I really need some help figuring out what is going on in my basement. I've had 4 cases of condensation in the past month. From what I've read, houses with central a/c don't usually have this problem so I'm not sure what is going on.

I have a basement apartment which is about 500 sq ft and has tiled floors. There are air ducts on the ceiling of each of the three rooms (bath, bed and living rooms) and a return in the kitchen, which also has an exhaust fan above the stove.

I've owned the house for two years and the basement was empty last year. I didn't notice any condensation or water problems until late May. Initially I found my tenants had opened windows during humid days, but they are all closed now.

Each time I turn the ac temp a little lower and run the fan continuously until it clears up.

I had another report of it this morning... temp was cooler so ac did not run overnight. I had a small window open upstairs which I don't think would cool things down enough to prevent the ac from turning on. In addition there seems to be some water in one of the exterior corners... there was rain last night and this morning.

I had previously checked the sump pump and its working fine. I do not see any cracks outside. The easetrough is split at that corner which could be a factor. However, since I had no problem last year I'm not convinced either of these are the cause. Plus there is a flower bed which is watered regularly but doesn't seem to cause a problem.

Before calling in a professional I'd like to make sure I've tried everything I can. The only other thing I can think of is that the tenants are using a plug-in air purifier. I don't think it impacts humidity but mentioning it just in case.

I would appreciate any suggestions you have.