Just make sure you wash the flannel first---shrinkage! Mom used to use old blankets for batting all the time. I've even seen her cut the worn parts out and piece the "good" sections together to make battings. The non printed feed sacks were often used for batting, old sheets, old blankets, worn out quilts, basically anything to make the quilt thicker and warmer. I love the "green" quilting ideas. I often shop at Goodwill. Top sheets don't get the wear and tear that bottom sheets do and often make wonderful backings, blocks or lightwieght battings.