Typically racks are kept in warmer than normal rooms often 80º or close to it. This provides correct ambient air temps and cool side temps and just a touch more heat to maintain a hot spot. It makes little to no difference under these conditions. this is the way most racks are set up. If you are attempting to run a rack in normal or below room temps most find that a secondary heat source to hold cool end temps. These conditions the closed body rack make a monster difference they hold much more heat I find my racks will hold a max of 6 degrees over room with out modifications as my room is below normal temps (66º and lower) I need all the help I can get.