How to Apply Insulator Board in an Attic

Measure the depth of the attic rafter spaces, then purchase rigid foam insulation that is at least 1 inch thinner than the rafter depth. The insulation comes in 4-foot-by-8-foot sheets of varying thicknesses.

Measure the space between two rafters and cut a section of insulator board to match. Because some discrepancy may exist from one rafter space to the next, you should measure each space before cutting a board to fit.

Place a straightedge on the cut line and use a utility knife to cut just outside the line. This cuts the board a hair on the wide side, which allows it to fit snugly between the rafters.

Start at the bottom of a joist space and insert a sized panel between the rafters, sliding it down until it makes contact with the top of the wall plate, but keep the face of the panel flush with the close edge of the rafters. Don’t push the board to the back of the rafter space.

Measure, cut and fit additional panels into the rest of the rafter spaces. If you cut the foam boards correctly, the panels will fit between the rafters without the need for glue. If you have one that’s a bit loose, you can drive a small finish nail into the inner edge of the rafter to hold the panel in place.

Install foam tape over the seams between the foam boards and the rafters. This creates an airtight seal. You’ll also need to apply the tape over the seams where the cut edges of the foam boards meet.