Step 1: Measure the Bays and Cut the Insulation Before installing the Rigid Foam Board, you'll need to measure and cut it to fit. Measure the cavity into which the board will be installed. Subtract a 1/4 inch from the length and width and use that new measurement to cut the board.
Take a 1/4 inch off the joist bay measurement to cut the board.
Step 5: Continue for each Bay Follow steps 2-4 to install a piece of rigid foam insulation into each bay, following by an application of expanding foam around the outside edges of the board, sealing it into place and providing an additional air barrier.