The most likely reason is that you have a regular, foam core, iron door. The best solution is to replace it with a Polar Shield™ wrought iron door with full thermal break from Donatello Doors. The difference between these two types of doors is huge - U 2.1 for regular iron door and U 0.21 for Polar Shield™ - that's 1000% better insulation!
However, if that is not possible then consider reducing the humidity in your home. Humidity is caused by the accumulation of moisture that evaporates into the air through the daily activity of showers, bathing, washing dishes, boiling and cooking food on the stove, drying clothes and even simply by breathing. A typical family of 4 will produce approximately 4 pints of water, per person, each day. This is over 100 pints of water a week per person. That is a lot of water and it all goes into the air in your home.
Humidity accumulation in our homes was not such a problem before our houses became airtight. Humid air would be replaced by fresh, non-humid air slipping in through poorly fitted windows, through vents, attics, doors left open etc. Unless you live in a very humid climate, in which case the fresh air outside is just as saturated with water as the air inside. Today, most homes are hermetically sealed to save on energy costs. As a result, this trapped, humid air when it comes in contact with a cold surface will condensate on windows, walls and particularly, your wrought iron door. Wrought iron will conduct the temperature outside to the inside surfaces of the door. So, when it’s cold outside, your wrought iron door will be cold to touch inside as well – no matter how warm you keep your house. In fact, by increasing the heat, the water vapor in the air, when it contacts the cold iron, will precipitate out forming condensation and even frost, depending on how cold the temperature is outside.
The University of Minnesota - Agriculture Extension Service made an extensive study of humidity and has provided some useful guidelines. For our purposes, we have simplified this information into a table that should be followed to keep condensation on your wrought iron door to a minimum:
Using a standard indoor air temperature of 70° F (21° C):
|Outside Temperature||Max Inside Humidity|
|+20° F (-7° C)||25% to 30%|
|+10° F (-12° C)||20% to 25%|
|0° F (-18° C)||15% to 20%|
|-10° F (-23° C)||10% to 15%|
If you have humidity levels that are higher than this, you should take steps to reduce it. You will need to purchase a de –humidifier. It should have a gauge to set the humidity level. Unfortunately, many of these gauges are notoriously inaccurate and will not tell you what the humidity is near your door – which may be different than in other areas of your home. A humidity meter can be bought for around $10 and can be wall mounted or placed on any surface within 6 feet or so of your exterior iron door (they are about the size of a small alarm clock).
Air leakage around your door in cold weather is another cause of condensation. If cold air is leaking into past the seals, it will dramatically cool the surface of the door, lowering the dew point and increasing condensation. The greater the temperature difference between inside and outside, the greater the likelihood of condensation forming on your exterior iron door.