As you stated in your question, there are many different Tyvek fabrics, some are only water resistant, whereas others are completely waterproof (such as is used for express mail envelopes and printing materials). There are lots of forums online that discuss the use of Tyvek tarps for backpacking, to determine how water resistant your fabric is you need to know which fabric you have.
The post below is from the thread, "Is Tyvek Truly Waterproof?" at www.backpackinglight.com
Tyvek waterproofness depends on the style
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
Rain resistance is measured and expressed as hydrostatic head in millimetres (mm). This indicates the pressure of water needed to penetrate a fabric. Heavy or wind-driven rain has a higher pressure than light rain. Standing on a groundsheet increases the pressure on any water underneath. Fabric with a hydrostatic head rating of 1000 mm or less is best regarded as shower resistant, with 1500 mm being usually suitable for summer camping. Tents for year-round use generally have at least 2000 mm; expedition tents intended for extreme conditions are often rated at 3000 mm. Where quoted, groundsheets may be rated for 5000 mm or more.
Wikipedia: Tent-General Considerations