After reading the entire paper at the web site given to us by Jay J, (an excellent site) I still feel that the Tyvec is the better material to use. I have pulled too much siding off of older homes and found smelly, moldy wood under the tar paper to continue using it. Maybe it was because of poor or non existent vapor barriers in those homes, but I need to do something different. I like the idea that tyvec repels water better, and has a consistent perm factor.