Yeah, that聮s a good point. Three-season tents such as the otherwise excellent Marmot EOS 1 ($225; marmot.com) often have too much mesh to be good winter tents. But it does have a full-coverage fly, so that should help quite a bit. If you聮re camped on snow, for instance, you can heap some snow around the perimeter of the fly to form a better seal. That will prevent wind from blowing up between the fly and the tent body, bringing snow with it.
Outdoor Research MicroNight Bivy
MicroNight BivyCertainly, taking some sort of bivy bag or sleeping bag cover wouldn聮t hurt. But I wouldn聮t get a fully bivy bag; it聮s too heavy and expensive. Outdoor Research (outdoorresearch.com) makes just the right thing: the MicroNight Bivy. It聮s made with Pertex Endurance fabric, which is very light, breathable, and water repellent. It聮ll nicely shake off any snowflakes that find their way into your tent. Cost is $119, and weight is less than 20 ounces. REI (rei.com) also makes a no-frills bivy, called the Minimalist Bivy. It聮s made from REI Elements, a proprietary waterproof-breathable material, so it would offer a lot of moisture protection. Price is $89.
Myself, I聮d worry a bit more about temperature than moisture. It聮s gonna be chilly in that well-ventilated EOS 1, particularly seeing as it聮s a solo tent so there聮s no one else generating any body heat. I聮d pack an extra sleeping pad (a lightweight foam pad such as Cascade Design聮s $35 Z-Lite is perfect). And certainly, use a sleeping bag with a rating that matches your anticipated low temperatures.
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