Roger:
At one point I had the old ICBO Report wherein Tyvek was approved as an air barrier and not a WRB, unfortunately I never thought to download it to my hard drive thinking it would always be available. A few years ago I was writing a report on a leaking house built during the time of the older ICBO Report and it has disappeared from the Legacy Reports, I contacted ES Reports asking for it and was told it had been replaced by the new ES Report and the old Legacy Report was no longer available. Strange because there are a ton of Legacy Reports available.
Tom Butt, FAIA, did a good paper on the history of Tyvek's approval process, I just went to his site to present it here but I see it's been taken down, about the same time
he did this but it is minimal without the history.