Malt whisky develops up to 70% of its character during the aging process, which takes place in different types of aging warehouse. The traditional type is dunnage, with racked warehouses originating in the 1950s - 60s. Dunnage certainly looks very different to racked warehouses. But with most distilleries using both, the question is whether there are differences in the environment they provide, the key factors being temperature, humidity levels and air circulation.
Dunnage warehouses are low-rise with stone or brick walls, earthen floors and a slate roof, with casks stacked on top of each other a maximum of three high (any more and the weight bearing down would be excessive). Racked warehouses are more high-rise, using brick, cement blocks or a steel clad structure, with concrete floors and tin roofs. Casks are stacked up to 8 -12 high on tall racks fitted with steel rails.
The thinner walls and tin roofs of a racked warehouse transmit temperature changes more readily than the thicker walls and slate roofs of a dunnage. Additionally, racked warehouses can have a larger range of temperatures between the floor and roof (depending on the height). Nevertheless, average annual temperatures are similar in both types of warehouse.
The significance of temperature is that it prompts a vital part of the aging process known as a 'cycle.' When the temperature rises in spring and summer the spirit expands within the cask and penetrates into the oak staves, which contain various flavour compounds. As the temperature cools in autumn and winter, the spirit contracts and exits the oak, carrying flavour compounds (which add vanilla notes, for example) back into the 'bulk' of the spirit.
Temperature also influences the rate at which water and alcohol evaporate from the cask, oak being porous. Evaporation helps develop complexity, and as the temperature rises so does the evaporation rate. As average temperatures are similar in dunnage and racked warehouses, so are the annual evaporation rates, at around 2-2.5% of the contents of the cask (although the influence of different evaporation rates is one of the least understood aspects of aging).
The temperature is also influenced by air flow, with dunnage warehouses typically having a window at either end (fitted with bars and mesh), while racked warehouses tend to have air vents (a slatted section allowing air in but not rain) along the walls. It's the degree of ventilation that counts, with windows and air vents able to provide equally good air flow. This helps maintain more stable temperatures throughout the warehouse, which is believed to promote a more consistent aging process.
Dunnage warehouses generally have higher humidity levels than racked. This is partly because dunnage have earthen floors which release moisture, with the level of concrete limited to a few paths, while racked warehouses usually have concrete floors. Some distillers believe higher humidity promotes a slower (and therefore preferable) rate of maturation, though the influence of different humidity rates is still being researched.
Another key factor when comparing dunnage and racked warehouses is that both types vary. Some racked warehouses limit the amount of concrete to paths, while also being insulated which promotes more stable temperatures. Similarly, some dunnage warehouses have concrete walls (ie. thinner than stone or brick), together with concrete floors and air vents.
Moreover, as the climate varies across Scotland conditions within a warehouse are also influenced by location. Even within a distillery site different areas are more sheltered/exposed, more/less humid, etc. Furthermore, each warehouse comprises varying microclimates (eg. cooler nearer the doors). Consequently, each warehouse should ideally be assessed individually.
Then again, it's difficult to isolate the influence of a warehouse. One reason for this is that every cask is individual, and even the same batch of spirit filled into the same types of casks on the same day, aged next to each other in the same warehouse will show differences. These can be subtle, or significant differences. This explains why casks are considered the primary influence on the aging process, and the warehouse secondary. Meanwhile, whether, and the extent to which dunnage and racked warehouses can each influence the character of a malt is the subject of on-going research.
Comparing dunnage and racked warehouses also raises practical differences. Dunnage are more labour intensive to operate than racked, as moving casks in a dunnage is essentially a manual task, with small hydraulic ramps usually the only mechanical assistance that can be used. This is because the doors of a dunnage are too small for fork lift trucks, which can easily get through the larger doors of racked warehouses. Consequently, a job requiring an hour in a racked warehouse could take up to 3-4 hours in a dunnage. Racked warehouses can also be more expensive to construct, although more efficient in terms of using the available space. One important similarity is that both types of aging warehouse always contain a range of casks spanning malt whisky of different ages. This practicality prevents the loss of an entire year's production in the event of a fire or other calamity in a warehouse.