Rab’s Infinity bags are the warmest for the weight the company has ever made and the 500 is the warmest of the three models. It has a new construction method called Insotect Flow that allows the down to loft more efficiently. This consists of vertical rather than horizontal chambers with partial inner barriers to stop the down moving about too much plus a differential cut with the inner smaller than the outer. The shell is also very thin, which reduces weight, prevents the fabric from compressing the down very much and allows the bag to drape round you well, cutting out cold spots. The result is a bag that is very thick for its weight. Its other features are standard ones – thick baffle behind the zip, filled draft collar and angled footbox. The zip is only half-length, which means you can’t vent the lower half of the bag.

The shell of the bag is made in China but the down is put in by hand in Derbyshire, which is part of the reason for the high price, along with the top quality down, shell fabric and complex construction.

The Infinity 500 is very comfortable and very warm for the weight. Although it hugs the body well there’s room to move inside and you could wear extra clothes in extreme cold. At -2ºC I didn’t need to do up the hood. For cold sleepers it’s an excellent bag. Warmer sleepers, like me, will probably find the Infinity 300 adequate for three-season use. This has a comfort rating of +3ºC with a limit of -2ºC and a weight of 650 grams. .

The Infinity 500 is expensive but for backpackers looking for a very light weight bag for the warmth it’s excellent.

First published: July 2013