You'll need to be conscious of all humidity producing behaviors. Boiling water, taking showers or baths, all need to have a vent fan running. Since you tightened up the windows, keep a roof vent just barely cracked open to allow warm, humid air escape. Wet clothing and towels also need to be allowed to dry in vented bathroom if possible. The cubic footage available is just too small to handle all the moisture.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI