The Xero 550 is one of the warmest bags tested [see July 2013 issue] and suitable for sub-zero temperatures for warm sleepers. As expected from Mountain Equipment it’s beautifully designed and made from top quality materials. The side seams and the zip are at ground level so the top of the bag is larger than the base. The top is also warmer as it has Mountain Equipment’s EXL elasticised stitching which makes the bag hug you, cutting out dead air space, and also allows the down to loft fully by pulling the fabric away from it. Mountain Equipment reckons EXL adds 2ºC to the warmth. The base doesn’t have the EXL stitching however and the bag is fairly close-fitting so if you turn over with it, as I often do, the warmer top doesn’t fully cover you. If you sleep on your back there’s no problem. This would only be noticeable on really cold nights anyway.

The Xero 550 has all the expected features and they’re better designed than on most bags. The hood fits snugly and the shaped footbox is very warm. The zip is just long enough to stick your feet out if they get too hot. The shell fabric is soft and comfortable.

The Xero range is Mountain Equipment’s lightest and the 550 is the warmest Xero model. Cold sleepers might well like it for summer use. I’d find it too warm. Using the European EN standard it has a comfort limit of -8ºC. However, Mountain Equipment also give their own rating, based on field tests as well as scientific research, which it says is, “the minimum temperature that we feel the bag should be comfortable to for an experienced user”. For the Xero 550 this Good Night’s Sleep rating is -11ºC. 

The next model down, the Xero 300, with an EN comfort limit of -2ºC and a Good Night’s Sleep rating of -5ºC, would be a better three-season bag for me. It weighs less too at 755 grams. The cost is £300.

First published: July 2013