Air in the natural environment contains a certain amount of moisture as is necessary for plants and animals to exist. In car storage the aim is to control the amount to a manageable level. The ratio of the amount of moisture in the air compared to the maximum that the air can hold at a given temperature is called Relative Humidity or RH. It is expressed as a percentage. For any given amount of moisture, the higher the air temperature the lower the RH%. Equally the lower the air temperature is the higher the RH%. In winter the air appears to be noticeably wetter than in summer.
It is the cooling of air that causes the problems. As air cools it can hold less moisture until a point is reached where the air is fully saturated (100% RH) – known as the dew point. If the car, tools, or anything else in this environment, is at a temperature below the dew point of this air, moisture is then given up by the air as condensation. This manifests itself as wet car surfaces, rust on tools, damp paperwork etc. Because the moisture condensed out from the air is pure distilled water it is also the ideal environment to encourage the growth of moulds and fungal spores, something you certainly don’t want to have to worry about with long term car storage.
The solution to the problem is to reduce the dew point to a level below the temperature of any part of the garage and its contents AND to keep the RH% within controlled levels.