Insulating materials are bad conductors and so this reduces the heat loss by conduction. The material also prevents air circulating inside the cavity, therefore reducing heat loss by convection.

Does insulation affect convection?

Insulation means creating a barrier between the hot and the cold object that reduces heat transfer by either reflecting thermal radiation or decreasing thermal conduction and convection from one object to the other.

How can we stop convection?

Convection happens when air moves around, carrying heat from one place to another. So in order to stop convection from happening, you just have to get the air to stay still. Warm air will rise relative to cold air even when nothing else is moving it, so keeping air still is not so easy to do.