Sea to Summit Trek TkII

Published · Researched 2026-09-21

2026 lineup note: Current product (2026) — Sea to Summit's comfort-cut 3-season down bag. Tapered rectangular shape (roomier than a mummy), RDS 650+ Loft ULTRA-DRY Down, comfort -1°C (30°F) / limit -8°C (18°F). Available in Regular (ATK2-R) and Long (ATK2-L), left/right zips that pair with S2S women's bags. Note: EU/US listings split between an older generation (~980–1,080 g) and a newer lighter spec (760–850 g) — see Buying traps; buy by the spec on the current Sea to Summit page.

Version lineage (2023–2026)

• 2023–2024: classic Trek TkII — the established spec: tapered rectangular, 650+ Loft ULTRA-DRY Down, -1°C comfort / -8°C limit / -25°C extreme, regular 980 g (34.6 oz), long 1,080 g (38.1 oz), fill weight ~500 g (reg) / 560 g (long), 30D nylon shell + 20D lining, #5 YKK two-way side zip + separate foot zip, Ultra-Sil compression bag included.
• 2024–2025: EU retailers began listing a revised spec — regular 760 g, long 850 g, fill weight 350/400 g — suggesting a generation refresh (lighter shell fabrics, recycled content) mid-decade. Generation split is retailer-evidence only; Sea to Summit's official current page is the arbiter. UNVERIFIED which generation is current at US retail.
• 2026: Still an active line on seatosummit.com and EU dealers. The "RW" item suffix (ATK2-RW) seen at some EU retailers appears to denote the recycled-spec generation.

Overview

The Trek TkII is the bag for people who find mummy bags claustrophobic but don't want a quilt. The tapered-rectangular cut is the headline: 152 cm shoulder girth (reg) that lets you sleep on your side with knees bent, while the hood, 3D side-wall baffles, and draft tube keep it thermally honest to -1°C comfort. Sea to Summit's signature details are here — vertical chest baffles that stop down migration, a 60/40 side-block fill ratio keeping more down on top, a separate foot-box zip so the bag converts to a quilt/blanket, an oversized anti-snag zip draft tube, and a large internal security pocket. The two-way #5 YKK main zip couples left/right to S2S women's bags for doubles. It's a "traditional comfort, anything-but-traditional design" bag — heavier than a mummy at the same rating, dramatically more livable.

Key Specs

Newer retailer spec (760 g reg / 850 g long) exists — see Buying traps.

Pricing

• MSRP: Sea to Summit's published MSRP UNVERIFIED in this research (no live manufacturer price captured). Historical US pricing for the Trek line sits in the ~$279.95 range for the TkII — UNVERIFIED, do not quote at checkout. EU pricing observed: 4,199 SEK (kayakstore.se, long, sold out).
• Street: Mid-tier down-bag street pricing; deep discounts are rare for S2S. Live street price UNVERIFIED — re-verify before publishing.

In the Box

Sleeping bag, Ultra-Sil™ compression bag, storage/mesh sack (retailer-listed "storage bag" at some dealers).

Warranty

Sea to Summit warranty term UNVERIFIED in this research (one EU dealer lists 2 years; S2S's consumer warranty may differ by region). Do not quote a term without checking seatosummit.com.

Support & Parts

Sea to Summit runs a repair program (wash/loft/zipper service) in its major markets. Compression sacks and zipper sliders are serviceable; the bag's baffles and fills are manufacturer-serviceable. The coupling zips are standard #5 YKK — any gear-repair shop can handle them.

Community Verdict

The Trek TkII is the quiet default recommendation in the "I hate mummy bags" corner of backpacking forums: owners praise the room-to-move cut, the foot-box zip that turns it into a quilt on warm nights, and S2S's honest EN ratings (the -1°C comfort rating is conservative in practice). Recurring critiques: it's heavy for its warmth (a 650-fill mummy at the same rating is ~300 g lighter), the 30D shell is thin for the bag's "comfort" positioning, and the price sits close to lighter technical bags. Qualitative pulse: bought by couples (pairing zips), side-sleepers, and taller hikers — not by ounce-counters. No fabricated scores.

Images

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Research date: 2026-09-21. Specs cross-verified across kayakstore.se, macpac.com.au, sportokay.com, tradeinn.com (trekkinn), en.aventurenordique.com. Generation split (classic 980 g vs newer 760 g) documented from retailer listings — re-verify the current generation at seatosummit.com before publishing.

