Pika is right.
Specifically the Tyvek sold by Tarptent as groundcloth is of the Homewrap variety but the one used on the Sublite is 1443r, and that is the cloth/soft Tyvek. There are 18 different versions.
http://www2.dupont.com/Tyvek/en_US/products/tyvek_styles/tyvek_styles.html
Some have made a bivy out of Tyvek (using different versions and I think often unaware of the others...)
Google "Tyvek bivy"
Franco