I live in an high desert where it gets very cold, -20 in the winter,with 95 degree days in the summer. I have a beverage cooler in a building with a new shed attached to it to protect the compressor. The shed is stuccoed with a metal roof. I have a vent in the upper portion of the shed, no windows. I will install an exhaust fan in the spring, but now I need to insulate the shed for winter. What is the best material to use and who do I avoid mold or condensation issues?
You don't need to insulate this shed. Also, it's a bad idea to have the compressor in a shed in the first place. Compressors are designed to be outdoors because they need to reject heat. In sunny 95f summer weather, your "insulated shed" is really an oven. In winter, insulating a building will not actually raise interior temperatures unless there is a source of heat on the inside (insulation slows heat movement, but it does not generate heat where none exists). High deserts are sunny, but you mentioned that your shed has no windows to allow sunlight in. So insulating the shed will not actually meaningfully raise the temperatures that the compressor is exposed to.
Next, you will not be able to prevent condensation in -20f weather. However at that temperature the moisture-holding capacity of the air is incredibly low--especially in a high-altitude desert (I live in one too; I know what it's like). Despite this, if there is a source of moisture, it will condense somewhere; the only way around that is to keep all condensation-forming surfaces above the dew point which is impossible in the winter without a source of heat, irrespective of how much insulation you have.
Finally, your concern about mold is misplaced. Mold won't grow in -20f weather. The time to worry is during the summer and the swing seasons, but during those times of year, the problem won't be condensation because you generally won't have interior surfaces below the dew point. The real cause of mold is not condensation but rather water leaks from a faulty roof or bad flashing somewhere. Monsoon rains can be brutal so the roof especially needs to be leak-proof. The intense sun in your climate will do a great job of killing mold if you let it!
Here's what I would do: remove the shed's walls and make your shed essentially just a roof to protect the compressor from rain and snow. This will ensure enough airflow around the unit in summer to do its job and allow the sun to warm it in winter and kill any mold that grows as a result of monsoon rains.
I would go with styrofoam. You can get it in sheets at most local hardware stores like Lowes and Home Depot. You can also get it sprayed on if you are looking to hire a company. Plus it is extremely easy to work with.
This is also how people are insulating their garages by adding them to the aluminum doors.
Styrofoam (sheathing) is commonly used under stucco and styrofoam also breathes well and tends to be mold free. I have never seen a styrofoam cup with mold. :P
Here is a link to sheathing at Home Depot