Oh, because it's light and cheap. I've used Tyvek HomeWrap and it worked fine. No moisture made it through. Liquid won't penetrate. Gas/vapor will, but didn't condense. It's noisy but breaks in over time. I've used space blankets, tyvek, plastic sheet, pastic tarp, all good.
It may be silly, but I don't want to add to the humidity inside the tent. Silly, in that the water vapor let through the tyvek may be negligible. I don't know if it is or not. If it does make a difference, it wouldn't be one of those things that you notice through experience. Unless, for example, you covered half the floor with Tyvek, and the other half with plastic, then layed a sleeping bag directly on each half, then measured the amount of water vapor in each sleeping bag (though most people use pads/mats).
I want to get Tyvek, because it is lighter. I just don't want to add the humidity in the tent by doing so. I'm hoping for the vapor impermeability of plastic, with the lightness of Tyvek.
Durability and protection are also nice. Maybe I'll just stick to the 6 mil plastic I'm currently using? I had some thorns I didn't notice make it through the 6 mil plastic, but I suppose those thorns would make it through just about anything: just means I need to be more careful clearing off the ground.
Side note: I learned that it is easy to fix big agnes air core with the provided glue: patch wasn't even necessary.
I'd give the Tyvek a try. I normally sleep in a hammock if there are trees, or under a silnylon tarp if not, so ventilation is everything. If you have any ventilation at all you shouldn't have any condensation problems. For lightness, you can also opt for coated nylon ripstop, say 1.9 oz.. It won't let anything through. I DON'T like using plastic because it takes forever to dry when I pack up in the morning. A little gas permeability and everything is dry when I pack up.
Oh well, I already ordered some 2 oz plastic. I did call DuPont, however, and they gave me a number to Material Concepts, who recommended "Hard Structure" type for tent floors. I see this is just type 10: http://www2.dupont.com/Tyvek/en_US/products/structure_types.html Old news, I guess. All Tyvek is gas permeable by its very nature.
I have read about others adding a coating, but that it peeled off after not too long. I have some "camp dry", that is 13% silicone, which I think would result in the same.
Yes, the ground cloth, itself, would dry quicker, if it is gas permeable.
If you have any ventilation at all you shouldn't have any condensation problems.
My tent has good ventilation. I will be trying Tyvek after my 2 mil plastic is worn out.
I think....you are over thinking it. I've NEVER had a wet bag because of a ground cloth. Back when camping with my dad, we used WWII USMC issue wool blankets. One on the ground, one to cover up with. The only time that doesn't work is on wet ground or during/after a rain. We rarely used a tent, always under the stars, which I prefer to this day. If the ground was wet, we used heavy tent canvas or whatever we had around....like a shower curtain. The "technical" side of camping just didn't happen back then as there were no fancy materials to worry about. You just laid down and went to sleep. Tyvek is a breathable material like Goretex or E-vent. They make kayak drysuits from both those materials because they breathe, but don't leak liquid. My drysuit, liquid has never made it in to my fleece undergarments. I've used Tyvek on wet ground and it worked fine.
All the materials that allow water vapor through but not water drops, do so because there is a humidity difference on either side of the fabric. Since only vapor can pass through, and not liquid, if there is no vapor on one side, none will pass through.
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