Measuring the water vapor rate transmission (WVTR) of barrier materials at low levels (<10-4 g/m2/day) is challenging. This talk will describe how WVTR values should be understood and some of the existing methods used for characterizing materials. In particular, we will discuss the National Renewable Energy Laboratory’s electrical calcium test method for measuring WVTR. We will examine some of the challenges in making high quality, repeatable measurements. As no WVTR standards exist below ~ 10-2 g/m2/day, as time permits, we will present some of our work in developing standards for WVTR levels between 1 and 10-6 g/m2/day.