You must be in a rather cold climate as I'm just shy of Canada and we just picked out tomatoes. I'm guessing Colorado. If it is below freezing outside, an uninsulated garage wall with no intentional heat will certainly be cold enough for frost. Air leakage would normally be the other way, from the garage to the house with stack effect. With a forced hot air system they may be blowing conditioned air into the garage. If there is leakage to the outside on the return side, the excess hot air has to go somewhere.
Do they park vehicles in the garage, if so they need to air seal for CO reasons as well, including the ceiling/attic boundary. If the vehicles are being brought in wet or snow covered that is another moisture source.
Basically, isolate the garage from the house (air sealing) and control any other moisture source. You didn't mention bath and kitchen fans.
New York but close enough. Just shy of Canada, almost.
There are two bath fans and a kitchen fan, all properly vented. I didn't think about the snow from the cars as they do park two cars in there. I've proposed air sealing the basement and attic to separate the garage from the house. The weather strip on the door between the two is in rough shape and will be addressed also.
The customer wants the exterior walls of the garage insulated but I don't want him to waste his money. He does not plan to add a heat source.
The house is only 9 years old.
We have only had a couple of frost nights so far in Bangor so figured you were out west.
Moisture in a garage is always a problem if you can't drain it off properly. Insulation and sheetrock may help hold the temp up a bit, but the moisture still has to go somewhere. Most garages leak air to the outside fast enough to take the water with it.
If they want to go the finished route with a bit of heat, I don't believe they can extend the forced hot air out there. That would be connecting the garage directly to the house air supply.
Not a lot of easy choices.
How effective is the venting on the dryer, bath and kitchen vents? Do they effectively vent outside? Might run some CFM testing on them. See if what leaves is what went in. I would consider a ventilation fan in the garage if they are not intending to heat. Less than insulation.
Install only after air sealing between house and garage. Other wise you just put air from the house to the garage.
I work with a radiant barrier coating that can be used as an interior / exterior paint. Let me pass on something I have experienced that sounds identicle to what this garage is experiencing.
Radiant heat energy is going to enter the garage from the walls and door and warm the air.
That heat energy is going to radiate towards the much colder exterior walls. When that happens, it carries whatever humidity is present with it until it runs into the walls, the heat energy goes through the walls but the humidity forms as condensation. Ergo; wet walls. I might even venture to say that the garage 's exterior walls might even be wetter closest to the walls of the house than they are farther away from them.. Ask the homeowners if this is the case.
You are in a very cold climate and you are getting wet walls that are freezing, frost as a result. We have a much warmer climate here and we get MOLD in the winter time because of the exact same reason.
I have two cases I will cite where I painted the outside walls on one side of the houses to reduce the heat gain in the summer and keep it warmer in the winter. We did just that in both cases. What they did NOT tell us in either case, at first, was that they also had a horrible mold problem on the inside surface of those same walls..
Once the exterior walls were coated, they had ZERO mold the next winter and have had ZERO since, three years now..
I asked why they had not said anything about it and they said they had never dreamed it would have any effect. We proved that stopping the radiant heat transfer of the walls also stopped the condensation forming and creating wet interior walls which allowed the mold to grow.
I will attach the statements from the homeowners.
Just a note;
The Rubles had stucco exterior walls, 2 x 4's, no insulation and wood paneling as the inside walls.
The Gusmans wall was foundation block, concrete filled, no insulation. The interior was the other side of the foundation blocks.
Both of these are here in my area of California. I have had a few others but did not gat a statement. We have MANY of these cases from Texas but there, no statements.