Hvac Small Room on Slab

I have a small room that was added on or partioned off from the back of a garage that is attached to the home.  It is about 6 feet by 12 feet.  There are two outside walls, on common wall to the house (door) and common wall to the garage (door). There was no insulation.  I added R13 value to the 3 walls, there is some in the attic as well.  The room has a hvac vent on the interior wall and I had added a small electric strip heater.  The room with the hvac vent does not stay the same temperature as the home.  I can leave the door open to the house and it does seem to balance otherwise with it closed which is preffered  it is about 10 degrees less with heat, I have not tried cooling and it did improve obviously with the insulation but not what I was expecting.  Any ideas?  thank you..