A. Don Fugler, senior researcher at Canada Mortgage and Housing Corp., responds: Let’s answer the easy question first. In my opinion, you could put housewrap on the back side of the second-floor kneewalls in a well-ventilated attic. This would reduce wind washing of the insulation. The installation is roughly in keeping with the standard practice of using housewrap on exterior surfaces.
As for the attic floor, I would not put a housewrap and plywood over the insulation. DuPont, the manufacturer of Tyvek, advises that their Homewrap is not recommended for horizontal applications.
to read the full article.