Although Tyvek is breathable and technically not waterproof, it is in fact functionally waterproof. Tyvek housewrap is very durable and commonly used for groundsheets, but its heavy . "Soft structure" Tyvek (Type 1443R) is the stuff Tyvek clothing is made of, weighs 1.3 oz/sq yd (the same as silnylon) and is functionally waterproof. My wife and I have been using it as a groundsheet for years, and Jim Wood got the idea from us. Big Sky International is now making their tent footprints of 1443R Tyvek, and one version of Tarptent's new SubLite tent is made of this same Tyvek.
You don't need to debate whether its waterproof or not, it is. We have been using it for 8 years, with no problems. A good source is
Into The Wind $3.50 per yard.
The Tyvek used for envelopes is #10, which is light but stiff. Type 1443R is very soft and much better for groundsheets. You can also make an inexpensive bivy out of it; just sew it up like an envelope with one end open.