Place plywood over two or more joists under your work area to prevent damage to the ceiling below.
Measure the distance between your rafters and between the rise. Start your measurement at the top of the vented area at the wall plate and measure to the crest.
Cut one-half-inch, foil-faced, rigid foam into strips the width of the space between your rafters. A utility knife is the best tool for this.
Push the foam strips into place between the rafters. Staple several times on top of the wall plate to secure.
Staple through the foam every 4 inches to the top. For thinner strips, these staples can be placed in the middle of the foam. For strips larger than 24 inches, a staple every 4 inches on each side is recommended.