Hydrostatic head is a measurement of how waterproof a fabric is. It refers to the height of a water column the fabric can support before leaking. It depends on which Tyvek style you purchase as to what the hydrostatic head is. The inches hydrostatic head is less than 15 (water resistant) except for the following styles:
Homewrap = 82.7, 1025BL = 53, 1059B = 56, 1073B = 59, and 1422A = 40.
The above values are in inches H2O; to convert to mm H2O, multiply any of the above in H2O values by 25.4. Other than the following exceptions, Tyvek versions are less than 381.0 mm H2O (water resistant at best):1025BL = 1346.2, 1059B = 1422.4, 1073B = 1498.6, and 1422A = 1016.0. Homewrap, which has printing, is 2100.0.
Unlike most light shelter materials, Tyvek maintains its original HH head through multiple wash cycles. You can compare these Tyvek mm H2O HH values with the common light weight shelter materials mm H2O HH I tested in the thread http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=45026