A: Unfortunately this happens in our area a lot more than you think, there are several causes, the most common ones are. Poor airflow in the duct, not enough insulation on the duct and seams and connections on the duct not properly sealed. When the hot humid air we have here comes into contact with cold leaky ducts it cause dew (sweating duct). The problem with this (besides the obvious issue of higher utility bills) is it can cause floor joists and sub floors to become rot, so, pay very close attention to it. The local utility companies have programs with TVA that can get you an inspection and even some money towards the repairs and of course your A/C contractor can help with is as well. Be comfortable!!