The North Face Hyvent DT uses a thin layer of water loving (hydrophilic) polyurethane. This is the same technology used in most low end WPB garments. TNF specs this membrane at 650 using the ASTM E 96 B Upright Cup Method (g/24hrs/m2). They state that, "While each individual test is standardized, there is nothing that requires a company to use one method over another. Unfortunately, since there is a different mechanism for moving moisture within each test there is no way to compare numbers. The North Face has historical data and has been testing via the Upright Cup for years and will continue to do so."
Contrary to their official position, I can help you blow away their smoke and provide you a number you can compare to my previously posted eVENT and Gore-Tex Paclite Water Diffusion (Breathability) g/m2 24 hours-JIS L1099 B-1 numbers. This material tests~ 11,000 versus ~21,000 for eVENT and ~17,000 for Gore-Tex Paclite.
The hydrostatic head for Hyvent DT is ~18,000 versus 30,000 for both eVENT and Gore-Tex Paclite. Although lower, it is an adequate waterproofness for Thunderstorms