Highly-neutralized acid polymer compositions having a low moisture vapor transmission rate and their use in golf balls

The present invention provides high moment of inertia golf balls comprising a low specific gravity core layer formed from a moisture resistant composition. The moisture resistant composition has a moisture vapor transmission rate (MVTR) of 12.5 g·mil/100 in2/day or less and comprises a highly neutralized acid polymer. Golf balls of the invention have a moment of inertia of 85 g·cm2 or greater.