What is the best way to insulate an existing frame floor that sits on an on-grade concrete slab? The floor cavities are uninsulated.
The building is located in Philadelphia; mixed climate.
If your framing is minimal, say 2 x 6 or less, I would recommend filling the cavities with Icynene foam insulation. This insulation system offers many benefits that urethane does not, particularly that of limiting potential damage due to its expansion characteristics. Perhaps installers can enter the floor cavity from the rim board at the exterior perimeter of the building.
This is the Cadillac of insulations, but your feet will thank you when it's cold. If you have any additional questions, don't hesitate to contact me directly.