humid basement - how do I find the source?

Hello,

I live in a relatively new construction (2002) and the basement is humid in spring, summer and fall. Mold is growing on items that are stored in the basement.

There is no obvious water leakage. The builder claims to have installed a high quality vapor barrier. Our gas-based water heater (which heats water for our baseboard heat) is located in the basement as is a second refrigerator. The water heater manufacturer has sent out a rep who did not see any possible vapor from the heating system as the cause.

We are one unit in a two unit building that shares a single foundation. Our neighbors have no humidity problems in their basement (the sump pump is located on their side of the property - if that matters).

We are stumped.

Any suggestions would be appreciated.

Lanhsin