condensation on metal porch roof

Hi,
I have a metal roof on an outside porch attachment [no ac, no heat] and the roof gets a lot of condensation on the underside in the morning, and sometimes drips and gets things wet. Can I just seal the underside with latex paint or elastomeric paint? Will that make any difference? Do I need to install some sort of vent fan?
thanks,
itchy
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The water in the room air is condensing on inside of the cold roof. Surface treatments to the roof will not fix this. Either lower the temperature of the inside to match the outside, or dehumidify the room air to an extreme point, or insulate the underside of the roof.
    Una
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On Thu, 2 Dec 2010 11:30:50 -0800 (PST), internaughtfull

Is it open (screened) or enclosed?

You need to figure out where the moisture is coming from. If it's enclosed, the moisture may be coming from inside, and a better seal would help (windows, doors, even walls). If open, the moisture could be coming from the ground or from vegetation, including potted plants on the porch. If it's screened and not too large, perhaps you could cover the screens on nights when you know it will drip, to stop moist air from entering the porch. (But that won't work if the moisture is coming from plants on the porch.)
I'm guessing that the roof is exposed to the sky -- this is why it gets colder than the air underneath and causes condensation on the underside. And it probably drips on clear nights when the low is at or near the dew point.
If you add insulation under the roof, then you have to make sure the moist air cannot get through the insulation, condense on the underside of the roof, and just get the insulation wet instead of what it drips on now. You would need a vapor barrier on the underside of the insulation, and you would have to make sure the vapor barrier is unbroken. Even a small gap could be enough to get the insulation wet.
Currently the underside of the porch is releasing energy through the roof via condensation and then (probably) radiation. You want to change the first part to conduction instead of condensation. To do this, you have to stop the moist air from contacting the cold roof.
Ventilation will help if you can arrange it to bring in drier air, for example from above the roof. If it brings in the same moist air that's resulting in the condensation now, then the only way it can help is if it moves the air fast enough that it can't cool enough for the moisture to condense. That would take a lot of air movement.
Ventilation will definitely help if the porch is enclosed and you have plants on the porch.

That might protect the roof, but will not stop the drip. Might slow the drip just a little, but only in proportion to the R-value of the paint, which is going to be way less than R1.
What color is the top of the roof? Dark colors radiate more heat. If the roof top is dark, painting it white or making it shiny might help a lot. Putting another roof over it would probably stop the problem entirely. You might still get condensation on the underside of the roof-over, depending on where the air between the layers comes from, but it would only drip on the lower roof. You would have to be careful with the roof-over structure, since it would get damp or wet a lot -- definitely all PT wood and non-rustable hardware.
Does it drip randomly? Does it get rafters wet? If the drips are in predictable locations and don't wet the rest of the structure, perhaps you could just arrange to move the drip water out. A few funnels and some tubing ... funky, but cheap. Perhaps even some strands of caulking to channel the condensation before it drips.
Edward
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