You might be thinking about the SOL EMERGENCY bivy. That's basically a mylar sheet. The ESCAPE bivy is spray coated with mylar inside and made of a fabric that let's moisture escape but keeps out rain and snow. I havent tried it in these conditions (I've read reviews that state it does) but for a fact it doesn't build up moisture. I've used it quite a few times in the summer desert. Also keep in mind that I only slip into it in the wee hours of the morning when I wake up a bit chilly, so that might be a factor. Plus side, that shit packs down TINY
From product description: "With the Escape Bivvy, condensation is no longer an issue, and you never again have to choose between staying dry and staying warm. The proprietary fabric lets moisture escape at the same time that it keeps rain, snow, and wind on the outside - all while reflecting your body heat back to you. Waterproof seams plus a drawstring hood closure and side zip mean you can seal out the elements entirely or use the bivvy like a traditional sleeping bag"