Assuming you are using fiberglass batt insulation, it's a poor choice for floors (also a poor choice all around, but that's a different conversation). There are three inherent problems with fiberglass batt and floors.
1. Fiberglass batt will almost always fall down. Gravity is relentless and constantly pulls on the fiberglass, and air gaps between the subfloor and insulation not only negate the insulation value, but can be dangerous.
2. Fiberglass batt has to be in contact with the subfloor in order to work properly and to help prevent the spread of fire. See #1 for the reason why it doesn't always maintain contact.
3. In order to keep fiberglass batt in place on a ceiling, it ends up being compressed around wires, ducts, pipes and the wire supports used to keep it in place. Once you compress the insulation, the fiberglass looses restrictive (R) value.
The very best you'll get by using fiberglass insulation on a floor is grade III, and that's if you're lucky and skilled at installing fiberglass batt.
Originally Posted By: JC1
You don't need to fill the entire joist with insulation. Just put one regular bat in the joist. I would leave an inch or two in between the subfloor and insulation leaving an air gap.
Leaving an air gap is an extremely bad and unsafe recommendation. Not only is it a fire hazard, but gaps negate the R-value of the insulation. Having an "air gap" would cause the installation to immediately fail inspection in most areas.
