Warmest for weight1 / 8
Images: Sea to Summit and its retailers. Used for product identification.
Sea to Summit
Sea to Summit Spark -18 °C
850+ fill hydrophobic goose down and double draught collars: -10 °C comfort at 1.1 kg.
- Weight
- 1,113 g
- Comfort
- -10 °Climit -18 °C
- Packed
- 20 L sack, ≈ 40 × 25 cm
Best price found
£516
at Rock+Run · In stock · RRP £645.00
- TrekittPre-order · RRP £675.00 · Long size only£540View Deal
- Ultralight Outdoor GearIn stock · RRP £644.99£544.99View Deal
- Sea to Summit UKIn stock£645View Deal
Prices checked 31 October 2026. We may earn a commission on these links.
- 850 FP down
- Hydrophobic down
- RDS down
- Draught collar
- Box-wall baffles
Specifications
- Weight
- 1,113 g (LFTO measured 1,255 g with sack)
- Packed size
- 20 L sack, ≈ 40 × 25 cm
- Comfort
- Comfort -10 °C / limit -18 °C
- Fill
- 850 FP goose down, 840 g fill
- Shell & construction
- 10-denier nylon shell and lining; Box-wall baffle construction throughout
- Rating Standard
- ISO 23537 rating
- Fill Note
- 840g of 850+ loft RDS-certified goose down with Ultra-Dry Down hydrophobic…
- Zip
- 3/4-length #5 zip with zip-plow and two sliders
Pros & cons
- 'a phenomenal warmth-to-weight ratio' – -10°C comfort for 1,255g measured
- 850+ fill down 'fluffs up beautifully' and the zip 'flows very smoothly... rarely snags'
- Hydrophobic RDS down, double draft collars and genuine ISO ratings
- 'the £625 price tag seems inflated' – the most expensive bag here
- 10-denier fabrics 'feel thin and flimsy'; alpine mummy cut 'may feel a little tight for some'
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Premium pickCumulus
Cumulus Teneqa 700
700 g of 850 FP Polish goose down in a 1.15 kg bag rated to -10 °C comfort; direct from Poland.
- Weight
- 1.15 kg
- Comfort
- -10 °Cbrand's own figure
- Packed
- 33 × 20 cm
- 850 FP down
Full specs, 1 retailer →View DealFrom
£399
at Cumulus
2 photosBest all-rounderMountain Equipment
Mountain Equipment Glacier 700
Trapezoid box-wall baffles and traceable duck down for a -6 °C comfort rating at 1.2 kg.
- Weight
- 1.23 kg
- Comfort
- -6 °Climit -13 °C
- Packed
- 30 × 22 × 20 cm
- 700 FP down
- RDS down
- Recycled fabrics
- Box-wall baffles
Full specs, 3 retailers →View DealFrom
£322.12
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7 photosBest valueAlpkit
Alpkit SkyeHigh 900
900 g of recycled RDS down and a roomy cut for under £250, direct from Alpkit.
- Weight
- 1.4 kg
- Comfort
- -6 °Climit -13 °C
- Packed
- 24 × 37 cm
- 650 FP down
- RDS down
- Draught collar
- Box-wall baffles
- Two-way zip
Full specs, 1 retailer →View DealFrom
£249.99
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