Why? Because the simplest and cheapest way to insulate an attic is to add material to the floor. But if the floor is covered in plywood, you can't stuff enough insulation beneath it to do the job sufficiently—not even in warm climates. Plan to pull up the flooring and layer new insulation on top of the old. With the floor gone, you'll have to find a spot elsewhere for stashing those off-season clothes and that holiday decor.