To determine the square footage of insulation needed, multiply the length by the width of the attic floor. Buy vapor-barrier faced glass fiber or rock wool batts wide enough to fit between joists. Lay temporary plywood flooring and hang one or more work lights. Start laying batts, vapor-barrier side down, at the outer edges of the attic and work towards the center.
Make sure the batts butt tightly together. To cut a batt, set it on a board, compress it with a 2X4, and cut it with a serrated knife. Compress insulation to fit underneath wiring; leaving a three inch space around a recessed lighting fixture or a motor-driven device. Proper air flow above attic insulation is essential to prevent condensation. Do not block the eave vents, if any; install vents or power ventilators in the attic walls or roof.
If you use loose fill, install a vapor barrier by laying strips of 6-mil-thick polyethylene in the spaces between joists. Cut the strips 3 or 4 inches wider than the spaces, staple them to the side of the joists, then pour in loose fill to the desired thickness. To add more insulation, place new batts without vapor barriers, on top of the old, or pour in more loose fill.
If your attic has a finished floor , fit batts, vapor barriers facing you, between the rafters; staple the outer edges of the vapor barriers to the rafters at 6-inch intervals. Allow for at least 1 inch of space between the batts and the roof for ventilation.