This Thermarest Parsec -18 has one of the highest fill powers and is comfort rated at -10 degrees Celsius, but still comes in at the lightest weight. It’s filled with hydrophobic down in heat mapped zones, which is kept in place by horizontal baffles around the bag. It’s got a very cosy feel, with an insulated foot pocket that adds additional warmth. The shell material is lightweight and recycled ripstop, helping shave off the grams.

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Rating: 5/5 | Price: £459 | Weight: 1127g for regular, 88g stuff sack | Fill: 800 FP Nikwax hydrophobic RDS down | Shell: Recycled nylon ripstop with DWR finish | Zip: 2-way three quarter length with anti-snag main zipper | Length: 183cm for regular | Rating: Comfort -10c | Sizes: Small, regular & long

More info: www.thermarest.com

There’s a two-way zip that stops around mid-calf, and the main zip is anti-snag which does a great job of not catching the baffles or bag fabric. There are no additional ventilation options.

The hood is a good size, has a lot of fill power and so heaps of warmth, plus large draft collars top and bottom. It can be cinched tight using a popper, and a drawcord on the outside of the hood, on the opposite side to the zip. There’s another draft collar around the shoulders to keep heat in if you want your head out. The zip has a draft tube that runs the full length too, and an external pocket on the opposite side of the zip, which is big enough for a phone.

It’s a unisex bag and a typical mummy shape, but has a wider cut at the shoulders so there’s lots of room for sleeping on your side or front too. There’s removable straps on the back of the bag that allow the bag to be attached to a sleeping mat and keep it in place. It works well and means you can move around in the bag without it getting twisted up, but it is only suitable for mats that are three inches thick or less. These are a bit fiddly with small loops and could be tricky in a tent in the cold!

It comes with a cotton storage sack, and a compression bag, which does a great job of packing it down small. The compression bag is a long but narrow tube, which I found slightly harder to pack than a wider but shorter shape.

The Thermarest Parsec -18 is a super bag for winter camping, coming in warm and light, but with the price tag to accompany it.