents, particularly common in retrofits. They depend less on wind direction than most other types of venting and can provide nearly continuous ventilation air directly up along the roof deck when used in combination with higher venting, such as ridge or roof louvers, which promotes stack effect (i.e. buoyancy-induced) flows within the attic."

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