By JASON MAGNESS

“Elite adventure racers’ secret weapon” — that’s the phrase that caught my eye in MontBell Co.’s description of its U.L. Thermawrap Jacket. This was several years ago, and as a new adventure racer at the time, I could use all the help I could get.

Lightweight synthetic insulation. Warmer than fleece. Retains warmth when wet. Extremely packable. These were the touted features. I was convinced and ordered one.

At the heart of the MontBell jacket are two things — weight (or lack thereof) and warmth. The jacket weighs only 8.8 ounces. According to the company, its proprietary insulation retains only 1 percent of its weight in water and keeps more than 80 percent of its thermal properties when completely soaked. Down can’t do that. Most insulating jackets can’t, either.

The U.L. Thermawrap Jacket’s outer construction is nylon. It has a full zipper, collar, and elastic cuffs. There are no unnecessary embellishments, staying true to the MontBell mantra — “function is beauty.”

In this case, beauty costs $145.

The Thermawrap has gone through only minor design tweaks since its inception more than 10 years ago, with the notable changes being more pockets, an improved zipper, a micro-fleece lining inside the collar, and stretch panels at the wrists and side waist to give the jacket an improved fit. The nylon outer fabric has been changed to a new weave that is quieter during movement and has a softer feel.

I used the jacket for the first time during an adventure race in northern Minnesota. The race was a disaster, but somewhere in the middle of that miserable night, I fell in love with the Thermawrap jacket.

Ten years later the love continues, and there are a few new additions to my Thermawrap family — a vest, parka, and pants. The parka has a little more insulation and a hood; the pants have full side zips making them easy to put on over anything, and simple to vent if you get too hot. All of the pieces are variations on the same theme: lightweight, extremely warm, and minimally affected by water.

For years, I’ve considered the jacket part of my mandatory gear during adventure races, multi-day climbs, and expeditions. The parka and pants — what I have started referring to as my “Thermawrap Suit” — kept me warm during the bitter cold (minus-40 degrees F) of the To Cross the Moon expeditions, and took the place of my sleeping bag during the “4 Ways to Die Expedition” in Montana, and the “Too Much Fun” expediton this year on Mount Rainer.

All of these adventures were firsts — possibly due to our team’s innovative gear and penchant for suffering. In these expeditions, the Thermawrap suit also doubled as our wetsuits as we pack-rafted down the glacier-fed rivers, keeping us warm (enough) and alive as we paddled — and too often swam — in the 34-degree water.

The only downside to the Thermawrap family is that the lightweight construction leads to durability issues with the nylon shell. If you thrash your way up an ice-choked chimney in your Thermawrap suit, you’ll put some holes in your clothes. Don’t expect to bushwhack in the company’s ultra-light outerwear either.

That said, I am still wearing my original jacket from that first purchase ten years ago. It has held up through hundreds of days outdoors, and it still keeps me warm and protected when things get cold and wet. Bottom line, if you want lightweight insulation that performs well in an extremely wide range of conditions, the Thermawrap garments are a great choice.

—Jason Magness is a founder of the YogaSlackers.