RH problem

[Ric Brekne]

Hi folks

We just got done installing a humidity system in an organ room here at the University I work for.  The thing has a humidistat and it was set to 40 %.  During the last bit of the cold period here the outside temperature hovered around 36 degrees and we keep inside temps around 72 %.  The inside RH was at 25 % with the unit turned off, and going on max it came up to just under 40 %.  All seemed well. 

The cold period ended and the last 3 days has seen very warm outside temps... upwards of 80 degrees F. RH continues to be low ... outside around 35 %.  However we walked in the room this morning and found green fungi beginning to grow on the ceiling and obvious signs of condensation on the upper walls.  Fortunatly the humidifyer was pointed away from the organ and that part of the room seems  uneffected. 

What I dont really understand is why the condensation ?  Inside RH levels remained nomally over 40 % the whole time as far as we can see. The only thing that was different was the temperature.  That would suggest the humidistat was working correctly.   

Any good explainations  ??

Cheers
RicB