Do we agree that there is no gravitational forcing function to cause convection heat transfer in the downward direction beneath a heated floor? By conduction transfer, the warmest, lowest density air is already at the top of the crawl space, the between joist volume. Gravity is holding it there, by displacing the colder air to the bottom of the crawl space. So the heat loss downward is restricted to conduction and radiation phenomena. There is no convection loss downward, in the absence of an externally caused air circulation, such as wind thru open crawl space vents (which is not considerded a natural convection phenomena). I close mine in the winter, and have no conductive nor radiant insulation under my heated floor.
Thus, in the absence of natural convection heat loss in the downward direction, there is a negative value to using a bat type conductive insulation in this area, inasmuch as any bat material will have a higher conductivity than does still air. Bat type, volume insulation is only useful in preventing natural air convection.
When building I decided against the use of radiant, reflective insulation under floor because it is only marginally effective in reflecting this low temperature radiation when new and is rapidly degraded by any accumulation of dust on its upward facing reflective surface. Thus, long term not cost effective.
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