The Ascent Acl-4 is shaped something like a mid point between a mummy and a semi-rectangular sleeping bag. If you are not a fan of the former’s snug fit around the legs and feet, this may be a really good choice for you. The foot box feels spacious, giving you plenty of ‘wiggle room’.

Fill: 330g 750FP Ultra-Dry down | Shell: 20D ripstop nylon | Construction: side block baffle | Zip: 2-way, full-length on left, 2-way, 1/3-length on right | Length: 205cm (regular) | Rating: comfort +2°C, limit -4°C | Sizes: regular, long | Women’s version: no

It comes with a generous lofting sack and an ultralight compression sack. When packed down in the latter, it really is compact: about 25x17x17cm – the smallest in this test. 

The 330g of 750 fill power Ultra-Dry hydrophobic down certainly did a good job of keeping me warm on nights when the temperature dropped to around 3°C. Sea to Summit claims the Ultra-Dry water-repellent treatment allows this bag to dry out 60% faster than bags with untreated down – good for bivvying – though I wasn’t able to test this directly. 

The hood is generous but can be tightened down. One of my favourite features of this bag is the zip on the right-hand side. It made it really easy to change clothing inside the bag, and provides ample ventilation. There is also a foot box ventilation zip. A baffle runs along the length of the main zip and this foot box zip. However, in the bag I tested, its insulation did not extend along the length of the foot box zip. It was hard to tell whether this was due to the down ‘migrating’, but I did find my feet getting a little colder in this bag than in others. 

Overall, the Ascent Acl-4 is a very attractive option: warm, lightweight and suitable for a wide variety of conditions.