Do you need tyvek with spray foam walls?

I'm not sure what the code is, but for the cost, I would use it. Theoretically, the foam should seal out any drafts, but, if there were a small spot that got missed by the foam applicator any draft could find it's way in. In an area with a lot of wind it would be even more important. A wind against the side of a house actually applies a pressure to it and if there is even a small crack, that pressure will enter the wall and push cold air into any cavity inside the wall ...... or even into the house. An example would be when you take a cover plate off of an electrical outlet and you feel cold air blowing in. That's from a leak allowing it to be forced in. Also, there are places that air will leak that you can't apply foam to. Like along the top and bottom plates of walls.

Tyvek isn't expensive when you consider how much it may save in the long run. I'm not sure, but I think the last 100 foot roll I bought was about $75?? As with anything, it's only as good as the application though.