Sea to Summit Alpine Down Sleeping Bag
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Alpine Down Sleeping Bag
Sleep System · Extreme Cold · Mountaineering
Gear up for your next high altitude or colder latitude adventure with the Alpine Down Sleeping Bag. The expedition mummy fit provides ample space for winter gear, while the modified Free-Flow Zip system provides ventilation and versatility. Complex construction throughout and 850+ fillpower, RDS-certified non-PFAS Ultra-Dry Goose Down delivers maximum warmth retention for serious cold-weather pursuits.
-20°F Rating
ISO Lower Limit of -20°F / -29°C — rated for high-altitude mountaineering and extreme cold-weather use.
850+ Fill Down
Premium 850+ fillpower goose down — top-tier loft and warmth-to-weight ratio. Maximum thermal performance.
Water-Resistant Down
Ultra-Dry Down treatment (non-PFAS) protects down from moisture — maintains loft in damp conditions.
Free-Flow Zip System
Full-length + half-length zipper on opposite side for excellent temperature regulation and ventilation.
Expedition Mummy Fit
Roomy mummy cut provides space for performance clothing and gear that shouldn't freeze.
Sustainable Sourcing
RDS-certified down (Responsible Down Standard) — ethically sourced, traceable supply chain.
850+ Fillpower RDS Goose Down (NEW — PREMIUM INSULATION)
Premium 850+ fillpower goose down — top-tier loft and warmth-to-weight ratio. Fillpower measures the volume one ounce of down fills (in cubic inches); higher fillpower = more loft = more warmth for the same weight. 850+ is in the ultra-premium range (800+ is premium, 850+ is ultra-premium). Combined with RDS (Responsible Down Standard) certification, ensuring ethical sourcing and full traceability from farm to factory.
Ultra-Dry Down Treatment (NEW — WATER-RESISTANT DOWN)
The down is treated with Ultra-Dry Down — a non-PFAS water-resistant treatment that protects individual down clusters from moisture. Wet down loses loft and warmth (and takes a long time to dry); Ultra-Dry-treated down maintains loft in damp conditions and dries faster. Critical for alpine and mountaineering use where condensation, snow, and humidity are constant threats. Non-PFAS formulation avoids the persistent "forever chemicals" found in older DWR treatments.
Recycled 20D Nylon Shell with Non-PFAS DWR
The shell is recycled 20D nylon with a non-PFAS DWR (Durable Water Repellent) finish — light and durable, with water resistance for damp conditions. 20D is a balance of weight and durability; not the lightest (10D/15D), not the heaviest (40D/70D). The DWR finish beads light moisture without using PFAS chemicals.
Water-Resistant Hood & Footbox
The hood and footbox use a highly water-resistant, breathable fabric — extra moisture protection in areas most likely to contact snow, condensation, or frost. Ideal for bivy sacks or snow cave camping where the bag may contact frozen surfaces directly.
Free-Flow Zip System (NEW — VENTILATION TECH)
The Free-Flow Zip System combines a full-length zipper with a half-length zipper on the opposite side — gives you multiple ventilation options:
- Full zipper open = blanket mode (warm nights)
- Both zippers open = flat quilt (very warm nights, or want to use over a sleeping pad)
- Top only = standard mummy configuration
- Half-side only = foot ventilation (cold feet but warm body, or vice versa)
- Both closed = full mummy (cold nights)
The two-zipper system is significantly more versatile than single-zip sleeping bags.
Highly Breathable Liner
The interior liner is engineered for high breathability — moves moisture vapor away from your body to prevent the clammy feeling that builds up in less-breathable bags. Critical for multi-day expeditions where you can't fully dry out the bag between nights.
Technical Hood & Footbox
Both the hood and footbox are technically shaped for warmth retention. The hood cinches tight around the head (only face exposed). The footbox has extra space for boot-wearing feet but is shaped to minimize dead air space. Generous draft tubes along both zippers prevent cold air infiltration through the zipper teeth.
QuiltLock System (NEW — TECH FEATURE)
QuiltLock is STS's proprietary system for securing the sleeping bag to a quilt or sleeping pad — keeps the bag in place during restless sleep and allows the bag to function as a quilt when fully unzipped. Specific QuiltLock functionality depends on the paired STS quilt/pad model.
Temperature Rating (IMPORTANT)
ISO Lower Limit Rating (-20°F / -29°C)
The Alpine Down is rated to ISO Lower Limit -20°F / -29°C — the formal temperature standard for sleeping bags. The ISO Lower Limit is the temperature at which an average man can sleep for 8 hours without feeling cold. For women, the equivalent comfort rating is -2°F / -19°C (per the spec sheet). The bag is engineered for serious cold-weather use: high-altitude mountaineering, winter camping, alpine ascents, polar travel.
Regular / Long
Two sizes — Regular fits users up to 72.05 in / 183 cm; Long fits users up to 77.95 in / 198 cm. Long version adds ~6.5 oz of weight and slightly more fill for taller users.
