Laying a floor over insulation isn't that much different than laying one over a non-insulated floor. Many attics have multiple layers of rolled insulation or mounds or snowlike blown-in fibreglass covering the joists where the floor will be put down. It is necessary to remove excess and level out the insulation to make room for the boards to lay flat to the joists. Insulation is both dangerous to breathe for long periods and irritating when contacting bare skin, so proper care must be taken.
Remove any added rolled insulation laying perpendicular over the joists. Lay a few pieces of flooring down over the joists as a work platform and to protect you from stepping through the ceiling below.
Ensure the insulation is flat and doesn't extend over the top of the floor joists. For paperbacked, rolled insulation, lightly pack it down as flat as possible below the edge of the joists. For blown-in insulation, use a long board to even it out by dragging it over the joists to push insulation flat and into the spaces between joists.
Start in the farthest corner from the exit door. Snap a chalk line across the two walls where you're starting, as close to the wall as possible. Measure and cut the first plywood flooring board so that the long end lies across the centre of a stud.
Hammer in 3-inch flooring nails every 5 inches along the board and at a slight angle into the stud below. Get this board as straight as possible along the chalk line even if it doesn't align perfectly with the wall.
Lay the next boards, butted up tight against the board before it. Also cut the next boards to lie at the centre point of a joist. Place the nails into the joists to hold the boards in place securely.
For areas where there is electrical wiring or ducting under a particular section of floor, consider using screws instead of nails to make it easier to pull up that section.
Wear gloves, eye protection and a face mask when working with insulation. Take proper precautions when working with power tools.