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Sea to Summit down sleeping bags, part 2: the 3D sidewall and Reverse differential cut

This spring season, Sea to Summit has released a groundbreaking new line of down-filled sleeping bags. The Sea to Summit down sleeping bags are so packed with technology, it’s more than we can adequately describe on, so we’ll be examining one technological aspect each day all week. Curious how Sea to Summit’s designers have pushed the envelope with their new down sleeping bag designs? Read on. Today, we are looking at the innovative construction methods used, including the Reverse differential cut and 3D sidewall.


Reverse Differential Cut

High-quality down sleeping bags have long featured a differential cut, which basically means that the shell fabric on the outside of the sleeping bag is cut slightly larger than the lining on the inside. This “differential” between inside and outside provides room for the goose down to loft fully; your body fills this inner lining, and the down lofts toward the larger, outer shell.

For the bottom of their new down sleeping bags, Sea to Summit has ingeniously reversed this process, making the lining larger than the outer shell. This way, as the down expands, it lofts toward your body instead of away from it, filling the space between you and the inner shell and increasing thermal efficiency.

This sounds like a minor change, but it’s a critical difference on the bottom of a sleeping bag, where the down is compressed by your weight and typically least-effective. With the larger side against your skin, the down can loft upward into the contours of your body for maximum heat retention.

3D sidewall, side block baffles and offset baffle construction

As discussed above, down insulates best when it is uncompressed and fully-lofted. The Sea to Summit down sleeping bags utilize an offset baffle, which is more stable than a traditional down baffle and minimizes the loss of heat through seams. The result is more-effective insulation without the increased weight of more baffling (or more complicated baffling).

These sleeping bags also utilize a “3D Side wall,” which accounts for the distinct vertical sides, aiding in vertical loft along both sides of the bag. A side block baffle keeps the down in these areas from shifting. Fill ratio varies from baffle-to-baffle, depending on the optimum down density for each area on the sleeping bag. For example, there is more goose down in the top of the bag, where it is most effective, than the bottom of the bag where it’s compressed by your body weight. For the Alpine, Traverse and Trek series bags, this ratio is about 60:40.

The bottom line here is that the 3D Side wall and reverse differential cut insure maximum loft, even on the bottom and sides of the bag, while advanced baffling makes sure the down stays where it needs to be.


Yesterday we wrote about the 3D NanoShell; check back throughout the week for articles about the unique certification provided for down content and fill power, the temperature rating system, and the features included in each of the four lines of sleeping bags.

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