Field test first hour out of the package, ambient temperature at 29 degrees and west wind at 7 MPH (21 windchill). Hike along local river trail for 4.75 miles at a 3.5 mph pace. Sporting the new vest with a short sleeve T-shirt as base layer with a long sleeve 1/4 zip fleece 100 weight top. The initial impact right out the door, the core area under the vest was comfortable. It had no issues with current wind speed. After the first 20 minutes of the hike start to notice the heat in my upper body parts. Start to off load some of the heat with the zipper at the 30 minute mark. I felt this vest was a touch warmer than an older vest that I own which is insulated with 500 fill goose down. The Zonal Vest has a very thin profile and light in weight and in this initial test delivered more than enough heat. The ground cover today had a light snow cover and the sky was over cast so, no help from solar rays. The Zonal Vest was performing as advertized.
I purchased both the size Small(36-38)and Medium(39-41)because my actual chest size is 38 with 31 waist and was not sure which would be the correct fit for my frame. I sent the size small back, I could zipp it up but I felt I was maxing out the waist area.