When we completed the UL Shirt in 2019, I was already thinking about what might come next. Could we create something even lighter—an entirely new kind of wind shell?

It wasn’t that I was dissatisfied with the UL Shirt. It had become an exceptionally comfortable piece of gear and introduced a new way of dressing for the trail. But I wanted to go one step further into the realm of wind: to create a single layer that could remain comfortable as the terrain, weather, and intensity of movement changed.

For me, ultralight means more than reducing weight. It also means making things simpler. The more conditions a single layer can handle, the less hikers need to stop, adjust their clothing, or think about layering. They can simply keep walking.

A wind shell should do more than block wind. It must also release enough heat to maintain a comfortable environment inside the garment. Breathability was therefore central to what we wanted to create.

We began by collecting wind shells from around the world and measuring their air permeability. There were already products designed with breathability in mind, but most came with a certain amount of weight. Among truly ultralight wind shells, we found very few that were clearly breathable.

Unable to find the fabric we needed, we began developing one with Pertex, the fabric manufacturer we had already been working closely with.

At first, I assumed that greater air permeability would always mean greater comfort. Testing soon proved otherwise. A highly breathable fabric can feel cold and offer too little protection when the wind strengthens or the temperature falls. But when we reduced air permeability to around 20 cc/cm²/s, heat began to build during sustained, high-output movement.

The balance we were looking for was around 30 cc/cm²/s. Air permeability measures how much air passes through a fabric: too little traps heat, while too much provides insufficient protection from wind and cold.

Shadow Rip®, used for the UL Shirt, measures 34 cc/cm²/s. After two years of testing, we realized that the comfort we were searching for was already rooted in the same balance—not maximum breathability, but the right amount.

Our next challenge was to achieve that balance in an even lighter fabric. This proved far more difficult. On ultralight materials, even a tiny change in the amount of coating can significantly alter air permeability. A sample might achieve the airflow we wanted but lack strength or durability. Occasionally everything worked, only for the result to prove impossible to reproduce consistently.

Eventually, we stopped development. If we couldn’t make the wind shell we believed in, there was no reason to release it.

But I never completely let go of the idea. Instead of asking one fabric to do everything, perhaps we could place different fabrics exactly where each performed best. I can’t remember when that realization came, but I think the problem had remained somewhere in my mind the entire time.

The final design combines three fabrics, balancing wind protection and breathability across different areas of the body.

A low-permeability fabric protects the front, where the wind hits hardest. More breathable fabrics release heat and moisture from the back, sides, and other areas where warmth tends to build.

Along the sides of the body and inside the arms, we used a slightly structured, uncoated fabric. Its consistently high breathability also helps prevent the garment from clinging to sweaty skin—an especially important consideration around the arms.

The structure of this fabric also helped shape the hood. Hoods made from ultralight materials tend to collapse, but placing the more structured fabric around the neck allows it to stand naturally, maintaining protection without feeling flimsy. A highly breathable panel through the center of the hood releases heat from the head.

The arms also play an important role in regulating body temperature, almost like the radiator of a car. We shaped the sleeves so they can be pushed up quickly, allowing heat to escape without removing the garment. Developed through our work on the UL All-weather Jacket, this detail gives the Wind Zip Hoody a much wider usable temperature range.

Every refinement adds weight. Our earliest prototypes were lighter, but there were forms of comfort we were unwilling to sacrifice simply to reach a lower number.

We reduced weight wherever possible, but not at the expense of reliability. This led us to develop our own cord stopper for the minimal 1.5 mm cords and search beyond our usual suppliers for the lightest suitable zipper.

We tested the Wind Zip Hoody ourselves throughout its development. I wore prototypes during my morning trail runs and on hikes. During the most recent Trans Japan Alps Race, Daisuke Katano carried one through the exceptionally demanding course. Seeing it perform there gave us real confidence in what we had created.

By the time the Wind Zip Hoody was complete, six years had passed.

What we learned during those years extends beyond this garment. The project changed how we think about breathability, material placement, and the body’s regulation of heat—and those lessons have since influenced other Yamatomichi products. Taking one piece of gear as far as we can often moves everything else forward with it.

Is the Wind Zip Hoody more comfortable than every trail shirt in every situation? No. A shirt designed for a particular set of conditions may feel airier or resist clinging better.

But the Wind Zip Hoody is designed for something different: expanding the range of conditions that one light, simple layer can handle.

It is not intended to protect against storms or severe weather. But you can put it on along a windy ridgeline, descend into the forest, increase your pace, and continue wearing it without stopping to change layers.

That is our answer after six years.

I hope you will take the Wind Zip Hoody into the mountains, discover what kind of tool it becomes in your own hiking, and let us know where it takes you.

(July 2026)