[quote}But I would have thought that the key factor is the relative humidity. The higher this is, then the more moisture will condense on the metal of the car promoting corrosion? I don't see the logic of allowing as much atmospheric air as possible ventilate the garage when the ambient air has say 95%+ RH as it often does during the winter months.
I think the key factor is temperature differential. If there is no difference between external and internal air temps and humidity, then condensation will simply not happen
I can see why you would say this and it would be true of all materials in contact with the air were the same temperature. In reality, various materials allowed to equalibrise with ambient air will be cooler than the air. Just touch a piece of metal near you now. It will feel cooler than the air around you. This is why a car left out on the drive overnight, at colder times of the year, will often have dew on by the morning.
I'm going to copy and paste an explanation I've found explaining the relationship with humidity and temperature. Don't let the misspelling put you off, it makes sense and seems credable:
"Hi, a bit of info from when I used to take relative humidity reading a few years ago. Any figures are for example and are not accurate!
Humidity and temperature go hand in hand being that they form a "due point". The due point is the point where the air at a given temperature can no longer hold water vapor in it and the water vapor then condenses into due. Using a de humidifier removes water vapor from the air so the due point must therefore be lower, the due point being the air temperature that water condenses at a given relative humidity. So to answer a few questions here, at 0 degrees C, the air being 0 C, the water vapor then freezes and becomes a solid that is not part of the air. So if the temperature is 0 or bellow there can not be any condensation, the driest place on the earth is Antarctica followed by the Arctic. This is due to the extremely low temperatures so no condensation can form. Relative humidity 0%
Working out the due point:- this is done by measuring the "relative humidity" as a percentage and also the temperature, there are hand held gauges that do this, the old fashioned way is with a dry and wet bowl humidity checker. This consists of two thermometers, one is dry at the end and the other has a small amount of cotton wool round it to which a small amount of pure, not tap, water is added. The device is then spun round for a minuet. The dry bowl thermometer must therefore measure air the temperature, the wet bowl thermometer measures the temperature of the water as it evaporates into the air. There is then a complex equation that works out the relative humidity. The calculation includes logarithmic scales. A cross reference chart is normally uses where you can look up dry bulb say 18 c wet bowl 14 c RH 87%, from this the due point is looked up on another chart. Due point 12 degrees C. Now the air can hold 87% humidity fine but, if after taking two readings half an hour apart and the temperature is falling as you get to 12 degrees (the due point) the water vapor will condense and you get condensation! The problem with say your car or vice is that it is metal, or glass window, these are not as warm as the surrounding air, this is why water condenses on your windscreen and car in the early evening you will notice, as the air temp falls, but it is neither raining or misty. The car or vice is below the air temp of 14C and also bellow the due point of 12C. De humidifiers remove the water from the air and also warm the air. This has two effects, one is that the due point falls to well below the present air temperature as water is removed and also as the air is warmed the due point goes even further away from the air temperature as the air temp rises. A/C units do not warm the air. Most good de humidifiers have settings as to the relative humidity; mine has 70% 60% and 50% or constant. This is of course relative to the air temperature. As the air gets warmer it expands and so can hold more vapor. 50% humidity at 10C is far more water than 80% at 35c. Now when we come to paint something, we find that we see common failures to the protective paint coat on large structures, motorway bridges or gas tanks. You may notice that one end of a motorway bridge is rusting, this is due to the due point getting within 2c of the steel temperature at the end of the day as the air temperature falls. When this happens bare steel feels slightly clammy or just damp. The water is condensing on it and has formed a layer that is sealed under the first paint coat. The paint fails to adhere to the steel and no matter how many coats are put on top they never truly seal the steel from the air and water. Next time you paint anything make sure the temperature is stable or rising also the steel temperature is not close to 2C of the air temperature. Also make sure the air temp is not falling with a high humidity percentage.
This is only a very a rough guide to due point, relative humidity and condensation. Good cans of paint will say in the small print, do not apply if the substrait temperature is within 2C of the due point or the relative humidity is 68% and the air temperature is falling. Or words to that effect.
Sorry it's so long but now you know all about it."
