Baffles prevent insulation from restricting air movement.
While it may seem counterintuitive, even well-insulated attic spaces require air circulation. That means drawing air in from outside the home and allowing it to push through the attic. Baffles provide a channel for air to flow through certain parts of the attic. Baffles are also called wind baffles, venting chutes, rafter vents or insulation baffles. Typically made from polyvinyl chloride, rigid foam board or cardboard, they can be installed with a few staples.
Proper air circulation in the attic is all about preventing moisture buildup. During the summer, soffit and gable vents draw cooler air from outside the house to replace hot air that collects in an attic. Moist, humid air escapes, which prevents water vapor from collecting on roof sheathing and rafters. Moisture, if not allowed to escape, eventually rots wood and other structural components. The super-heated air inside the attic can warp and damage roof shingles if not forced out through gable or roof vents. In winter, the natural convection air flow through the attic prevents ice dams from forming on the roof. Baffles correct these problems by allowing air to circulate under or near insulation.
Wind can blow up through soffit vents, disturbing insulation and moving it out of place. This is especially true with loose fiber insulation that has been blown into the attic. Once the insulation moves, it no longer provides protection from drafts or stops air from migrating out of the living space below the attic. A baffle acts as a screen that separates insulation from winds that blow up through the vents.
Installing baffles from a soffit vent and up the rafter bay allows you to insulate every available inch of attic floor space. Without baffles, you must keep vents clear of insulation, which means the space around the vent typically receives no blanket of protection. Manufacturers make baffles in 14 1/2 as well as 22 1/2-inch sizes to allow for 16-inch on center and 24-inch on center rafter placements. Baffles are typically 4 feet long, and they may be trimmed if necessary.
If you want to insulate the rafter bays rather than the attic floor in order to make the attic a heated space, baffles are even more important for proper air circulation. Baffles establish a channel for air to circulate between the insulation and the roof sheathing. As with attic floor insulation, moisture and heat removal are the reasons for creating proper air movement. Run baffles the entire length of the rafter bay from the soffit vent to the ridge vent, then add batting insulation over the baffles.