A silk filled comforter is always covered with a duvet cover for protection and prolonged use.
The comforter had better be encased in a removable duvet cover that can be laundered regularly. The covers are available in a vast array of materials from different weaves and grades of silk to ones made of organic cotton to a combination of materials. The choice is up to you, but I recommend a silk comforter cover for the silk comforter. Why skimp on the cover? But if the silk comforter you are buying is for a child, opt for a cotton cover because it is going to stand up to the wear and tear a child will subject it much better than a silk duvet cover will.
So don’t forget a silk duvet cover when you go shopping for a silk comforter set.