Breathability
The Marmot Isotherm Jacket’s Polartec Alpha insulation, Polartec Power Stretch Pro under arm and side paneling and open mesh interior back panel lining are all highly breathable but the 20d Pertex Quantum exterior shell is the bottleneck to vapor and heat escape. This makes the jacket best for more moderately aerobic activities in colder climates. 

Durability
The Pertex Quantum exterior shell proved highly abrasion resistant throughout the test, emerging unscathed from repeated bouts of squeezing between boulders and almost continuous use with backpacks. The Power Stretch Pro panels are thin and airy, making them prone to snagging, although they weren’t damaged throughout the testing period. 

Weatherproofing
The Marmot Isotherm Jacket scored high in wind resistance, which contributes to it being one of the warmer items tested. This jacket also has a massive storm flap behind the main zipper, aiding both wind resistance and warmth. The Pertex Quantum portions of the outer shell are DWR treated and shed light precipitation well compared to others in this category. Since this jacket is one of the warmer choices better suited to less intense pursuits, it received a minor ding for not having a hood available in the 2017 version. 

Fit/Comfort
The Marmot Isotherm Jacket is athletically cut, but still provides enough interior volume to use midweight layers underneath. The “Angel-Wing Movement” pattern keeps the wrists covered when reaching overhead without the use of overly long sleeves. The torso is a little short to stay tucked in climbing harnesses. The Power Stretch Pro panels are key for mobility as the rest of the jacket doesn’t provide stretch. The bottom hem is adjustable via a single cord lock on an elastic drawstring but the wrists are simply bound with elastic.

Compressibility
The Marmot Isotherm jacket is highly compressible for its warmth, stuffing down to just slightly larger than a 1-liter water bottle. This allows it to be considered a “just in case” insulating layer that can provide a lot of warmth while moving in exchange for a little corner of the pack.

Features
The Marmot Isotherm jacket has just the right number of usable pockets: two zipped handwarmer pockets (do not clear pack hip belts), one vertically aligned zipped chest pocket, and one interior mesh dump pocket that was perfect for keeping gloves and a hat warm and accessible.