When you have insulation in the attic, you have to be sure it doesn't block the flow of fresh air from the soffit vents below the eaves, up and out through the ridge vent (or gable vent in older houses). Attic rafter vents (also called insulation baffles, baffle vents or proper vent) are used to prevent insulation from blocking the space between rafters, which is needed for this ventilation.
These vents are installed starting where the attic ceiling meets the attic floor, and extending up several feet to create a clear channel for air flow. The vents are made from polystyrene foam which is stiff so they’ll remain rigid, and won't disintegrate over the years.
To install attic rafter vents, you staple them to the roof decking sitting on top of the rafters. The vents come in four foot lengths, and you must buy the right width for your rafters.
After the attic baffles are installed, you're ready to install your insulation out to the very edge of the attic floor. With blown insulation, you also need to block the upper ends of the rafter vents to be sure the insulation doesn't blow into the attic rafter vents.
Attic Rafter Vents For Do-It-Yourself
If you're planning to install any type of insulation in your attic, this government brochure, Do-It-Yourself Guide to Sealing and Insulating with Energy Star, can provide more information.