I use my semi-rec sleeping bags both ways, depending on my needs. 90% of the time I'm in top quilt mode. But there are nights when I have problems getting and keeping warm. This might be because of weather (low temperatures, draft / wind, high humidity) or because of physical issues (exhaustion, illness) - or both. This is when I really appreciate the option to zip up. It definitely improves warmth retention, although I can't tell you why for sure. I think it's a combination of factors. With the zipper closed, there are no gaps whatsoever through which heat can escape. Maybe the volume of air is smaller as well, because I can tuck the fabric around myself more aggressively without gaps opening up accidentally. It also eliminates micro air bubbles between my body and the underquilt. Even with a well fitting underquilt like the Wooki, small air bubbles remain. If you hang it under a see-through hammock you can clearly see them. Most people generate enough body heat to not feel them. I usually do as well, but sometimes my internal furnace is a bit sluggish. That's when each bubble sucks away the little body heat I have. Putting a insulation between my body and the hammock brings immediate relieve. Even though it's compressed somewhat.
The underquilt doesn't reflect heat. It just slows the heat transfer by inhibiting convection. I have used my quilts to keep frozen foods cold for longer in summer. It works both ways. So yes, if you want to be warm you need body heat in the first place. You just need to generate less than you would if you wanted to keep warm without an underquilt.
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