Is this physics or just noodling?

I mentioned to a friend that my fridge is mostly empty and he said I should fill it with water bottles to save energy. As I see it, he's probably half-way right. I think there are two things that a bunch of water bottles and gallon jugs would do: first, they would reduce the amount of air inside the refrigerator, meaning that when you open the door, less cold air could escape; second, water has a huge thermal mass, so it would stabilize the temperature of the contents.

I think the first part, that you reduce the amount of cold air that can escape, is useful. But here's the thing, since water has such a significant thermal mass, it also takes a substantial amount of energy to cool. Doesn't that mean you lose a lot of energy efficiency bringing the water down to the desired temperature and keeping it there? Perhaps it would be better to stuff the fridge with balloons filled with air since they have a lower thermal mass and would still reduce the amount of air in the machine.

CDs listened to today:

• Johann Georg Albrechtsberger: Concerto in D for Trombone
• Bebop Deluxe: Raiding The Divine Archive
• Juraj Filas: Sonata For Trombone and Piano (At the end of the century)
• Ludwig van Beethoven: Sonatas for Piano, disk 4
  
• The Disposable Heroes of Hiphoprisy: Hipocrisy Is The Greatest Luxury