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Alpine Down Winter Sleeping Bag (-5°F & -40°F)

This is the sleeping bag you'll be al-pining for!

$649.00 – $899.00

Size Guide

Sleeping Bag Temperature Ratings Explained

The EN Rating scale is a scientific test to determine the warmth of a sleeping bag. The test produces a Lower rating (for male sleepers) and a Comfort rating (for female sleepers). Both are listed in the features table on the product page.

A unisex sleeping bag with a Lower limit of 14ºF means a man should be comfortable down to air temperatures of 14ºF. The same bag will also have a Comfort limit, in the realm of 27°F, so a woman would be comfortable down to an air temperature of 27ºF. These ratings require a sleeping mat with an R-Value appropriate for expected ground temperatures – without this, the sleeping bag itself is unlikely to keep the sleeper warm enough.

A variety of factors impact sleeping bag warmth - the above temperatures are merely guidelines, not guarantees. It’s important to think of a sleeping bag, an appropriately insulating sleeping pad and a liner as a sleep system. For example - if you plan on sleeping in nighttime air temperatures of 27ºF / -3°C, choose a bag with a gender-appropriate rating of 23°F / -5°C, plus a sleeping mat with an R-Value of 2.5 to 3, and consider a Reactor thermal liner.



The Alpine™ down mummy sleeping bag is designed for the rarefied world of high-altitude and arctic conditions. It features cutting-edge materials, the highest quality Ultra-Dry down, and complex construction. Expedition mummy sizing allows you to wear extra clothing, and boot liners inside the bag.

If you are going above or beyond, the Alpine is the bag you will want to take with you.

Technical Features

Nano Shell™

Nano Shell™ is a 20 Denier Nylon fabric with a water-shedding DWR finish on the exterior and a microporous PU coating on the interior. This highly breathable, highly water-resistant, windproof shell fabric is used on the Sea to Summit Alpine sleeping bags and Cinder quilts where it protects the down from external moisture even during very wet conditions.

850+ Loft Ultra-Dry Down™

The highest-lofting down used in any of Sea to Summit’s sleeping bags. 850+ Fill Power indicates that one ounce (30g) of down will re-expand to over 850 cubic inches after compression. Higher loft means that less down can be used to achieve the same temperature rating – important for performance sleeping bags. The loft and down quality/composition are independently tested by the International Down and Feather Laboratory (IDFL) – a copy of the certification is supplied with every Sea to Summit sleeping bag.


The down is responsibly sourced and RDS certified and treated with the environmentally-friendly Ultra-Dry Down™ treatment.


All of which means you can rest easy.

Responsible Down Standard (RDS)

All Sea to Summit sleeping bags use only down which is certified according to the Responsible Down Standard. The RDS is helping shift the entire down and feather industry towards more humane animal rearing and handling practices. Overseen by the industry leading Textile Exchange, the RDS requires independent, third-party assessments of all aspects of animal rearing and handling throughout the entire supply chain. By using only down that meets the audited requirements of the RDS, we can have confidence that each phase of the collection process has been completed with the welfare and ethical treatment of waterfowl in mind.

To learn more about RDS, visit

Free Flow Zip

Our Free-Flow triple-zip design ensures your sleeping bag retains maximum warmth and efficiency when needed and multiple ventilation options when it's not. 

The half-zip on the right side of the bag (left side on women's bags) provides freedom of movement for both arms, additional ventilation and allows the top third of the bag to be folded down as a mini quilt when the mercury climbs higher than expected.

Tech Specs

Product Details

Best Use
Comes With
Ultra-Sil Compression Sack & Storage Cube
Ultra-Dry Down 850+ Loft
Season Rating
Shell Fabric
Water repellent NanoShell
Zip Options
Free-Flow zipper system
alpine-down-winter-sleeping-bag-5-f-40-f-regular.svg alpine-down-winter-sleeping-bag-5-f-40-f-long.svg
-40°F LONG
Alpine II
2 lbs 15 oz / 1340 g
9.3 L
L - 20L to 6.5L
28 oz / 800 g
6 ft / 183 cm
4 Season
10ºF (Comfort)
-5ºF ( Lower Limit)
Alpine II
3 lbs 5 oz / 1500 g
13.1 L
L - 20L to 6.5L
32 oz / 920 g
6 ft 4 in / 192 cm
4 Season
10ºF (Comfort)
-5ºF ( Lower Limit)
Alpine III
4 lbs 2 oz / 1870 g
14.7 L
XL - 30L to 10L
44 oz / 1250 g
6 ft / 183 cm
4 Season
-20ºF (Comfort)
-40ºF ( Lower Limit)
Alpine III
4 lbs 10 oz / 2100 g
19.5 L
XL - 30L to 10L
55 oz / 1440 g
6 ft 4 in / 192 cm
4 Season
-20ºF (Comfort)
-40ºF ( Lower Limit)


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