In general terms, mass is dense, and thus it conducts and transfers heat/cold through it, absorbing and radiating in a slow flux. Insulation on the other hand, is generally less dense, and thus has more air or space in it, and as a result, does not transfer heat/cold like mass does. Solid rock, for instance, is denser than sand (sand has air between it's aggregate individual grains). Sand will hold heat, but not as well as solid rock. Rammed earth, is much more effective of a thermal mass, than is dry loose soil. Clay is better than silt, etc because it is denser. Fibreglass insulation is not nearly as effective as an insulator if it has been compressed, because it becomes denser and is more towards the mass end of the spectrum, though compressed insulation is still much less massive/dense than sand, stone, rammed earth, clay or silt.
Mass absorbs and then releases heat while insulation acts to slow the transfer of heat.
So the floor joists from below show that there is no insulation? In that case, cutting & fitting the insulation to fill the spaces, and fastening the plywood on to hold it up, would be the best use of those materials.