Western Mountaineering Bag Expander – Down Sleeping Bag Expander for Mummy Bags

Increase the comfort and versatility of your sleeping system with the Western Mountaineering Bag Expander, a down sleeping bag expander for mummy bags. Whether you need extra space for broad shoulders, want more elbow room, or plan to share your bag with a partner, this simple addition transforms your sleeping bag into a roomier, more adaptable shelter for the night.

Designed to zip directly into any Western Mountaineering mummy-style sleeping bag, the expander adds width without compromising warmth. Like our premium sleeping bags, it’s insulated with 850+ fill power ethically sourced goose down for the highest warmth-to-weight ratio. This means you get additional interior space while maintaining the exceptional loft, comfort, and cold-weather performance that Western Mountaineering gear is known for.

The Bag Expander works best with 6-foot models, but it’s compatible with all lengths. Shorter or longer bags will simply have a small amount of overlap, as most length differences are found below the zipper line in the footbox area.

Available in Summer and Three-Season fill weights, you can choose the version that best matches your sleeping bag’s intended use. Both are crafted with the same attention to detail, lightweight fabrics, and premium materials that define our gear.

From solo trips where you want a little extra breathing room to sharing your bag under the stars, the Western Mountaineering Bag Expander is a smart, packable upgrade for anyone who values adaptable comfort in the backcountry.

Made in USA.