I am considering using Tyvek sheeting as a ground cloth under my Silshelter but I read somewhere that it is breathable as well as waterproof. Does this mean that Tyvek will soak through if a person lies on it on soaking wet ground due to the weight of a person forcing water through it? Would hate to get my down bag wet. Appreciate any feedback.

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

Edited by richard295 on 04/08/2012 12:22:49 MDT.

I am curious about this... bumping a 7 year old thread ftw.

Justin, it's been waterproof enough for my needs.

I use the soft grade Tyvek for ground sheets on all my tents/tarps and have never been wet. I also don't pitch my shelters in lakes or areas that will become lakes when it rains :)

I was recently out at Henry Coe and spent a night in constant heavy rain in a Hexamid Solo Plus (mesh floor) with my Tyvek groundsheet... had a dry spot under me the shape of my sleeping pad in the morning.

Locus Gear is making bivvies out of the same material, fwiw.

Tyvek Homewrap is waterproof enough for everything I've encountered so far. Years ago, I tried a lighter Tyvek from a kite company and it wasn't quite up to the task. But Homewrap is great. For several reasons, I prefer it to all the other groundsheets I've used including polycryo, Neat Sheets, silnylon, and various thicknesses of plastic sheeting.