Is Tyvek truly waterproof?

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

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.

Where do you find Tyvek 1025BL?

wow, a 15 year old thread with an awakening at 7 years and now another one : )

I put mineral spirits and silicone on a piece which makes it waterproof.  But it’s sticky – stuff clings to it.  The thought has occurred to me to replace it.

I googled it and 7 links down is this thread : )

I’m familiar with Tyvek 1025D (great stuff!) but not 1025BL.  What is it?

That’s what I’m trying to figure out. It’s mentioned in the second comment in this thread “1025BL = 53” and I’ve found it on a chart listing its hydrostatic pressure head at 53 in the comments of this Trailspace thread as well yet its supposedly only 1.25 oz/yd which is fairly light. Just wondering where do I find some of this magic stuff. https://www.trailspace.com/forums/gear-selection/topics/165408.html

From the chart in your link, 1025BL looks like it could be the Holy Grail of Tyvek.  At least it would save a bit of weight over my HomeWrap groundsheet.  Please post back here if you find any.

It’s a no-brainer to test Tyvek—or any tent floor or ground cloth—Just create a small pool of water in the yard with a water hose and place fabric atop this pool and sit down on it for 5 minutes—if any water sponges thru the ground cloth is not going to work.

You can also test 30 denier or 100 denier tent floors etc.  One of the best I tried was your basic Walmart type poly blue tarp.  My current ground cloth is a 10mil white poly tarp.