It will eventually get cold, for two reasons. First, the water bottle will compress the insulation and lose its heat to the bottom, so the hot water would cool more quickly.
Second, loft works by creating micro-climes...little bity pockets of air that can't move. If you have a big pocket of air, it'll have some convection and you'll end up losing heat. Your body will have to heat that pocket and keep it warm, which takes more energy than keeping the underquilt warm.
Plus it's pretty hard to get a completely non-ventilated air pocket in there...when you move you'd push some air out the sides of the UQ.
But you could try it out and let us know how it works for ya.
“Republics are created by the virtue, public spirit, and intelligence of the citizens. They fall when the wise are banished from the public councils because they dare to be honest, and the profligate are rewarded because they flatter the people, in order to betray them.” ~Judge Joseph Story
- My site: http://www.tothewoods.net/
- Designer, Jeff's Gear Hammock / Pack Cover
IMPOSSIBLE JUST TAKES LONGER