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Condensation on ceiling while painting?


The last time I painted the master bedroom in my home there was a spot on the ceiling where it looked like condensation was forming. The spot was diagonal from the window not above if that has any bearing. It had actually happened a few years prior the first time I painted after we moved in. Same exact area. Both times I believe it was in the colder months too. Only two times it has ever occurred. The last time I dried it off with a towel but it was Behr flat ceiling paint and it left a large mark from where I had used a towel. Had to tarp the walls and repaint the ceiling after the fact as touching it up was not going to help.

Fast forward a few years and this time we are paying someone to paint that room as well as our kitchen. Mind you this has never happened in any other room. I brought it up to the painter and he was sort of stumped...said maybe someone had used an oil based primer at some point? Said it could happen again but if it does to just let it dry.

I'm anxious about this because we are not paying him to do the ceilings as they look pretty good.
Each time this happened I remember having the window cracked open because it was hot in there, although maybe that was my mistake? it is not a huge room...10x15 and we are in an upstairs condo. The window is smaller and it can get stuffy in here. Always seemed to do so when I had painted!

Any suggestions? Keep the window closed? Leave the heat running? use a fan in there as he is working? I don't think it is an insulation issue. JUst really hoping that it doesn't happen this time around. Thought I would ask! Thanks :)
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Posted 2016-10-29T22:05:59+0000  by jk4321 jk4321
As you know, condensation occurs when warm(er), moist air comes in contact with something cooler.  Since you said it was the winter months I would assume that the heat in the house would be relevantly consistent and heated surfaces equally.  In other words, your ceiling should have been about the same temperature across the surface, maybe being a little cooler towards the outside walls.  The only thing that might make sense is if there was a spot of insulation missing or perhaps there is [improperly] buried electrical junction box in the ceiling creating a gap where cold air in the attic could come in contact with the drywall of the ceiling.  When you let outside air into the room, I suppose if it contained enough moisture it could condense somewhere.  Although cool air would sink to the floor of the room first.

I would let your painter decide how he wants to ventilate the room.  Since he's not painting the ceiling, it shouldn't be an issue.
Posted 2016-10-30T00:37:59+0000  by Adam444

Remember that during the actual painting, a lot of moisture is entering the air from the water based paint itself. During my contracting days in Chicago in winter, special measures had to be taken during really cold weather, especially in older, less than well insulated homes. Often exterior walls or ceilings near walls would begin to sweat to the point that the paint would begin to run. there are a couple things that can be done to stop this: first, turn the heat up . Secondly, if possible, set the furnace fan on constant "run". I always carried a good box fan in my truck, even in winter, just for this purpose. The more air moving, the less the chance of condensation forming.

Opening the windows when the outside air is zero degrees will not raise the relative humidity in the house, as there is absolutely no relative humidity outside at these temps. However, when it is in the 30's outside and the relative humidity is 80 or 90 percent, opening the windows may actually raise the interior relative humidity.Do keep in mind, however, that you are putting much moisture in the air via the paint. 

If the ceiling is under an attic, look up there to make sure the insulation is sufficient in that area. Over the years, under insulated areas will begin to show up as gray, dirty shading as the airborne dirt sticks to the slight condensation on the cool surface. 
Posted 2016-11-02T22:16:07+0000  by ordjen

Thank you for the responses. Maybe the issue was from the temps outside and having the window cracked. It was definitely colder out, maybe 40's? It I supposed to be in the low to mid 50's this time around. Is there still a need to run the heat or use a fan in the room to keep air moving or no? I'm glad a pro is handling the job this time and that it wont be really cold out. Still nervous about that happening again but my fingers are crossed!
Posted 2016-11-03T21:16:29+0000  by jk4321
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