insulation in crawl spaces

ok, my research has shown that insulation in an unconditioned crawlspace can be done in either of two ways.

  1. if the crawl space has water piping and uninsulated duct work that the walls should be insulated.
    a. close vents and use bat insulation from mud sill to floor and extend two feet onto the floor.
    b. use 6mil plastic to cover the floor (using correct installment procedures)
  2. if the crawl space has no piping then insulate the underneath of above flooring. (using correct installment procedures.)

My question is if there is a furnace in the crawlspace it will need supply air so, you cannot cover or close the vents to that area. This tells me that I would have to insulate the water pipes, the heating ducts and then insulate the floor above. What do you think?