Expert: Stephen Major (Owner--Major Design Group) - 4/12/2007

Question
I am having trouble with condensation on the interior of a vaulted ceiling. The ceiling is attached to the rafters with the top unfinished...the joint is covered by a 1x4.  I have had moisture run down all along the 13 foot span.  There is insulation but there are gaps at the very top where the ceiling meets at the top.  I was wondering if it was possible that cold air at the very top of the ceiling could be causing the condensation.  That is where the moisture is coming from.  I don't believe it is from any type of leak since the moisture is all on the surface of the drywall and will wipe off without any stain and there is no sign of any soft/wet drywall. There is a window about 3 feet below the peak and there is never moisture on the window.

Answer
Cliff,

There is one primary reason that condensation forms on a drywall ceiling -- the temperature of the ceiling surface (or a portion of it) is below the dew point of the indoor atmosphere.  In other words, the ceiling is cold and your room is warm and moist (relatively).  I can't say how to fix the problem without seeing it and/or understanding the way the ceiling assembly is constructed, but it is pretty obvious from your description that the insulation is inadequate to maintain a reasonable drywall surface temperature.

Something you can do is get a humidistat and try to keep the indoor relative humidity around 40% or less, either by ventilation, mechanical dehumidification, increasing the room temperature, or a combination of these methods.

It is common for vaulted ceilings to fail in this regard.  It takes a bit of finesse and planning to put them together correctly, and each case varies as to what is correct for the building and it's location.

Steve