John in LA: Nice dissertation! Condensation is a lesson in basic physics.
1 Any material will radiate to the night sky. If it is a clear night the radiation will be greater. Grass does it, windshields, steel, wood, shingles etc. The radiation depends on the difference of the fourth power of the temperatures (absolute temperature of the material squared then squared again) and the dark night sky is close to absolute zero so heat will leave the item looking at the sky. Insulation on top of the roof does not reduce the radiation but it helps keep the material underneath warmer.
2 Condensation only happens when the material temperature is lower than the dew point of the air in contact with it. If you have a cool day and warm moist front comes by everything will get wet except those things in a closed building where the warm moist air does not get. Open a big door and the moisture will enter a cold building and wet what it touches until the dew point of the air is lowered by condensing water out of it, or the air is cooled enough to lower the dew point. If you walk outside with cold glasses they will fog up only if the dew point of the air is higher than the temperature of the glasses. Any open roof will drip if warm moist air moves in after a cold night. The bottom of cold insulation will still get wet if high dew point air contacts it.
3 Under roof insulation works because most of the time the roof does not get cold enough long enough to chill the bottom of the insulation. You don't get much moisture between the insulation and the metal because the air can't get to it. The little bit of air trapped between the layers will condense a small amount of moisture until a few hot days (all summer) drives the moisture out of the gap. If there is no air flow between the insulation and the steel that moisture will not be replaced. Felt or porous material will tend to absorb the moisture until the temperature comes up and then it can evaporate into warm air instead of dripping.
4 No insulation works in open sheds in many climates because the dew point is low overnight and only a little moisture condenses on the bottom of the roof, not enough to drip off come morning. We don't usually have a problem in Oklahoma, but my hay barn has rained many times this fall, for a few minutes. Then it warms quickly enough the water on the hay and equipment evaporates with no damage.
5 If you have something you absolutely need to keep moisture off, air conditioning to keep the dew point below the material temperature is probably necessary. Bagged feed in a well insulated building will get wet when the door is opened on a warm day after a long cold spell if a warm front came through bringing high dew point air in. one way to condition the air is with heat, you can also condition the air by cooling it to condense the moisture then reheating it slightly (what we normally think of as an "air conditioner")
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Today's Featured Article - Talk of the Town: The Saga of Grandpa's Tractor - by The following saga is from the Tractor Talk Discussion Forum. Someone. The saga starts with the following message: Hey guys I have a decision to make. I know what you all will probably suggest and it will probably agree with me way down inside, but here it is. I have a picture blown up and framed in my "tractor room" of a Farmall M. It was my Grandpa's tractor, of which whom I never got to meet. He froze to death getting this tractor out of the barn to pull a truck out of the ditch before I was born. Anyway my dad and aunt had to sell it at the auction,
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