Long-term durability & warranty-claim reality (enrichment — Sept 2026)

The TkII's 30D shell / 20D lining is the delicate end of mid-range — this is not a bag that survives ground-sleeping without a liner or pad discipline. The ULTRA-DRY down treatment helps against condensation (the bag's real enemy in its tapered-rectangular volume), but like all hydrophobic down it has a finite wash life. The full-perimeter #5 YKK zips are the long-term wear items — keep them clean and waxed. Warranty-claim reality: Sea to Summit's repair program is the practical warranty — S2S will re-loft, re-zip, and patch on a fee basis, which matters more than the headline warranty term for a bag you'll own a decade. Exact warranty term remains UNVERIFIED; check the regional S2S site.

Version history — year by year (enrichment — Sept 2026)

• 2023: Classic generation in full distribution — 980 g reg / 1,080 g long, 500/560 g fill, comfort -1°C.
• 2024: The Trek line's recycled-spec transition begins appearing at EU dealers (RW suffix); weights drop toward 760/850 g in some listings.
• 2025: Both specs coexist across retailers — generation ambiguity peaks; US and EU assortments may differ.
• 2026: Current product; the refreshed lighter spec appears to be the going-forward generation, but retailer evidence is mixed — UNVERIFIED.

Head-to-head vs closest rivals (enrichment — Sept 2026)

• vs REI Co-op Joule 21 (/library/gear/sleep-systems/rei-coop-joule-21/, this round): the Joule is the women's mummy equivalent (21°F/−6°C, 700-fill) — lighter but narrower; the TkII is the roomier unisex comfort cut. Both are "comfort first" bags; the Joule only exists used now.
• vs Aegismax Wind Hard Twilight (800FP quilt) (/library/gear/budget-brands/3f-ul-gear-floating-cloud-quilt/): the quilt is lighter and cheaper but draft-prone and narrow — the TkII is the enclosed, no-draft alternative for restless sleepers.
• vs Therm-a-Rest Questar 20 (/library/gear/sleep-systems/therm-a-rest-questar-20/, this round): the Questar was the direct comfort-bag competitor (W.A.R.M. fit, 20°F) — discontinued, so the TkII now owns the roomy 3-season down-bag slot among current products.

Care & maintenance (enrichment — Sept 2026)

Store loose; never compressed. Wash only with down wash (Nikwax Down Wash), warm hand wash, rinse by kneading, dry flat then tumble low with dryer balls — per S2S's own care guidance. The hydrophobic down treatment survives several washes but degrades with regular detergent. Keep both zips clean; the foot-box zip sees grit and is the first to fail. Re-treat shell DWR with spray-on products.

Buying traps (enrichment — Sept 2026)

• Generation confusion: the 980 g classic and 760 g refreshed spec both circulate. If the listing doesn't match the current S2S page, confirm which generation you're buying — weight differs by 220 g.
• RW suffix: ATK2-RW vs ATK2-R at EU retailers — appears to denote the recycled-spec generation; not a different product line.
• Temp ratings are °C-native: -1°C comfort is 30°F, not 20°F — buyers comparing against US 20°F bags will find the TkII less warm than the number suggests at a glance.
• Left/right zip: bags are handed; confirm you buy the zip side that pairs with your partner's bag if doubling.
• No US price captured: S2S US pricing for this item was not verified in this research — check seatosummit.com; EU prices don't convert cleanly.

Community Verdict — additional voices (enrichment — Sept 2026)

Additional sourced voices (Sept 2026): Trekkinn/Trekinn shoppers rate the line ~4/5 (retailer aggregate); Macpac and EU dealer copy emphasize the "traditional comfort, anything-but-traditional design" positioning. Backpacking-forum sentiment (qualitative): the TkII is the most-cited bag in "roomy 3-season down bag" threads, ahead of the discontinued Questar; the consistent caveat is weight — "great bag, just accept it's not light." No fabricated scores.

Key Specs gap-fill (enrichment — Sept 2026)

Comfort -1°C/30°F, limit -8°C/18°F, extreme -25°C/-13°F. 650+ Loft RDS ULTRA-DRY Down; fill 500 g (17.6 oz) reg / 560 g (19.8 oz) long. Total: 980 g = 34.6 oz (2.16 lb) reg; 1,080 g = 38.1 oz (2.38 lb) long. Newer refreshed spec: 760 g reg / 850 g long (UNVERIFIED current). Shell 30D nylon DWR; lining 20D nylon. #5 YKK two-way side zip + foot zip (quilt mode). Girth 152/146/102 cm (reg). Ultra-Sil compression bag included. MSRP UNVERIFIED (~$279.95 historical, do not quote).