Why is humidity a factor?
Above 70% relative humidity, mould can start to grow on surfaces. Above 60% relative humidity, the speed at which rust can form and grow increases rapidly.
Solution by ventilation
Adding more ventilation to a garage space may assist with drying a car put away wet, but with the average humidity in London around 80%, this will never prevent mould or rust.
Solution by heating
Heating will increase the amount of moisture that the air can hold and lower the %RH. This will not stop a cold car getting condensation on it, and heating is by far the most inefficient and expensive way to lower the humidity and you will not want to heat a garage when it is already warm outside.
Solution by dehumidification
Using a dehumidifier physically removes the moisture held in the air and lowers the %RH. Removing the moisture uses much less energy compared to heating (around 2.5 times less) to lower the %RH.
Dehumidification: Desiccant vs. Refrigerant
There are two main types of dehumidifier Desiccant (that uses a silica coated wheel to absorb moisture and an electric heater to dry it) or Refrigerant (that uses a compressor based heat pump to cool a surface that the incoming air passes over and the moisture condenses onto, before being reheated). Desiccant units are ideal for very low temperature (below 5°C) and/or very low humidity environments (below 40%RH) and their performance does not vary that much in usual garage conditions. However they use quite a lot of energy compared to the amount of extraction they offer. Refrigerant dehumidifiers are optimised for UK conditions, so will operate down to 0°C and will use less energy than the equivalent capacity desiccant unit. You will also get the benefit of the unit giving out 2.5 times the energy input as free heat to the garage.
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