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?
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.
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.
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