Certain housewraps, like Tyvek, are most certainly air barrier materials per ASTM standard testing for air barrier materials. The problem with using a housewrap in this application is the noise that can be created by the flapping of the material. If it is installed tight along the bottom of the rafter and not draped, then it might solve the noise problem. Using a housewrap seam tape along the rafters where it is being stapled would seal up the staple holes and eliminate any potential airflow in those areas.
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