Humidity and condensation

This article will explain how to protect your valuable belongings from mold, mites, mildew, rust, paper rot and wood degradation while in storage. While we will discuss things in depth below, the main point is that maintaining a Relative Humidity (RH) below 65% will prevent the growth of mold, mites, rot and wood degradation.

The self-storage industry has historically offered “climate controlled” units to tackle the humidity issue through temperature control and, in turn, relative humidity (RH). The industry norm for “climate controlled” units is a maintained temperature range of between 60 and 80 degrees Fahrenheit. Under normal circumstances, this range will produce RH’s below 65%, which will prevent the hazards listed above.

A more direct and effective method of preventing mold growth is through dehumidification or simply reducing the moisture content of the air below 65% RH.


Air contains water vapor, whose content depends on the temperature of the air and barometric pressure. Warm air can hold more moisture than cold air, so as the air temperature falls, the maximum amount of water the air can hold also falls. Air at lower barometric pressure can hold more moisture than at higher BP.

Three elements go into calculating moisture in the air (humidity) and how dangerous it may be to stored items – Temperature, Relative Humidity and Dew Point. The first two are fairly straightforward; however, dew point gets a bit complicated because it varies depending on the air pressure and water vapor content.

Relative Humidity

Relative Humidity is the “closeness” the air is to saturation. RH is the amount of moisture in the air compared to the maximum amount of moisture the air can “hold” at a given temperature. As temperature rises, molecules in the air expand and can hold more moisture. When the temperature drops, molecules in the air contract and can hold less moisture.  A Relative Humidity of 65% simply means that the air is holding 65% of the maximum amount of moisture it can hold before condensation begins. When RH reaches 100%, the air is said to be saturated and condensation begins.

Mold cannot grow below a Relative Humidity (RH) of 65%. See for yourself – the University of Rochester has a handy Dew Point Calculator.

Dew Point

The dew point is the saturation temperature for water in air. More specifically, the temperature at which the moisture (water vapor) in the air at constant barometric pressure condenses into liquid water at the same rate at which it evaporates. At temperatures below the dew point, water will condense and leave the air in the form of dew, fog or clouds.

You can find today’s Dew Point on most weather reports ( For example the dew point at the time of this writing is 56°. This means that the air is holding 56%


Humidity affects both thermal comfort and indoor air quality. Damp air:

  • facilitates the growth of fungi (mold) and bacteria that can cause respiratory problems and/or allergic reactions.
  • provides the conditions for dust mite populations to grow, which can affect asthma sufferers
  • results in odours in poorly ventilated spaces because of fungal growth will result in condensation forming on windows, walls and ceilings that are colder than the air temperature and potentially damaging building materials.

Maintaining a comfortable humidity range
North Carolina has a wide seasonal range of daily relative humidities. In April the daily average RH ranges from a low of 37% to a high of about 85%. In August, the range is a low of about 50% to a high of over 95%. There is no period during which the RH is naturally below the safe 65% level for a full day.


Condensation occurs when warm, moist air comes into contact with a colder surface such as glass window or a block wall. The air temperature in contact with the colder surface suddenly drops, reducing the amount of moisture it can hold. This results in moisture formation, or condensation, occurring on the cold surface. This often produces “sweating” which may be a white powdery substance – the lime in the block coming out with the condensation.

Condensation can occur on all surfaces that are cold enough and becomes apparent by mold growth.

Condensation can be controlled in two ways: first, by reducing humidity so that air is less likely to become saturated; second, by reducing the likelihood of warm air coming into contact with cold surfaces. This can be achieved through insulation.