Zeus has the right idea. I do tend to jump the gun with these people.
I just get a little miffed when I hear 1/3. That's a lot of money for not having done a darn thing!

Talk to the head guy and try to get where you need to be.

In the end, if it states to apply Tyvek, then it is going to assumed that it will be installed as per mfg. recommendations... but be careful with teh "assume" part.