What temp to keep garage??

This is my first year with heat in my detached garage. 1,100 sq ft. What minimum temp should I keep in the garage when I'm not working out there for a few days. I'm not an expert on dew points etc. My idea is to keep at around 45f and turn to whatever comfortable when I'm out there. Will 45 keep cars from sweating? Should I be monitoring the outside temp? I'm in MD near DC. Before I put in the heat I did have some problems with tools etc rusting from moisture attracted to cold metal. Seemed more a problem in late fall and early spring than winter. Anyone with better knowledge they'd be willing to share? Bottom line, I'm too cheap to leave it at 65 all winter.

For those that don't know what I talking about there are times where moisture condensates on the cold metal objects in the garage. I suspect it's caused by the metal objects getting cold during the cold nights and the moisture condensating on the metal like a cold beer in the summer.

 

Had the problem when I had no heat in the garage. Just installed the furnace/unit heater this year to prevent as well as make it better to work. Cold nights followed by warm days give you a condition where you have cold metal and warm moist air. Sort of the definition of a dehumidifiyer. The moisture would attract to the cold metal. Probably not as big a problem with attached garages. You wouldn't see the moisture on the cars unless you looked in hidden areas of the frame etc which didn't warm up as fast as the exterior sheetmetal. Common cause of rust in collector cars also made some saw blades etc rust up a bit. Just wondering how much heat do I need to stop it? I know you can also prevent this by moving the air with a fan but hate to run a fan 24/7 in the winter.

My Propane unit heater is vented through the roof so should probably be sending most of the moisture out the vent and maybe drying the air.

 

I suspect it has to do with the dew point as Fasterisbetter suggested. I checked and todays dewpoint is 7 degrees F so I should be safe for the next couple days. I was doing a little searching and did find this from some company that sells some sort of inflatable airbag to keep your car in, I'm not sure if the dew point outside would be the same as the dew point inside my garage? I don't know if I'm buying everything they are saying but here it is:

The Carcoon Airflow System is the only system in the world that actively isolates, cleans and ventilates the air around the vehicle whilst garaged
Dampness in the average garage is often the result of a build-up of condensation. This, along with poor ventilation, can create the worst environment to keep any vehicle, even for a short period of time.

The moisture problem is exacerbated each time the air temperature within the garage drops below the dew-point (ie. the temperature at which dew begins to form, which varies with the humidity and temperature of the atmosphere), when a little more moisture is absorbed into the building.

Later, as the air temperature starts to rise again, the potential for drying is greatly reduced, usually entirely due to the poor air circulation and inadequate ventilation within the building.

Imagine it working like this: Equate the air within the garage to a large sponge. The warmer the sponge becomes, the more water it can take on. Conversely, the colder it becomes, the more water it has to dispose of.

In the mid to late evening, as the air cools and drops below the dew-point, it immediately starts to rid itself of excess moisture. The colder it becomes, the more moisture will be expelled. Several hours later and it’s a new day. The sun starts to rise, and the air temperature within the garage starts to increase, and begins soaking up some of the moisture deposited only a few hours earlier.

Poorly insulated garages (in reality, most) will be subject to increased temperatures, sometimes even higher than the external ambient temperature. This means that the air within the garage will not only take on moisture from inside the garage environment, but under certain climatic conditions it will also take on moisture from outside, as the external ambient air finds its way into the building.

Under these conditions there will be more moisture supported within the air in the garage than is outside in the open air. Poor ventilation will keep the moisture locked within the building. Over ventilation will let in more moist air than necessary early on in the proceedings. Subsequently, again, as the temperature starts to drop, the garage environment will be the first to reach the dew-point, and could end the day with more moisture than it started with.

This is all part of the normal 24-hour cycle during wintertime. Some people simply heat the garage in an effort to keep it dry, or more importantly to keep the vehicle dry. As long as the garage doors remain closed, and the temperature is maintained throughout the 24-hour cycle, this is an adequate - if not ideal - solution.

However, heating the garage doesn't remove water from the air. The resulting drop in relative humidity after turning on the heating simply demonstrates the extra capacity of the air to support moisture.

A garage heated to, say, 26°C with a relative humidity (RH) of 50% will have 5.5 grams of water for every cubic foot of air. A typical unheated garage with a temperature of only 5°C, again at 50% RH will contain 72% less water - only 1.5 grams per cubic foot.

Therefore the rule of thumb has to be: If you heat your garage, keep the heat low and heat it day and night. Or alternatively, don’t heat it at all!

One thing is perfectly clear, the higher the humidity, and the greater the temperature drop, the heavier the condensation. Historically, only governments could afford the expense of pressurised temperature and humidity controlled buildings. These buildings cost millions to build, and are very expensive to run and maintain.

Running a dehumidifier in the average garage without controlling the airflow can be an expensive and futile attempt at controlling the moisture content within the garage. Sometimes it’s more like an attempt at dehumidifying the whole neighbourhood! If you really want to protect your car properly, keeping it dry is the obvious one, but in this modern industrial age there are also other important considerations.

 

That's a good question, I don't know? I have a couple friends that have the radiant floor heating and as long as you leave it at one set temp it's very nice. I don't think it's very good if you want to keep it low and just crank it up when you go out to work. It might even be worth looking into a geothermal heatpump to heat the water? I think if I were building a garage I might consider radiant heat and still get a unit heater to heat the place up quick when I need it. It only cost me $700 delivered for a 75K Btu propane unit heater.