Ivory keys and humidity.

[theodopolis]

This may not count specifically towards pianos, but my pipe organ with ivory naturals develops a rather unpleasant slipperyness almost like dew after a chilly morning where water condenses on the keys (i presume that's the explanation).  The moisture isn't superficial either: the keys require about a minute of rubbing with a handkerchief so they can be played without the fingers slipping and introducing a rogue glissando in the middle of a hymn.

Is this a problem with piano keys too?
I realise that draughty churches may be more prone to this sort of temperature etc.

Would something like a felt cover for the keys result in a prevention of this rather annoying ritual every Sunday during winter?

Thanks
Theodopolis