A properly built cellar is really a house within a house, all the way up to its tightly fitting front door. Keeping the carefully conditioned air behind that door takes a knowledgeable builder and meticulous construction detailing.

“People think they can cheat on the construction of the room,” says Dena Rose, of D’Vine Wine Rooms, based in Wall, New Jersey. But you can’t. Her firm has found mold in poorly constructed cellars, she says. “We had to get those bottles out of there, basically rip out the walls and start again.”

Savvy builders insulate the room carefully and install a vapor barrier to prevent water from condensing inside walls. “Remember,” says Charlie Griffiths, of Vigilant, Inc., a New Hampshire–based specialist, “if the power goes down for three, four, or five days in a well-insulated, well-built wine cellar, it’s not going to be the end of the world. As long as you’re not going in and out of there all the time, the temperature will creep up slowly.”

Rule No. 2: Think big. 

“It’s amazing,” says Art Bogue, who has designed thousands of these cocoons. “Once you have empty spaces where bottles should be, they will get filled.” Let Bogue (now a designer for Bilotta Kitchens of New York City) build a cellar of 3,500 bottles, and by the next year he’ll discover that the collector owns 4,000. “He’ll have cases stacked up on top of each other. No matter how big you do it, it’s just not big enough.”

Rule No. 3: Make sure that the design will exclude strong light, excessive vibration, and harsh odors

That means you’ll need mechanical equipment—and you won’t want that eyesore, an off-the-shelf refrigeration unit mounted on the wall. The best installations are custom ducted systems designed by a professional with wine-cellar experience. With careful planning, ducts and grills can be hidden inside walls and cabinetry so the system is essentially invisible.