Keep in mind that there are at least 12 types of Tyvek, the one that is breathable (for humans not houses...) is the 1443R also known as Cloth Tyvek or Kite Tyvek.
1443R is water resistant but not waterproof particularly when in contact with another surface .
I would use the HomeWrap* (or silnylon) for the bottom and 1443R for the top.
Take a look here :http://www.milesgear.com/UberBivy.html
* to get rid of the noise of new HomeWrap just roll it up and twist it