jazzycamel @ instructables.com has published an Arduino based Hydrometer with OLED display. It uses an Arduino Leonardo Pro Micro, an Adafruit SSD1306 OLED display and DHT11 digital temperature and humidity sensor.
We have been having some condensation and mould issues in our apartment recently due to, I think, the humidity levels. Our property is relatively newly built and, as such, has very good insulation properties (double glazed windows etc.).
Quick Arduino Hygrometer with OLED display – [Link]