Something else to consider is to use the thermal camera on the outside of the house as well. If you do see a vertical stripe of heat loss, it might be easier to remove the siding in a few spots and drill into the cavity and fill from the outside.

If your siding is some kind of lap siding, like clapboards, you might remove the one at the intersection of the floor and the rim joist and caulk that seam from the outside as well as from the inside. Then will you not only block any infiltration, you will have any voids in this seam filled with dead air that will insulate.

Also shoot the attic to see if there are any heat columns coming up from around plumbing vents or any other penetrations in the attic insulation. This could significantly cut your heating bills, far more effective than just blindly adding more insulation.