Quote Originally Posted by keith3267 View Post
You misunderstood dj1. What he was talking about with the insulated ducts was flex ducts. They are a flexible duct with about 2" of fiberglass insulation around them. They are cheap and easy to install, but if mice or other critters get under your house, they will tear them up. You can see this stuff at any of the big box stores like Home depot and Lowes.

If you have cold floors, you could start with closing your foundation vents. That will warm up the crawl space, especially if your present ducts have any leaks in them. Insulating the rim joist around the perimeter will do more for you than insulating between the joists. If you do decide to insulate between the joists, the vapor barrier goes up and you don't need a lot of insulation.

Just like the walls and the attic, the warm air in the joist bays will rise to the top, but unlike the walls and attic, warm air rising is a good thing as it gets trapped against the floor boards. The air under the floor stratifies and insulates, you just need enough insulation to keep air movement to a minimum.
I can assure you this is not quite correct. I highly doubt the ( air leaky ) crawl space will provide warm enough air to rise and warm the bottom of the floor. Air that stratifies ( meaning it moves ) will not insulate.,only trapped ( non moving ) air would act as an insulator.
Adding insulation to the underside of the floor is most likely required by code and it does work to prevent heat loss from the living space above and help warm the floor to increase comfort.

If your subfloor is either plywood or OSB then you don't require a vapour barrier since those panels are considered a vapour retarder, I would recommend using unfaced batts and applying housewrap to the underside of the joists. The house wrap will be an air barrier preventing cold air drafts from the crawlspace entering the joist bays and reducing the insulation performance of the batts.

One area that's often overlooked is along the base of the wall. Sealing along the baseboard will prevent cold air infiltration and cooling the floor.