Widest tire safely fit on rim
Assuming you're measuring accurately, you can use this table from Sheldon Brown or this table from Schwalbe. 

The (inner) rim width is measured this way:



(Source: Schwalbe)

and for a 13 mm width, 18-25 mm tires and for 15 mm width, 23-32 mm tires. So, So, a 14 mm width (assuming you're measuring properly) should be fine with 28 mm tires; you can exceed the recomendations by a bit usually without adverse effects.