As mentioned briefly in my post on barrels, temperature and temperature change has an affect on the aging of bourbon. Hot Kentucky summers lead to expansion and bourbon seeps into the barrels, while winter brings with it retraction. The rackhouse (or rickhouse) is where the bourbon barrels are stored and aged, and temperature regulation within these storage houses is thus crucial in bourbon aging.
A rule of thumb is that temperatures that dip below 40 degrees in the winter may halt the chemical interplay between whiskey and wood, and summertime heat above 90 degrees may cause too much expansion too quickly. The storage houses take this into account.
Traditionally, "old-style" rackhouses are layers of heavy wooden floors topped with a tin roof. Each floor holds barrels of bourbon. The walls are thick stone dotted with windows, and the basement is exposed dirt. The exposed dirt isn't a result of cost-cutting; rather, it helps to add humitidy to the dry winter months and softens the temperature changes between seasons. These warehouses were also built with ventilation as a priority - with the help of the several windows, the rackhouses were designed to allow cross ventilation, or lateral ventilation through each floor, and stack ventilation, which is the cycle of air from the top of the house to bottom and back again. All of this attention to detail is to the benefit of the bourbon.
These traditional rackhouses are relatively expensive to build, and thus aren't always a viable option for new construction. Newer, cost-effective rackhouses use less expensive wood, less robust exteriors, and cheaper, but highly reflective, metal roofings that deflect heat and reduce extreme temperature spikes inside. These newer-type warehouses don't allow as much control over the interior temperatures as the traditional ones. So, to make up for the temperature swings, the bourbon barrels are rotated throughout the rickhouse. Generally, the barrels at the top and near the outside walls of the houses are most susceptible to the varying climate. To compensate, barrels are moved from top to bottom and from outside in as they mature. Ideally, the rotation keeps consistency among the thousands of barrels.
Other types of rackhouses are made with a view towards accelerating the aging process. These houses are constructed with dark roofs to absorb heat, and meager walls that welcome the cold of the winter. The idea is that allowing temperature spikes will drive the bourbon into and out of the barrel walls quicker than other houses allow, and by fast-forwarding the process, a distillery saves a lot of time. However, this speed has its price, and bourbons that undergo this accelerated process are said to be of inferior quality to those whiskies that have paid their dues and spent years and years on the dark floors of the rackhouse. But, you decide for yourself.
I hope that with the past few handful of posts you are seeing a theme: bourbon is profoundly natural. Of course modern technology has permeated the industry, but at no point has it stolen the lore from the bottle. The appreciation and enjoyment of bourbon goes far beyond the taste of the liquid, and should not end when your glass is empty.