It has been a standing practice for contractors to insulate the underneath of the floor joists in a crawlspace. The theory has been if you are going to ventilate your crawlspace with foundation vents, the only way to keep the home insulated was to place the insulation on the floor joists. Most of the time the insulation was installed with the paper vapor barrier facing the crawlspace instead of facing the floor as it should be. This was done so that the insulation could be easily stapled to the floor joists. When hot humid air would enter the crawlspace it would condense and the paper backing would become wet. The weight of the insulation would eventually pull the paper through the staple and the result would be insulation falling to the floor or the crawlspace. The other option would be to have the expose fiberglass facing down and be held up with metal rods. The rods would eventually come loose and then the insulation would fall to the crawlspace floor. In either case rodents usually aided in the deterioration of the fiberglass insulation. With the theory of sealing the vents and properly insulating the foundation walls, the moisture concerns are eliminated and the need to fix or replace the insulation every 7-10 years has been eliminated.