Western Mountaineering Sleeping Bag Expander

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The Western Mountaineering Sleeping Bag Expander was created for campers who just want more room. The Expander offers comfort for people who dislike the claustrophobic feeling of a mummy bag.

Western Mountaineering says

The Expander was designed to accommodate the varying needs of bicyclists kayakers rock climbers ultralight backpackers and college students. The Expander is available in 3-season weight and summer weight and can be used on the top or underneath your mummy bag. The Expander is made with a full-length 70 YKK #8 coil separating zipper black 50 denier Ripstop Nylon double layers of Polarguard 3D insulation with offset stitching and built in draft tubes along the zipper. The result is a highly compressible insert that is lightweight breathable absorbs little moisture and retains loft even when wet. The Expander is easily stored and laundered and is non-allergenic and mildew resistant.

Our Verdict

Western Mountaineering down gear has an awesome reputation in the US and that reputation is beginning to build on this side of the pond. Very high quality materials design and construction results in the lightest bags available for a given warmth.  

An example of their attention to detail is their quoted 850+ fill power down used in all their bags: WM quote this figure even though their down generally gets higher ratings in official tests this is because they feel that the official pre-testing preparation enhances the down quality!    The Bag Expander comes in two types Summer and 3 Season these weigh almost the same but the 3 Season expander does have significantly more down in it – so it gets a higher rating – the closeness of the weights is due to the different shell fabrics used on the expander and the fact that a substantial part of the weight is in the zip.  

The Bag Expander will fit all WM Mummy-shaped sleeping bags and adds about 16cm at the shoulder tapering to zero at the foot.  

`The bag expanders are really designed for use with the Mummy style bags rather than the Semi-Rectangular style bags that can be unzipped both along the side and across the foot.  

In general these semi rectangular bags already have a decent amount of space in them compared to the Mummy shaped bags where sometime extra space is required. There are wider options for a lot of the bags but the bag expander gives an option where folk may want a close fitting warmer bag for some uses and then also to have a wider bag in other situations without resorting to having 2 bags.  

The bag expanders are designed to work in the side zips if the Mummy bags and that said are also in just one length so they are `optimized` for a 6` bag and so for other lengths they are not going to be a perfect fit. Its not a perfect solution but one that works in most cases and provides the WM bags with a bit more flexibility.`

– Reviewed by Ultralight Outdoor Gear

Features

• Summerweight: Taffeta Shell (Extremelite Series)
• 3 Season: Microfibre shell
• Highest Quality minimum 850+ fillpower down

Specifications

• Summerweight expander weighs: 158g
• 3 Season expander weighs: 168g
• The small amount of outside fabric used (Microfibre used on the outside of the 3 season expander vs. taffeta on the summer expander) provides a part of the difference in weight.
• There is a little over 1/2 ounce of difference in down filling which is a substantial portion relative to the amount of fill in each of these expanders.

More About Western Mountaineering:

• Western Mountaineering Construction
• Western Mountaineering Materials