Humidity Control

[Algonquin]

Attached is information I posted on another site last year. I thought it might be beneficial to post it here for forum members.

I purchased my current home six years ago with an unfinished basement. A dehumidifier was the first item I purchased. I then installed my layout, and then finished the basement around the layout. At the time, I did quite a bit of research on room environments for collectibles and specifically trains. The following is some of the information I acquired.

Condensation is a major concern for the storage of trains. All air carries water vapor. The amount of water vapor air can hold, is very dependent on the air temperature. The warmer the air, the more water it holds as vapor. The amount of water vapor in air, relative to its maximum capacity at its current temperature, is called Relative Humidity, and is usually specified as a percent of the maximum capacity.

Water in air in vapor form will not harm our trains. The concern for our trains is when the air temperature drops, and the air can no longer hold the amount of water vapor it is carrying. As the temperature drops, the relative humidity of the air rises until it exceeds 100 percent. At this point, the water vapor in the air begins to condense on surfaces. In a basement, this can be on the cold walls or anything stored in the cold basement that is at the cold temperature. This fine moisture causes rust on the various metal surfaces associated with our trains (gears, exposed wires, track, etc.). In addition, if the area is dark, mildew can also result from the condensation.

A dehumidifier removes water vapor from air, effectively reducing the relative humidity of the air, such that the air at the colder temperature will not be forced to condense.

As an example, take a summer day with an outside air temperature of 80 degrees F, with a relative humidity of 80 percent, and a basement temperature of 65 degrees F. When the air from outside flows into the basement, it will cool to the basement temperature. This will force water vapor to condense out, since air at 65 degrees cannot hold as much water vapor as the warmer outside air. The opposite occurs in the winter. The colder air from outside, say, 30 degrees at 70 percent relative humidity, would warm up to the higher indoor air temperature 65 degrees, and the air is capable of holding more water at the higher temperature. The relative humidity drops, and the air feels dry because it will actually absorb water.

The goal in preserving trains in a basement is to maintain the relative humidity below 100 percent for the coldest spot in a basement. Generally, a safe room air relative humidity in a basement is about 65 - 75 percent. The exact number required depends on the condition of the basement. If the basement is unfinished, the walls and floors will be much colder than the general room air temperature. Boxes stored on these cold surfaces will generally cool down to those temperatures. A lower general room relative humidity is required to ensure that moisture will not condense out of the air at the cold surface temperatures.

If the basement is finished and well insulated, the temperatures at the room walls will be more near the general room temperature and a higher general room relative humidity can be maintained without condensation occurring.

Sizing of Dehumidifiers:
Dehumidifiers are rated by the number of pints of water they collect per day. The Association of Home Appliance Manufacturers provides a sizing recommendation chart for humidifiers (http://www.aham.org/mfrs/cert/dehumcert.htm). As an example, it recommends that for a moderately damp area (unfinished basement) of less the 1500 square feet, a dehumidifier rated at 25 pints per day is required.

It should be noted that basements are not the only storage areas that can be a problem. Even a dark, cool closet in a living area, can be a problem in humid parts of the country, when the dwelling is not air conditioned to keep the relative humidity low.

One thing to remember, unfinished basement walls and floors are generally much colder than the basement air temperature. The dewpoint in the room must be maintained at a higher temperature than the wall or floors to ensure moisture will not form. A box of trains stored on the floor or next to the basement wall will be colder than the general room temperature. If the box temperature drops below the dewpoint, condensation can form inside the box(s).

All dehumidifier units have internal fans for moving room air across the condensing coils to remove moisture from the air. Most rooms in homes have enough natural air circulation as to not require additional circulation (fans). Generally, if you have a unit sized large enough for it's room, separate fans are not required (the bigger units also have bigger fans).

I hope this information is useful.
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Tim P