I have a 12 Georgetown. Had a year, no leaks. went to Fl. for winter. the water would pool on the dash from so much water. I worked on the roof ,windows lights all winter long could not get the leak fixed. Go home to Ohio in mid Apr. put it in the shop. NO LEAK, they did find NO insulation in the area you are talking about and added 15 ft. of 3 1/2 insulation to stop the dampness and water running down the window from it. Cold outside like 45 hot inside, heater on , humidity high . yep $330.00 later, 3 1/2 of insulation, rain like heck this spring and NO LEAK.
All that to say , YES insulation it for more than one reason. BTW heading to In. for new dash in July all because someone forgot to insulation:h