Like keith said the roots don't need to be buried.
What they would prefer - if you like to fuss- is to have them sitting on the media and then every day with a spray bottle you give them a little spray.
Let them dry out of bit, have one or two roots buried in the media but those bulbs should be exposed more.
Imagine each bulb is a babies head. Would you bury them in the ground?
The roots will grow into the pot.
But the plant needs to be hydrated quite a bit.
So daily spraying (and letting the roots dry) is the best way to provide moisture. Get the roots nice and wet and let them dry between.