I went ahead and took some measurement with a digital temperature probe,
with and without the insulation.
Here are my findings:
Test Procedure:
-Car parked in shade (attached house garage).
-Outside temperature approximately 83 F.
-Probe was located in center left vent. Temperature taken at vent output. Readings on this probe update every 5 secs.
-A/C in max fan speed mode, with the 'all upper vent' mode set on the dial (max air output on top vents, no air coming out of bottom vents).
-Recirculation mode ON; also set to OFF to test both conditions.
-Took readings after temperature had stabilized (i.e., when no change in temp after 20 secs the temp reading was taken and the test stopped).
-Tested with insulation, and without insulation. The last (small) portion of the run was not ever insulated.
-The material used was the highest grade 'rubber' plumbing insulation sold at Lowes. I also have the lighter grey foam material, seemingly lower grade/quality and certainly much cheaper, but did not test it.
WITHOUT INSULATION:
-Temp at vent, recirculation ON: 43.3 F
-Temp at vent, recirculation OFF: 60.9 F
WITH INSULATION:
-Temp at vent, recirc ON: 43.5 F
-Temp at vent, recirc OFF: 60.2 F
As you can see, there is not much difference. A variation of +- 0.5 degrees is probably within the error margin, so I consider these results ~ NO DIFFERENCE.
Note: I did not take 'time' readings, which I should have done. So, we have no way of knowing which condition cooled faster, but I can tell you I noticed the 'insulated' version to be cooling faster than the non-insulated. Sorry I don't have exact numbers in this regard.
Conclusion: No significant difference to final temperatures at vent, when adding insulation. Adding insulation however appears to cool the car faster.
Other tests to perform: test with car outside directly under sun on a hot day. Test with different foam material.
PICS showing material, test procedure, etc:
