question for bivy campers

I assume you're talking about if snow falls on you while sleeping in your bivy. If this is the case then yes, six inches of wet snow will compress your insulation.

Fortunately all you have to do is move your bivy bag around a bit to push the snow off. Consider that in a four season tent you would still have to clear the snow off your tent in order to keep it from collapsing.

If you are referring to stand-alone bivys w/ waterproof fabric on top and bottom, then Chad's response answers your question.

However, many people on this site use a tarp in conjunction with their bivy (often they are lighter than stand-alone and many have water-resistant tops, see mountainlaureldesigns.com). In that case there would only be minimal need for pushing snow off the top, although it may drift all around the sides of the bivy (depending on the shape of the tarp). Sorry if you know this already.

Yes I meant stand-alone bivys w/ waterproof fabric on top and bottom -I should have been more clear but you guys got it. Thanks for clearing up my concern.