This bag is so light that it can feel as though you’re sleeping in nothing at all – except that you can feel the warmth. As the name suggests, it’s designed to be used inside another bag except on the warmest nights. I find it okay on its own in temperatures above 10°C. The design is minimalist, as you’d expect, with no hood, just an extended base, and no zip. The bag is tapered and there is a shaped foot. The down has the highest fill power of any tested and the shell fabric is gossamer thin. As well as using it on its own I used the Filler inside other bags last winter and it certainly makes a huge difference. PHD reckon it can add up to 10°C and I agree. It could be used to add warmth to a three-season or summer bag that wasn’t quite warm enough and I think it would make a good pairing with one of the lightest synthetic bags reviewed here for a combination of warmth and water resistance. I’ve found it comfortable both as a standalone bag and inside another bag. The price is high but the quality is excellent and the weight ridiculously low, as is the packed size. Indeed, when using it with another bag I found I could leave it inside and still stuff the two into the same stuffsack.

First published: July 2014