Determining How Much Insulation You Need
Measure the space between each joist in your attic with a tape measure.
Measure the length and width of the attic and multiply these numbers to calculate the attic's square footage.
Multiply the square footage of the attic by .90 if your joists are spaced 16 inches apart, and by .94 if they are spaced 24 inches apart. These numbers will indicate the square footage of insulation you'll need to cover your attic floor.
Place the insulation on the floor of the attic with the vapor barrier side down and the fiberglass up.
Lay the insulation in sections, beginning at each side of your attic and moving toward the center. Make sure the pieces fit snugly together and that they don't block any vents.
Cut scraps of insulation with the utility knife to piece the final sections together as needed.