Synthetic.
<<<<We've finally made our own Hammock Underblanket which is compact, lightweight, easy to set-up and suitable for use in temperatures down to -5C.>>>
That's 23 degrees F. I have AHE's New River UQ and it fluffs up a LOT more than DD UB.
In one of the videos, the presenter (in UK) said it had been quite chilly (don't remember if temps were given) and he was comfy.
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