Let’s talk humidity
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.
How does this affect my garage?
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.
Consistent temperature is key to store a car
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, 79°F with a relative humidity (RH) of 50% will have .2 ounces of water for every cubic foot of air. A typical unheated garage with a temperature of only 41°F, again at 50% RH will contain 72% less water – only .05 ounces 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 pressurized 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 neighborhood! 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.