"You will experience some condensation build-up in humid conditions."
"You" in this case would be, well, you. The "condensation build-up in humid conditions" no doubt covers your own experienceto recap, inside a tent on a damp night in the 30s, you got soaked.
The "why" is easy enough. While it's true Pertex Endurance finds its way into sleeping bags, in the case of the Endurance Bivy ($120; www.integraldesigns.com) it coats the fabricrendering the bivy totally waterproof and totally non-breathable. The moisture on your bag in the morning was actually exuded by you. It got through the sleeping bag cover just fine, but hit the waterproof shell, chilled, and condensed.
If you do, be sure it's one that is breathable. Mountain Hardwear makes a decent, inexpensive light bivy called the Conduit SL Bivy, which uses that company's proprietary waterproof-breathable fabric. It's only $105 (www.mountainhardwear.com). Or, Outdoor Research makes a bag shell specifically designed for use as a light bivy or inside a tent as a bag cover. It's simply called the Sleeping Bag Cover, and sells for $120 (www.orgear.com).
Until December 15, the Gear Guy is unveiling his ten favorite pieces of gear, and in the spirit of the season we're giving it all away. CLICK HERE to find out what makes his all-star Hall of Gear and enter the 2003 Great Gear Giveaway!