Close-up: A complex knit, water beads on some Delta fibers and absorbs into others
Close-up: A complex knit, water beads on some Delta fibers and absorbs into others

Complex Knit: First Look At Polartec Delta ‘Cooling’ Fabric

For years, runners donned wet cotton shirts or soaked hats to cool down in hot weather. New this year, technical-fabric maker Polartec puts science to work making a better wet T-shirt.

delta fabric by polartec

A water-soaked cotton T-shirt has long helped runners cheat the heat. Evaporation cools the shirt, with wet fabric a stand-in for sweat on naked skin.

But clinging cotton is far from comfortable, and hot-weather exercisers more often default to polyester, nylon, and synthetics touted to wick sweat and quickly dry as they bike or run.

Velocio Radiator SL Base Layer for women
Velocio Radiator SL Base Layer for women

A new option this year, Polartec LLC launched a fabric called Delta. It’s advertised to “cool you down in the hottest conditions” via a tangle of fibers that hold water yet let the fabric breathe.

Polartec ‘Hot Weather’ Fabric

Polartec is known mostly for its fleece and cold-weather apparel. Delta is the company’s first warm-weather play.

I tested it for a month in heat up to 100 degrees. The material, which soon debuts in shirts from multiple brands, is knit with a raised texture designed to manage heat.

Unlike treated or chemical-based cooling shirts, which can feel cold to the touch, the Delta effect is more than skin deep. The company calls it “metabolic cooling” versus the “sensory cooling” offered by many brands using a treatment that feels cold when wet.

With Delta, no chemical treatments are used. Polartec instead knits together hydrophobic and hydrophilic fibers, the contrasting result dispersing moisture, increasing airflow, and (unlike wet cotton) reducing friction against the skin when you move.

outdoor research t-shirt
Outdoor Research t-shirt made of Polartec Delta material

Polartec Delta Review

In my review, where I biked and ran in Delta shirts, sweat soaked into the fabric, allowing for noticeable cooling. It was especially prevalent on a bike, where stronger air movement perpetuates the effect.

While running, the technology was not as obvious. But the shirt remained comfortable even when soaked through with sweat, its raised texture never clinging.

The fabric is made of Tencel lyocell, a form of rayon which consists of cellulose fibers (for its hydrophilic properties), and polyester (for its hydrophobic properties).

Close-up: A complex knit, water beads on some Delta fibers and absorbs into others
Close-up: A complex knit, water beads on some Delta fibers and absorbs into others

There are different versions of Delta. We reviewed the Radiator SL Base Layer from Velocio and the Outdoor Research Gauge Tee, as well as a Polartec-branded Delta shirt that will not come to market.

Each were slightly different in look and feel, with the Velocio top extra thin and the Outdoor Research more like a normal t-shirt.

(We also tested the Radiator Tee from Kitsbow, which uses a Polartec fabric called PowerDry for its body and Delta in strips under the arms and down the sides.)

kitsbow shirt
Kitsbow employs Delta under the arms on its bike-oriented T-shirt

Run Wet, Stay Cool

I’ve long been a proponent of the “run wet” philosophy in summer. (See my detailed “Tips to Beat Extreme Heat Outside this Summer” article for a list of tricks.)

With Delta, Polartec gives an easy option for anyone in need of an edge in hot temps.

Look to brands including Outdoor Research, Westcomb, Kitsbow, Oiselle, MacPac, Rhone, and Velocio to adopt Delta this year. The fabric is no panacea in the heat. But give the material a try this summer on the hottest days. You may never run “dry” again.

Stephen Regenold

Stephen Regenold is Founder of GearJunkie, which he launched as a nationally-syndicated newspaper column in 2002. As a journalist and writer, Regenold has covered the outdoors industry for two decades, including as a correspondent for the New York Times. A father of five, Regenold and his wife live in Minneapolis.