55* with a humidity of about 65-70% The most important thing is stable temp as temp flucs will make your wine breath in and exhale through the cork with expansion and contraction. If you have bad temp fluctuations in your house and no way of getting around it then synthetic corks may be a better deal for you but for the most case perfect agglomerate are best as they will let a wine age and develop. Wineries use a lot of synthetics because of this reason but their wine has typically aged around 2 years already. Too high a humidity and your corks will develope mold, too low humidity and the corks will dry out even if they are on their side but this will take quite some time, probaly beyond whet we need to worry about.