What you do, apparently, is empty out the down filling from your sleeping bag and fill it with this stuff.
I am reliably informed by Someone In The Know, that it will keep you warm down to about minus twenty five although some users have complained about the filling “moving” about in the night and others have said they heard strange noises when they’ve rolled over in their sleep.
Its also best if the sleeping bag outer is made of waterproof material.
It is, however, breathable – or, rather, breathing and has a certain amount of intelligence.
I give you – Puppytex. Made from real fur from sustainable sources…