Expert: Daniel Humphrey - 1/15/2009

Question
We put an addition on our kitchen 5 years ago. Every winter we get water on the ceiling at one corner were the old and new addition meet.I thought it was the roof with the angle that the new addition hit the old roof and siding. We had a new roof put on and extra flashing under the siding but it still leaked.(only in the winter when we had snow and ice on the roof) I also put a heated roof wire on the roof and gutters to cut down on the snow on the roof.
This week we had allot of water spots on the ceiling in the kitchen and it started to get soft so I took a saw and cut out a hole to look through. I could see the steel I-beam they used as a header for the new addition. The corner of it above where the ceiling was ruined was all wet and dripping and it looked like it was sweating due to condensation. Can that much water come from condensation on an I-beam or do you think I might have an ice build up somewhere that is dripping on the I-beam?
I left the hole in the ceiling open for a day and the I-beam dried up. Do you think that is proof that it is condensation? If it is how can I stop this from happening?
The I-beam is in an area that used to be the corner of the attic where the ceiling meets the roof. There is insulation between the joists but I have an area about 3ft by 3 ft, 25 ft long that I could blow in some insulation. Do you think that would help?  
We do have allot of snow and ice on the roof and this seems to happen only when we have snow on the roof so I can't say for sure if it's condensation or not. Thanks.

Answer
Tom,

I think you're onto the right track with the condensation theory. Yes, that much water can come off an I beam that has been collecting frost for a while and suddenly lets it all go when the temperature rises a bit.

Leaving a hole in the ceiling probably warmed up the I beam to above the dew point. Adding more insulation will probably just increase the problem, ironically. To eliminate condensation there, you would need to reduce the amount of moisture entering the attic and increase the ventilation. Wrapping the I beam with foam would help too, if that is possible.

Enough moisture can come through even one light fixture to do what you're seeing, especially above a kitchen or bath. Foaming all light canisters and electric outlets, sealing all air leaks into the attic, and caulking and re-painting with a vapor-guard paint will all help. Make sure no plumbing vents or air ducts terminate in the attic, and look at ways to exhaust moist air from the house more efficiently.

Every winter about this time, I get calls about roof and wall leaks that turn out to be condensation. Frost builds up on the cold attic surfaces if there isn't enough ventilation up there, and it melts on a warmer day and comes dripping down all at once. The worst cases I've seen appear to be ice dams because they occur where the roof meets the ceiling--same place as ice dams--but they are caused because the insulation has taken up any vent space. I have to actually go up and check for roof leaks because it's so hard to rule out an ice dam without checking the roof itself. One way to check is to keep the roof clear of snow for a while in cold weather and see if the drips stop.

So, you can't rule out a roof leak until you go up there when the snow is on the roof and dig the snow off and expose any water.  Your other guess about the water coming down the siding is plausible because that is so common. But your new roof and flashing should have taken care of those possibilities.

Daniel