A reviewer at Outdoor Video Magazine describes camping a few nights with an Integral Designs eVent Expedition Unishelter Bivi. eVent fabric is supposed to be as breathable as it gets, which isn’t much, according to this review:

Night 3

Sleeping on VERY wet snow. Temperature was above freezing and raining hard before bedtime. Over night, rain turned to 3 inches of snow, and temperature dropped to -8C. Sleeping bag damp to the extent where it was effecting loft in my opinion. Glad I didn