How to Install Rigid Foam Under Siding

Measure the height of the wall you are insulating with a measuring tape. Butt the tape's body against one of the short ends of a rigid foam board. Unravel the tape across the board's length. Stop the tape's clip at the height of the wall. Dig the clip into the foam board's face.

Draw the body and clip across the board's width to score a cut line across through the foam's face. Depress a box cutter's or utility knife's blade button and push the blade from the tool's body. Extend the blade to roughly half the thickness of the rigid foam board. Pass the blade through the score that runs through the board.

Set the board on a flat surface. Hang the score over the edge of the work surface. Press the hanging edge downward to snap the board along the score. Remove the snapped portion of rigid foam from the board. Lift the cut board and position it against the wall. Position the long edges of the board over the wall studs' centers.

Align the board's top and bottom with the wall's top plate and bottom plate. Stabilize the board's position and press the tip of a nail into one of the board's upper corners. Drive the nail through the board and into a stud until the nail's cap rests flush against the foam board's exposed surface.

Drive nails every 12 inches around the board's edges. Drive nails every 16 inches into studs that run through the center of the board. Measure and cut subsequent rigid foam boards to precisely fit against previous boards.