Thermarest Fly Creek UL 25F Down Sleeping Bag

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Redefine backcountry comfort with the Fly Creek UL 25, the ideal ultralight bag for all your minimalist adventures.

Big Agnes says: 

Premium materials, like ultralight NetPlus® fabric made from recycled fishing nets and high-quality 850-fill down, along with a sleek design featuring Big Agnes Signature Diamond Quilting, produce a lighter, more packable bag. From the ergonmic footbox to the insulated draft collar, every aspect of this sleeping bag is designed to maximize thermal efficiency, without the need for extra weight and bulk. The result is a technical sleeping bag that is a backpacker’s dream, perfect for fastpacking, thru hiking, or any overnight trip in three season temperatures.

Our Verdict

The Fly Creek UL 25F from Big Agnes is a tidy blend of ultralight design and reliable insulation, coming in at 702g (Regular) with a -4°C lower limit. It’s well-suited for changeable spring and autumn conditions, especially for those with pack space and weight as top priorities.

The 850-fill DownTek down lofts quickly and holds warmth effectively, even in damp air. The quilted upper and baffled lower sections keep insulation in place, and the ergonomic footbox adds comfort without bulk. The half-length centre zip trim reduces weight and works well for venting, but it does compromise flexibility compared to a full-length option.

The 10D recycled nylon shell is soft and breathable but needs gentle handling. The minimalist hood cinches securely, though the cordlock could be easier to manage with cold hands or gloves. The fit is snug, as expected from a mummy cut.

For those who value sustainability, the use of recycled shells and PFAS-free treatment is a welcome bonus. While the design leans minimalist, the warmth-to-weight performance is convincing.

– Reviewed by Ultralight Outdoor Gear

Features

• Premium materials and minimalist design produces a lighter, more efficient bag
• Ultralight nylon ripstop made from recylced fishing nets
• Big Agnes Signature Diamond Quilting prevents down migration while using less stitching than traditional quilt lines; saving weight and allowing better loft
• Minimalist hood with low-profile cordlock. Cordlock is made with renewable source materials from organic waste.
• Half-length center-zip keeps weight low and provides easy temperature regulation
• Mummy fit, generous draft collar and zipper draft tube increase thermal efficiency
• Ergonomic footbox construction provides more loft and better fit
• Storage sack & stuff sack included
• We recommend trying this product with a Big Agnes Compression Stuff Sack or Super Light Girdle for adventures on which space matters

Specifications

Regular

• Weight: 702g
• Temperature rating: 25°F / -4°C
• Fill Type: 650 Downtek™ PFC-Free
• Fill Weight: 482g
• Compressed Size: 20 x 24cm
• Stuff Sack Size: 19 x 38cm

Long

• Weight: 777g
• Temperature rating: 25°F / -4°C
• Fill Type: 650 Downtek™ PFC-Free
• Fill Weight: 539g
• Compressed Size: 20 x 25cm
• Stuff Sack Size: 19 x 38cm

Material & Construction

Fabric

• Shell/Lining: Ultralight recycled 10D nylon ripstop with PFAS-free water-repellent finish. Fabric made from recycled fishing nets

Insulation/Construction

• 850 fill Downtek™ PFC-Free repels water while maintaining insulating value using bluesign® approved PFAS-free water-repellent chemistry
• YKK #3 zipper
• Diamond quilt-through construction on top. Baffled construction on bottom and sides.

Sustainability Notes

• This product contains 100% Certified RDS Down Certified by Control Union CERT 879262
• Downtek™ PFC-Free uses bluesign® approved PFAS-free water-repellent chemistry
• PFAS-free WR finish on fabrics
• Fabric made from recycled fishing nets

Help & advice:

• The Ultimate Guide to Lightweight Sleeping Bags
• Time-Saver Guide to Choosing a Sleeping Bag
• The EN13537 Standard
• Down Quality
• All About Down
• Hydrophobic Down
• About Big Agnes
• Big Agnes Sleeping Bags Buying Guide
• Why is ‘PFC Free’ Important?
• What is Bluesign?