Thomas is correct and that is what I experience in Finland. The relative humidity of the air in the room when drying the film needs to be as near as possible to 50%. In winter in Finland, the dry air brought into the house and warmed up to 20c has a relative humidity of 20 to 25%. The film curls like hell.
Originally Posted by Thomas Bertilsson
Flattening the sleeved film under a pile of books after it is TOTALLY dry can improve it a lot. Beware that the film can get damaged if still slightly damp, because the pressure "embosses" the texture of the sleeve into the emulsion.