Rack location in wine cellar

[atrent32]

After almost a year in delays my wine cellar is almost complete. The only thing left to install are the racks. Here is my question, can the racks be atatched directly to the walls or must there be space between back of rack & wall. I have heard everything from al least 4" to no space needed at all as long as the cooling system is the proper size for the room. Please advise.