Rigid foam insulation is rated in "R" values just like all other forms of insulation. However, it is manufactured in large sheets similar to sheet rock, thus making it much quicker to install. In addition to installing the insulation boards on walls, you can install it on an interior ceiling or on the ceiling of an attic. Hanging the board requires only basic tools, but having an assistant is a good idea to help hold the sheets up.
Walk to one side of the room and place two ladders approximately 4 feet apart. Climb up onto the ladder carrying a piece of rigid foam insulation and have another person climb up on the other ladder.
Place the foam insulation sheet flat on the ceiling and push it all the way into the corner so that it butts up against two walls. Position the foam board so that it lies perpendicular to the wooden ceiling joists. Ask the other person to hold the board in place.
Secure the boards to the ceiling joists using ring-shank nails, also called roofing nails, spaced at 12-inch intervals along the foam board. Use a hammer to insert the nails, making sure that they penetrate the wooden joists.
Climb down off the ladders and reposition them directly next to the first panel. Climb back up with a new panel and install the second panel as close to the first one as possible in the same manner.
Continue installing additional foam boards in the same manner until the entire ceiling is covered.
Go to one side of the room and locate a seam where two pieces of the foam board meet. Place the end of a roll of house wrap seam tape over the seam and unroll it until you reach the opposite end of the ceiling. Cut off the excess tape with a utility knife. Repeat to cover all remaining seams with the tape to create an airtight seal.
To cut rigid foam board, draw a line on it using a straight edge and a pencil. Then hold the straight edge on the line and draw a sharp utility knife along the board to cut off the excess.