When used as a bivy w/o tarp, how much rain protection does M90 offer? I see it used in a lot of bivys but it's not listed as WPB, just DWR. What's your experience?

SJ

Richard tested some for me and it gave a result for Hydrostatic head of 632 mm.

So... That would make it more water resistant than waterproof?

Sorry, I am interested in this as well, and am not well versed in what the Hydrostatic Head means. Wikipedia mentions 1000-1500mm as being shower proof. That correct?

If so, the this material is really only usable for non-waterproof applications such as jackets and bivies, right?

632 mm refers to a pressure, expressed as a height of water, that the water resistent/proof material would resist.

632 mm = 0.632 meters = 24.9 inches = 2 feet depth of water.

Now, you are rarely under two feet of water, right? And if you are, a little leakage in your outerwear is the least of your worries. But you can actually get pretty high pressures under pack straps and where creases and folds form.

So don't take the water column height literally, but use it as a comparison. Ideally get to a location where you can compare different garments (I know REI in Seattle has a rain booth - basically a shower stall where you can test rainwear under a downpour). But generally, the more waterproof, the less breathable.

Personally, I tend towards something pretty waterproof but with pit zips and other ways to ventilate.

Very, very waterproof (and very, very cheap): trash compactor liner bag.

Very, very breathable: take off the trash compactor liner bag.

Above the collar, I like a Seattle Sombero in cold weather or an unbrella in mild / warm weather.

M90 is neither waterproof nor waterproof breathable; you need a primary shelter.