Tyvek is a wonderful material for not just ultralight trekkers but anyone who's looking for a rugged, very light material. Essentially a white plasticized material, DuPont markets it mainly for wrapping the frames of houses under construction, as well as covers for boats, BBQs, you name it. It's extremely durable as a groundsheet, and now comes in various grades as well (soft or stiff, etc.). Tyvek vestibules for your bivy sacks at last?
While finally becoming more common in local home improvement/DIY stores, it still generally comes in long rolls 3' wide, far more than you'll ever use. Until recently the only other option was to scrounge smaller pieces from construction sites. Luckily, there are at least two online sources now.
For the relatively soft "kite" Tyvek, Hang 'Em High Fabrics sells it by the yard. (Thanks to Janet and Will for the tip!). Great stuff! Bear in mind it's not as durable as traditional Tyvek, I usually have to replace my soft Tyvek ground cloth every two seasons or so.
Andy is selling traditional Tyvek pieces (cut to order) for essentially his cost in providing it to you, as a service to fellow backpackers. I can vouch for him - good fellow, runs a very nice site, and we've traded plenty of email (and yes, Tyvek). :-)
Reader "Anonymous Joe" has submitted the following technique for taming "classic" Tyvek's incredible crinkly noisy nature. Maybe experimenting with a Tyvek VBL isn't such an outlandish idea now - at least it'd be cheap to try! Thanks Joe!
"On the noisy tyvek vestibule. I've used tyvek for a prototype Jardine style tarp and a prototype 1.5 lb 2-man tent of my design. It's surpisingly durable and strong. The trick is to crumple it up pretty well with your hands, then put it in an upright clothes washer with cold water for about 10 minutes. Don't put in soap, and the spin cycle won't help get much water out so you can stop it early if desired. Have a bucket to put it into because it entraps a lot of water, take it outside and hang it on the fence or a line. Then sew it up to your heart's content and it won't be very noisy at all. If you "wash" a finished article it would probably work if not too large."
Important note - Reader and longtime backpacker Jeff reports that his piece of Tyvek ended up a frayed annoying giant debris-trapping spider web during his 80-mile trip. Best guess is that it was due to a little detergent being added in during the wash, but we don't know for sure. Either way if you're washing your "hard" Tyvek to soften it up, best beware and consider the "don't put in soap" directive to be a requirement. (Thanks Jeff!)