How to Install Soffit Vents for Knee Walls

Use rafter baffles with knee wall construction.

Airflow in the attic reduces heat and humidity that can accumulate and damage rafters and the roof decking. Standard attic venting features soffit vents under the roof eaves and additional vents near the ridge of the roof. Finishing out an attic often includes building a knee wall that blocks the narrow floor space at the edges of the attic. During a remodel, you will add rafter baffles to redirect the airflow from the soffit vents to the upper roof vents. Rafter baffles are lightweight panels that attach to the underside of the roof decking starting at the top of the knee wall.

Things You'll Need

• Tape measure
• Reciprocating saw
• Saw blade
• Cordless drill
• Rafter baffles
• 1/4-inch crown stapler

Instructions

Install Soffit Vents

1. Center the vent in the middle of the eave, which is the flat horizontal board that runs from one side of the house to the other on the underside of the roof.

2. Hold the vent in place and draw around its perimeter with a pencil. When the drawing is done, take the vent down.

3. Draw a second pencil line 2 inches inside the first rectangular line. This small rectangle is the cut line.

4. Fit a reciprocating saw with a standard wood blade and cut along the line of the small rectangular.

5. Fit the soffit vent on the eave evenly along the original pencil lines. Turn the vent so the louvers face the house.

6. Insert the screws that came with the soffit vent through the designated holes and into the wood of the eave. Soffit vents usually require four to six screws.

7. Install additional soffit vents along the eave in the same manner.

Install Rafter Baffles

1. Start at the top of the knee wall, because the attic space is open below that point and doesn’t require baffles. Position a rafter baffle between two rafters. The side of the baffle that bulges outward should face you.

2. Insert staples with a 1/4-inch crown stapler along the sides of the baffle, which are flush with the underside of the roof decking. Insert one staple about every 6 inches on both sides.

3. Add rafter baffles to the undersides of every rafter space where you finish out the attic. After the baffles are in place, insulate the rafter spaces and hang drywall.

Tips & Warnings

• The number of vents you need depends on the height and pitch of the roof. Typically, a soffit vent is installed about every 8 feet.

• If the attic framing features a small flat ceiling at the peak, you can install static roof vents in the space above the ceiling. If the attic ceiling is completely vaulted, you’ll have to install a roof ridge vent that runs along the top ridge of the roof. This will provide an outlet for air that rises through the rafter baffles.

• You can install perforated soffit, which features a continuous perforated panel, instead of individual soffit vents.

• The middle of a rafter baffle features a 1- to 2-inch gap that allows air from the soffits to filter upward, unhindered by insulation between the rafters.

• Rafter baffles are about 4 feet long, and they cut easily with a sharp utility knife.

• Use only one-fourth inch staples to attach the baffles to the underside of the roof decking. Longer nails can penetrate the roof’s surface and increase the risk of leaks.