There are 3 major aspects for keeping wine barrels in a cellar: temperature, relative humidity and air movement. Some winemakers believe in extra “factors” having impact in wine aging such as: light, noise, cellar shape, location, orientation (North facing, South facing etc), but in general the cellar temperature, the relative humidity and air movement have a major impact on wine aging and on barrels too.
Humidity I would say is the major factor for aging wine in barrels. Usually humidity level must be maintained between 60% – 75% all the time. If humidity is too high (over 85%) mold start growing outside the barrels specially around the bungs. In time may have a major impact on wine quality (mold can “migrate” trough bung hole/staves in wine), giving a “moldy” taste. In general is happening in the summer time when temperature and humidity have the optimum value for mold developing. Also, when humidity is very high one strange phenomenon may occur, such alcohol content dropping. If humidity is low (less than 50%) the wine losses are higher. At lower humidity in the cellar, the losses for one barrel can be 1L of wine per month. It has also and advantage by raising the alcohol concentration in wine. And don’t forget humidity is changing temperature values too.
The best cellars with natural humidity control are those built at certain underground levels, at least 5 meters underground, where humidity and temperature can be steady all year around. Some cellars have a specific mold growing on the walls for keeping certain humidity levels. Others, they have gravel on the floor (without concrete barrier), so the floor can “breath” by giving/taking moisture out of the cellar.
Air movement is the second important factor for an wine cellar. Must be kept a constantly air movement in the wine cellar. The air flow must be a very slow natural air exchange between inside and outside of the cellar. When the air flow is created artificially (fans, forced air venting systems), need more attention because the excess of air flow between the barrels can increase the wine losses during aging process. Air movement controls humidity and temperature and also control the corner areas where air had tendency to “stuck”. That’s why most of the underground wine cellars have ceiling rounded and less corners.
The best air ventilating system is the natural one. When the cellar has build-in long chimneys for outside air intakes (bringing air at the bottom of the cellar) and air vents at the top (in ceiling). So, by natural temperature exchange, the warm air is going up and in the same time the new fresh cold air it gets inside close to the cellar floor. With a natural venting system the wine cellar is “breathing” all year around and keeping fresh air inside (slowing down the mold developing on barrels).
The temperature in wine cellar it is another important factor for aging wines in barrels. The ideal temperature should be all year around at 12*C. Keeping the wine at 12*C, the oxidation process is very slow and the microorganisms have slow growth too (acetic bacteria, white film on top etc). To keep a steady temperature all year around, the cellar must be underground at least 5 m and must have an air movement system (for temperature and humidity control).