Of all the conditions essential for optimum storage of your fine wine, humidity is one of the most important to be mindful of. The structure of your wine is completely dependent on the environment you provide it with. Unfortunately, humidity is also the most difficult element to control.
Proper humidity is essential in keeping the wine's cork moist, preventing the wine from evaporating to a damaging degree whilst in storage. However, corks do not completely seal a bottle of wine, so almost all bottles laid down for extended periods lose some of their contents due to evaporation. If the air is too dry, evaporation speeds up, reducing the level of your wine and increasing the ullage, or the gap between the bottom of the cork and the wine. When ullage increases significantly, it automatically decreases the value of your wine. A look at Sotheby's sales guide clearly demonstrates how important ullage is in determining the value of a wine. Over time in less than perfect wine storage conditions, low humidity can also cause the cork to dry up to such an extent that it loses its elasticity and lets air into the bottle - ruining wine through oxidation.
The ideal humidity level for wine storage is 70 to 80%. When stored in optimum conditions, fine wine can be laid down for 10 to 30 years and, according to wine experts, even as long as 50 years. This is why wine has traditionally been stored underground, natural humidity levels allowing for more cost effective and predictable control over the optimum storage conditions. To attempt to maintain the correct humidity level for wine storage anywhere above the ground, for the entire length of that period, is virtually impossible.
Lack of humidity is not only one of the biggest problems, but also the most difficult to solve. Ambient humidity in the UK fluctuates between 57 and 88% across a typical year. To artificially humidify the air might help the wine, but is considered commercially unviable by wine storage providers. Excessive humidity in storage can also cause a bottle's labels and packaging to discolour and deteriorate. This affects the presentation of the wine and, in today's market, would definitely decrease its investment value.