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Bivvy bag and fly is very cold

Despite a tent being very thin it does trap a fair bit of heat and does make a huge difference to your comfort

The next level is a tipi with an inner.
Traps 50mm...

Kuiu Mountain Star
Brilliant tent with decent head height and proper vestibules
Exoskeleton means that it pitches nice and tight, doesn't flap in the wind and doesn't need lots of guy ropes around...