• All air contains a certain amount of moisture. 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 the Relative Humidity or RH and is expressed as a percentage. For a given amount of moisture, the higher the air temperature the lower the RH%. Equally the lower the air temperature the higher the RH%.
  • It is the cooling of air that causes the problems. As air cools it can hold less and 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 then moisture is 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.
  • 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.


    The aim is to arrive at a stable environment between 40% and 60% RH independent of outside temperature. Below 60% bare metal can be left out in the open and rust/corrosion all but stops. Below 40% RH leather and wood can dry too much and will become brittle and crack. This target environment can be achieved in several ways:


    By heating the garage the air becomes warm enough to hold the moisture and condensation stops – well nearly stops – if you bring a cold car in from outside into a nice warm garage then condensation occurs immediately (like taking a milk bottle out of the fridge). Heating also has one other serious drawback – cost. To heat the average garage sufficiently will require several kW of power either as electric heaters or radiators. This heat will need to be continuously applied through cold weather to stop condensation and this will be very expensive to run.


    This can take the form of natural ventilation built into the garage or forced ventilation by installation of a suitable extraction or positive pressure device. Ventilation will remove the stale high humidity air from the building at little or no cost. One current commercial system on the market incorporates a small heater to aim to lower the RH% of the incoming air. The reality of these systems is that the air that enters the building from outside to replace the exhausted air will still be high in humidity. Claims that for every 1 degree increase in temperature the RH% will reduce by 5% are frankly inaccurate, as the relationship between temperature and RH% is not constant. Furthermore, any temperature rise has to be sustainable throughout the whole of the garage volume – think how little a 3kW fan heater does to the garage temperature. There is no control over absolute humidity level in the garage – only of the air entering the garage. The average humidity level in the building can still remain above the point where deterioration can occur.


    Several manufacturers supply systems that enclose the car completely in a plastic ‘bubble’ and provide low volume ventilation airflow that evens out the humidity within the bag and maintains its shape. These systems stop condensation quite successfully and are relatively cheap to run. The downside is that the systems make no attempt to control the RH%. The contents of the bubble can still be at a RH% well above safe levels. Another disadvantage is a practical one – these systems are fine if you want to move the car very infrequently. If you use, or work on, the car in the winter then getting it in and out of the bubble can be laborious and adds to the wear and tear of the bag itself. It is also obvious that only the air within the bubble is controlled – everything else within the garage area is unprotected – tools, stored parts etc.


    By removing the moisture from the air in the garage with a car storage dehumidifier everything stored in the garage will remain condensation and corrosion free. Dehumidifiers are cheap to run, quiet in use and largely maintenance free. Suitably sized units will reduce and then maintain the humidity within the 40-60% RH target environment. In-built humidistats ensure that the air does not become too dry or too wet by turning the machines on and off as required. A correctly chosen dehumidifier will tolerate periodic changes of air such as opening the garage door or internal door. Furthermore, the action of dehumidification will bring the building as well as the contents into moisture equilibrium after a few weeks. If you then bring a wet car into the garage after a run in the rain it will actually dry it out for you!
  • There are two main types of dehumidification equipment on the market – each use a different method to extract moisture from the air.


    By far and away the most popular type of dehumidifier, certainly for domestic applications is the condensate type. A condensate dehumidifier uses the principle of dew-point to its advantage. Air is passed over a cold coil and is cooled to below dew point thus giving up its moisture as condensation on the coil. This condensation drips from the coil into a collecting tank below or is piped away to a drain. The dry air is then passed over a hot coil before exiting the unit. The air coming out of the dehumidifier is a degree or two warmer than that going in. Dehumidifiers using this principle are small in dimensions and easy to install – just requiring a mains power point. If you want the water to drain away continuously then a pipe can be attached through a removable blanking plug on the machine. The machines require no regular maintenance other than periodic cleaning of the built-in air filter and can be expected to last for 10-15 years. All modern machines use CFC free refrigerants.
  • A word of warning – Do not use standard domestic dehumidifiers in an unheated garage environment. They will not work when the ambient temperature drops below about 10C. The condensing coil becomes so cold that it freezes solid and the dehumidifier stops working. All the machines Dry-it-Out supply have ‘Hot Gas Defrost’. Machines with Hot Gas Defrost detect the onset of freezing on the coil and either reverse the flow of refrigerant or vent warm air internally to automatically defrost the condensing coil. A dehumidifier with Hot Gas Defrost will operate at ambient temperatures right down to freezing point and most will stand -5C without machine damage.


    To control the larger volumes of, for instance, a commercial car store, a different type of dehumidifier has traditionally been used – an adsorption dehumidifier. Adsorption dehumidifiers use a completely different principle to extract the moisture from the air.
  • Air is passed through a high surface area rotor (rather like a catalytic converter to look at) that is impregnated with silica-gel. As the air passes through the rotor the moisture is adsorbed onto its surface. Part of the rotor is separated off from the main airflow. Through this part a small amount of air is ducted in from outside the building, heated and passed through the rotor. The moisture on the rotor is given up to the hot air which is then exhausted back outside the building as hot wet air. The newly-dried section of the rotor then re-emerges to the main airflow ready to adsorb more moisture. The rotor turns slowly making the operation continuous.
  • The adsorption principle has important advantages. Firstly, its action is independent of temperature. An adsorption dehumidifier will operate as efficiently at -20C as +30C. Secondly the adsorption principle makes it possible to reduce the humidity levels to lower levels than condensate types. Larger adsorption dehumidifiers are permanently mounted units. However, Dry-it-Out Limited have a range of small portable completely self contained machine for domestic storage applications, such as the DD8L.
  • Dehumidifiers offer many advantages over alternative storage systems so it is little wonder that dehumidification is fast becoming the most popular solution to effective car storage in the UK.


  • We also supply and fit fully automatic control systems if required. Monitoring humidty and temperature we can supply a system that will automatically start and stop a dehumidity within an adjustable humidity range and all data is output to a secure link allowing remote moitoring. Contact us for details
  • To find out more about dehumidification for your storage area contact Dry-it-Out either by telephone, fax, e-mail or by completing the enquiry form. We can provide expert advice on all aspects of car and commercial storage and supply a wide range of machines to suit all applications from small workshops to large commercial dehumidifiers. Dry-it-Out also supply dehumidification and monitoring equipment to many other commercial applications including power stations, museums, the maritime, defence and food industries and agriculture.