Is this door made of metal and comprised of several hinged panels? If so, I have insulated some of them in the past by purchasing rigid, 4' X 8' sheets of either plain styrofoam insulation panels, or the same size urethane insulation panels and cutting them into sections (batts) that will fit into the spaces of the door interior panel "voids". I used 100% silicone caulking to seal the styrofoam batts to the door panels (last step). Most of the door panels I worked with had an angle lip (stiffening structure) that ran all the way around the interior edge of each door panel. To get a good tight fit of the styrofoam sections to the door panel voids requires cutting the styrofoam panel into two pieces lengthwise which can be inserted one at a time into the void. Insert each section into the void edge lip first, then press the centers of the sections together for a tight fit. I then used 1/4" wooden "lath" strips, cut to fit between the upper and lower lips of each door panel to hold the styrofoam batts from falling out. Some door panel voids were deep enough to allow 2 layers of 3/4" thick styrofoam batts. After getting the batts in place and secured with lath strips, I sealed the edges of the batts to the door panels. After completing the job on a couple of doors, I had to add some tension to the springs that assist lifting the door when opening. That was simply a matter of cranking down on the tensing eyebolts that the springs attached to. The doors were all "manual operating". If a door opener is present nothing needs be done to compensate for added mass to the door.

If the problem is mainly in the summer, I think it results from warm moist outside air coming into the cool garage, resulting in near 100% relative humidity in the garage. Insulating the walls should help some, as the insulation will help retain heat in the garage whigh will result in a lower relative humidity (relative humidity is dependant on temperature). Sealing the gaps (to the extent practicle) to prevent moisture laden outdoor air from entering will also help. But a dehumidifier might be something that you will need to run when it is particularly hot and humid outside.

Dave, I am German but have lived in the greater NYC area for the past 6 years. I recall when I was a kid we insulated the attic with styrofoam. But fire codes are much stricter here due to the very combustible building materials. A house on my street, 100m from my house, burned down last week. Dryer vent caught fire. I am somewhat cautious now.

OK. Since the adjacent walls to the house are already insulated, what am I gaining by insulating the remainder? My garage is not conditioned (heat or cooling) and as I said, there is no living space above. True, I may keep the temperature swings in the garage itself lower, but is there any real benefit here? Isn't it possible that once the heat of summer or cool of winter "sinks in" to the garage (which it will, albeit more slowly with full insulation) that it could actually them take LONGER to remove the heat (or cold) BECAUSE of the new insulation? My real hope here is that by completing the attic insulation, I would somehow get a benefit to the adjacent living spaces from the garage walls. Thus, I am still not quite seeing all the ins and outs of this. Also, why the blow-in type? Wouldn't R-30 between the joists work nicely? If yes, I'm thinking unfaced since there is no "heated space" to place a vapor barrier toward. Any more thoughts are greatly appreciated.