Because I use the bivy I rarely use a ground sheet. Sometimes a Heat Sheet in the winter if needed. I carry the heat sheet but only use it when I need to...which is never as my bag system is damn warm.

The bivy is an REI minimalist. With the tarp, I don't need any more than that. I've always gotten by with the relatively small tarp/poncho because leaving my feet hang out doesn't really put me in harms way...it's just a little cooler if I get rained on.

I put my MEC Kelvin short sleep pad inside the bivy for comfort and warmth. In winter I use a Thermarest RidgeRest as well between the bivy and the ground/snow. That pretty much is my ground sheet and keeps me from melting into hard snow.