What would cause that? The most likely answers are (1) there is inadequate ventilation in the attic so hot air is not able to escape; (2) bathroom exhaust fans are venting moist air into the attic. In the picture below, the problem was related to poor attic ventilation. Often, it will be related to both of those causes.

If you look at the photo below (where the problem is much less than one often sees) you will notice that the insulation has blocked the soffit vents. Proper attic ventilation should include both low and high venting. In this case the low venting is blocked so there is no "cross-circulation" of air coming in and then going back out at the ridge or roof vents. I have seen extreme cases of this where the attic has significant mold. Often the sheathing is falling apart.

There is another possibility too. That is that the sheathing was stained at the job site prior to it being put in place. To come to that conclusion, the inspector needs to rule out the other possibilities and, even then, it is an educated guess.

Steven L. Smith