Under a tarp, I may use a MYOG bivy. In a tent, I don't. Either way, I haven't had a problem with drafts under my quilt. I've got bags that I can open up and use like a quilt and that works, too. My bivy is more like a fitted ground sheet. Its purpose is mostly to keep my quilt from spilling onto the (damp or wet) ground. It is coffin shaped and has a 40d PU coated bottom and a 30d DWR top with a center zipper to about 5/8ths of the way up.