Our 1997 built home has what appears to be a water stain in our living room, yet we cannot find the cause and the symptoms themselves are puzzling.

The stain itself is about six feet long and about five inches across its widest part (at the center). The stain is very symmetrical and runs perpendicular to the 18" floor trusses which support the second story. The stain is also near (but not centered on) an a/c duct which runs between the trusses.

I cut a one ft square of sheetrock out to see if I could determine the cause. The sheetrock was not stained on the truss side nor did the truss of a/c duct work show any signs of moisture. I could reach some of the drains associated with the upstairs baths so we tried several experiments with running water in the upstairs baths, filling and emptying tubs, etc. No evidence of moisture could be found. I also reached over the the nearby a/c duct and didn't feel any moisture there either.

The only time I've seen a situation like this occur was when I had my garage re-roofed. The roofer put in a ridge vent even though we didn't have any other vents for the garage attic. In winter, cold air would fall down through the ridge vent and chill the attic space. Warm, moist air from the cars would rise and apparently condense along the ceiling sheetrock seams. But in our case, the stains aren't along the seams. We have an open concept floor plan at the back of our house so the kitchen and LR flow into each other, but the stain is well away from the stove, sink, and d/w areas.

I began my investigation some months ago. The stain remains about the same size (unfortunately, I didn't measure), but that may be attributable to the fact that my cutting open the ceiling changed air circulation.

Right now, though, I'm totally stymied. I don't know what the cause of the stain is or what can be done to address it. One the one hand, I'm tempted to enlarge the hole, wrap all pipes in the nearby truss area with insulation, patch & paint, and then hope for the best. On the other hand, I'd REALLY hate to revisit this problem in the future or leave a problem for some other home owner to cope with in the future.

I would very, very much appreciate ideas on how to proceed.