Should Sourdough Starter Be Airtight?

While the temperature and surroundings of a starter are crucial to its outcome, the sourdough starter does not need to be sealed in an airtight container. It's still helpful to cover the starter with some sort of a lid, to prevent any mess from ensuing (via The Perfect Loaf).

Do sourdough starters need air?

Oxygen: Fermenting sourdough starters will produce carbon dioxide. The starter should be loosely covered in order to safely release the gas, but the culture does not require oxygen.

Should sourdough starter be covered or uncovered?

Your starter will grow to at least double in size, sometimes more, and you'll need a jar to accommodate this. You can cover it loosely with a lid, plastic wrap, or even a small cloth. ... Keep in mind, the jar might burst if the lid is on too tight which means you'll run the risk of getting glass shards in the mixture.