You need a little more on top. With the sun beating down on it in the summer, and heat trying to escape through it in the winter, attics require more insulation than your basement or walls.
Like walls and basements, you’ll need both vapor barrier and the insulation itself. But you’ll also need to ensure proper ventilation. This carries any warm air or moisture that does get beyond the insulation away from the house.
Now, if you’re not finishing your attic, you’ll have the easiest time, since you have lots of room to install batts or loose fill.
If there’s no floor, start the insulation between the floor joists, and add more on top (the second layer of batts should be perpendicular). With this method you’ll need to run vapour between each joist below the insulation.
Take care around electrical fixtures and heating vents. Keep the top free of insulation and install blocks at least 3″ either side to keep insulation from crowding the fixture and causing it to overheat. Surround chimney pipes with a metal sheath and a few inches of non-combustible material.
Here’s another option. Lay a continuous sheet of vapor barrier over the joists (or floor), and lay the batts on top of this, keeping your first layer perpendicular to the joists. At the outside edge, remember to insulate and seal the space between the joists.
Install vents up near the peak. These can be gable vents; standard roof vents; turbines, which spin in the wind to help draw air through the channels; or ridge vents, which provide for ventilation along the entire ridge.