I don't see much difference between using t-shirts and flannel, both are fairly thin. So, one or 2 layers of t-shirt material may work for a quilt. It would take more layers to make a hot pad, I would probably start with 5 layers. We have a bunch of t-shirts that have worn out. If you make a t-shirt quilt, you're using just the graphic or text on the t-shirt, so there is the rest of the t-shirt that can be used for something. Since t-shirts would end up with a seam in them in a quilt, I think they would be best for things like rag quilts or other blocks that are sewn together after quilting the block (including QAYG).