I have made several shower curtains with heavier cotton, some lined and some not lined. I have not made a pieced shower curtain, however. What I can tell you is that I like the unlined shower curtains best because they laid better and dried quicker. If I was making a pieced shower curtain (and this does sound pretty), I would line it with a light weight cotton and SID. How are you doing the top? I would recommend a separate piece for buttonholes or grommets and that piece I would interface or use extra lining to stiffen. Can't wait to see it.