Before you add insulation, air-seal your attic. Move the insulation out of the way. Run a bead of expanding foam over the top plat of each wall on the interior of your home. Build a rigid styrofoam box around things like bath exhaust fans. Build a rigid styrofoam box around recessed lighting. Seal around electrical box, pipe, tube, and vent penetractions.
Around the perimeter of your attic, inspect the eves. Seall the top plate on the exterior wall and ensure your attic venting is correctly set up. You probably have soffet vents. Make sure then are not obstructed, but they should vent from the outside only, NOT from the walls, etc.
Then, get bags of cellulose insulation. If you buy 10 or more bags from HD or Lowe's, you can rent the blower for free. this is a two person job, one to feed the machine with bags, one to crawl around up above and blow. Go for even, complete coverage. 12" is a good starting point.
Sealing first, though. If you don't stop the drafting, all the insulation in the world is just an air filter.
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