Temperature Rating
ISO Lower Limit (Men's)
-20°F / -29°C
ISO Comfort (Women's)
-2°F / -19°C
Climate Category
Extreme Cold / Mountaineering
Dimensions & Weight (by size)
| Measurement | Regular | Long |
|---|---|---|
| Total Weight | 51.5 oz / 1460 g | 58.0 oz / 1644 g |
| Fill Weight | 35.27 oz / 1000 g | 40.04 oz / 1135 g |
| Compressed Volume | 12.6 L | 14 L |
| Recommended User Height | 72.05 in / 183 cm | 77.95 in / 198 cm |
Materials & Construction
Shell
100% Recycled Nylon (20D, non-PFAS DWR finish)
Lining
100% Nylon (highly breathable)
Down Fill
90/10 Goose Down (RDS-certified, 850+ fillpower, Ultra-Dry Down treatment, non-PFAS)
Stuff Sack
100% Nylon (included, excluding trims)
Zippers
Free-Flow Zip System (full-length + half-length on opposite side)
Hood/Footbox Fabric
Highly water-resistant, breathable
Draft Tubes
Generous draft tubes on both zippers
Performance (STS scales)
Weight Class
Midweight (heavier than ultralight summer bags — appropriate for -20°F rating)
Climate
Extreme Cold (most extreme category on STS scale)
Compatibility
Use Cases
High-altitude mountaineering, winter camping, alpine ascents, polar travel, snow cave camping, bivy trips, expedition climbing
Sleeping Pad Pairing
Pair with a high R-value insulated sleeping pad (R 5+) for full -20°F system warmth
Sleeping Bag Liner Pairing
Compatible with STS Silk Blend, Reactor Fleeceweight, and other standard sleeping bag liners for added warmth
Profile
Brand
Sea to Summit (Australian outdoor gear, founded 1994)
Category
Sleeping Bag / Down Sleeping Bag / Mountaineering Sleeping Bag
Activity
High-altitude mountaineering, winter camping, alpine ascents, polar travel, expedition climbing
Season
Winter / Extreme cold (rated for -20°F / -29°C use)
Cleaning (NEW — DOWN BAG SPECIFIC)
Hand wash or machine wash (front-loading washer only, no top-loaders with agitators) in cold water with a down-specific technical wash like Nikwax Down Wash or Grangers Down Wash. Regular detergent will strip the natural oils from down and degrade the Ultra-Dry Down treatment. Wash infrequently — most down bags only need washing once every 5-10 trips. Spot clean between washes with a damp cloth. Do not dry clean — solvents strip the down treatment and damage the shell DWR.
Drying (NEW — DOWN BAG SPECIFIC)
Tumble dry low with 3-4 clean tennis balls or dryer balls — the balls break up down clumps as the bag dries, restoring loft. Drying takes 2-4 hours minimum — don't rush. Ensure the bag is completely dry before storage — any retained moisture will cause mildew and down clumping. Can also air dry in sunlight on a dry day, but fluff by hand periodically. Never store a damp down bag.
Storage (NEW — DOWN BAG SPECIFIC)
Store UNCOMPRESSED in a large cotton storage sack — not the included compression stuff sack. Long-term compression damages down loft permanently. The included stuff sack is for transport only. STS typically includes a large cotton storage sack with their down bags. Store in a cool, dry place away from direct sunlight (UV degrades the shell fabric and the down).
Ultra-Dry Down Treatment Care
The Ultra-Dry Down treatment is durable but will wear off with years of use and washing. To refresh: use Nikwax Down Proof or similar down-specific water-resistant treatments after washing. Apply to clean, dry bag and tumble dry low with tennis balls to activate the treatment.
Shell DWR Refresh
The shell's non-PFAS DWR will wear off with use. Refresh with a PFC-free spray-on DWR (Nikwax TX.Direct Spray-On, Grangers Performance Repel, Gear Aid ReviveX). Apply to clean bag, let cure for 24 hours before use.
Zipper Care
Both zippers (full-length and half-length) should be kept clean and lubricated. After exposure to sand, salt, or dirt, rinse with fresh water. If zippers stick, apply a small amount of beeswax or a graphite pencil tip to the teeth — avoid silicone-based lubricants which attract dirt.
Draft Tube Maintenance
The draft tubes (insulated tubes along zippers) prevent cold air infiltration. If a draft tube shifts or develops gaps, you can hand-position it back into place. Don't aggressively stuff the draft tube back inside the bag shell — it can damage the stitching.
Hood & Footbox Care
The hood cinches tight around the head — inspect the drawcord and toggle periodically for wear. The footbox is shaped for boot-wearing feet; if the stitching fails, contact STS warranty service.
Long-Term Loft Maintenance
Down bags lose some loft over years of use as the down clusters break down. To refresh loft: tumble dry low with tennis balls for 30 minutes every 6-12 months. The mechanical action separates down clusters and restores some loft. Replace bags showing significant loft loss (typically 10+ years of regular use).
Repair
Small shell tears: STS repair patches or any silicone/PU repair tape. Large tears, baffle failures, or zipper failures: contact STS warranty service. Down bags are significant investments; STS offers repair services for out-of-warranty bags.
Expedition Tip
For multi-day mountaineering expeditions, pair the Alpine Down with a high R-value sleeping pad (R 5+) — the sleeping pad insulation is just as important as the bag for cold-weather warmth. Sleep in dry base layers (not the clothes you hiked in — sweat will cool your body overnight). Stuff a water bottle with hot water and place it in the footbox for a "hot water bottle" warming effect. The Free-Flow Zip System lets you vent your feet if you overheat, or vent your body if you need to cool down.
Shipping Policy
Components:
All component items (wheels, drivetrains, suspension, brakes, etc.) ship within 2-5 business days of purchase.
Bicycles:
Complete bikes typically ship within 2–3 weeks due to build, tuning, and inspection times. You will receive tracking information as soon as your order is ready to ship.
Riding Gear & Apparel:
Riding gear and apparel orders (helmets, jerseys, gloves, etc.) ship within 2-5 business days.
Returns & Exchanges:
If you received the wrong size or wish to exchange an item, it must be returned in original condition—unused, uninstalled, and with all packaging intact.
For full details, please see our Refund Policy before initiating a return.
