Re: Water drips from ceiling but no rain outside
It surprises many people to learn that it is relatively rare that the water source is from the outside, although it could be due to bad flashing or ice damming. Much more common is that warm moist air escapes from the house below into the space between the ceiling and roof.
Originally Posted by wecurtis
This moisture laden air moves towards the vents but hits the freezing cold roof before getting to the vent and deposits its moisture. The ceiling is not well enough sealed and too much moisture leaks into the attic for the cold climate to handle.
Low drying capacity, not the same as high relative humidity, is a measure of the absolute amount of water that air under certain temperature and humidity conditions can accept before reaching saturation. Did you get that?
Or more simplified yet, cold air does not dry out water problems as fast as warm air will.
Below freezing temperatures and well insulated attics mean that water vapour quickly freezes (frost, not condensation). Ice evaporates more slowly than water, slowing down still further the drying process in the attic.
Uninterrupted periods of time under these conditions lead to serious accumulation of frost. If the above conditions exist, but are interrupted by occasional warm spells, the small accumulations we be carried off during the warm spells and you will have no problem. If there are no warm breaks a small leak can become a serious problem. However, when the frost build up is high and the outside temperature warms rapidly all that frost melts and drips water down toward the ceiling --- this is where many folks think the roof is leaking.
Where does this moisture laden air get into the attic?
Through light fixtures. Through plumbing chases. Through electrical holes. Through cracks in the top of partition walls. Through any hole in the ceiling. Bathroom fans that do not exit outside are serious sources of moisture.
Serious moisture problems have to be solved by cutting off the source of moisture -- sealing the house at the level of the ceiling. If you could see in the attic on a freezing cold day -- you would see the frost accumulation because it will be white. If what you see is generalized all over -- you have a generalized problem. If you see a cluster of it, look almost straight down and you will usually find a light fixture or some other hole to the house below. Seal off that hole so that the air cannot get into the attic, and you will cut off the moisture and not have to worry about trying to ventilate the moisture out.
"" an ounce of perception -- a pound of obscure "