Dark is good... cotton sack is good... but any warmth is bad...
Be sure to keep them in a cool place... even a hall closet or than upstairs room that's too cold to sleep in...
Here in the U.S., potato farmers nerby that raise thousands of tons of potatoes each year store them in a semi-underground bunker and they keep until the next years crop are ready to store. Simply keep them in the dark and somewhere around 35 degrees (F) works every time.