Long-Term Report:
Zyflex Thermal Shirt And Pants

Summary
I wore the Zyflex Thermal top and bottom as a base insulating layer and the top as a outer layer on numerous mountain backpacking trips in cool and cold weather. I found them not particularly warm when worn in static conditions as a base insulating layer. Thin and medium weight microfleece or wool base layers are warmer and lighter. However, I discovered that the Zyflex Thermals perform fairly well as an outer layer at low to moderate exertion levels in cool weather, and seem to be quite durable for that purpose.

Long-Term Report
Test Period—Late May 2006 to late September 2006.

Test Location—Southern Colorado Mountains.

Testing Conditions—The testing environments were forests and high elevation alpine country. Extreme conditions included: snow showers and one snow storm, strong winds, thunderstorms with heavy rain, low temperatures, and high elevations with strong UV and intense thunderstorms. Terrains and conditions were forest trails, off-trail bushwhacking, scrambling, and walking in rain and snow. Elevations ranged from 6,000 to 13,500 ft (1829 to 4115 m) and nighttime temperatures ranged from 28 to 55 F (-2 to 13 C).

How The Zyflex Thermals Were Used—The Zyflex Thermals were used for mountain backpacking, day hiking, and car camping where they were worn on the trail, in camp, and in my sleeping bag - situations where extra warmth was needed.

Amount of Use—Over the four-month test period the Zyflex Thermals were used on a total of 10 trips totaling 27 days as follows:

Measurements And Data—On each trip I recorded the following data in relation to my comfort in the Zyflex Thermals: 1) temperature, 2) estimated wind speed, 3) estimated humidity, 4) altitude, 5) precipitation (including what form), 6) clothing or sleeping system the Zyflex Thermals were used with 7) clothing worn over the base layers, and 8) how well they performed in terms of warmth, utility, and comfort.

Factors Evaluated
Following are the specific factors on which I evaluated the Zyflex Thermals.  

Fabric—Except for stating that the fabric is 59% Polyethylene, 35% Nylon, and 6% Lycra®, Zyflex does not give a detailed fabric description. It has a lot of horizontal stretch, but very little vertical stretch. The fabric is woven in a fine grid pattern (see photo). It is claimed to be warm, comfortable, highly breathable, and water-resistant.

Construction, Quality, And Fit—The fabric and sewing quality are very good; I could not find any flaws in either the fabric or sewing. The seams are serged rather than flat-felled, except the collar. The design is very good - the sleeves are Raglan style to provide greater freedom of movement, the neck opening is right-sized, and the body is well-proportioned, amply long, and has a dropped tail.

Both the shirt (size XL) and pants (size L) fit me very well. I normally wear a size large shirt, but I needed a Zyflex XL (according to their size chart) to get the correct fit, which turned out to be correct. The shirt fits slightly relaxed on me, and is roomy enough to use it as a second layer. The sleeves are a little short, and (for me) it would be nice if they were an inch longer. Articulation is good; when I cross my arms the sleeves don’t move, and I can feel the fabric stretching across my back. However, when I extend my arms above my head the sleeves retract about one inch (2.5 cm) because the fabric has very little vertical stretch.  The pants fit like tights and stretch to fit my legs without feeling too tight. When I squat or raise my legs the pants stretch with me and don’t inch up or down.

Performance As A Base Layer—When mountain hiking, my normal routine when I stop for the day is to put on long johns to avoid chilling. I put my hiking clothes back on over the base layer. Worn against my skin, the Zyflex Thermals were comfortable, but not as soft as microfleece. The same was true for warmth; they were somewhat warm, but not as warm as microfleece (which weighs a little less). On one late May trip, I had evening temperatures in the high 30’s F (2-4 C) and a morning low of 28 F (-2 C), and the Zyflex Thermals were noticeably NOT warm. On numerous other trips I wore the Zyflex Thermals in 40-50 F (4-10 C) temperatures in camp and found them to be marginally warm enough.

On all of my backpacking trips, I found it necessary to also carry a pair of synthetic insulated pants to wear in case the Zyflex thermal bottom was not warm enough in camp or my sleeping bag. I ended up wearing the extra pants over the Zyflex Thermal pants about two-thirds of the time. Thus, I ended up carrying extra weight in my pack because the Zyflex Thermals were not warm enough. Normally I would carry a microfleece base layer and that would be sufficient.

When I hiked in shorts, I put the shorts on over the Zyflex bottom to wear in camp. That combination added a little warmth and the shorts protected the Zyflex bottom when I sat on rocks. The combination was sufficiently warm when temperatures were above about 45 F (7 C).

Performance In Combination With Other Performance Clothing—I wore the Zyflex Thermal top as a single layer hiking shirt and as a second layer. It performed quite well in both situations. The Zyflex fabric is very stretchy, breathes well, disperses moisture well, and is fairly durable. I found it to be quite comfortable while hiking at moderate exertion levels. At high exertion levels, especially in the sun, the Zyflex top was too warm and sweaty. In my opinion, the Zyflex Thermal top performs better as a hiking shirt, or layering piece, for hiking in cool weather than it performs as an insulating base layer.

The same may be true for the Zyflex Thermal bottom, but it was not cold enough during the test period to hike or bike in it to find out. The bottom fits like tights, so it could be an appropriate hiking or biking pant for cold weather. For that purpose, black would probably be a better color for appearance purposes. The gray color of the Zyflex Thermals tested looks a little too much like underwear!

One reason why the Zyflex Thermals were only somewhat warm as an insulating base layer may be that their breathability allowed them to lose heat too readily. I found the Zyflex Thermals to be distinctly warmer when I wore other clothing over them to help seal the heat in, but that would also be the case with microfleece or wool base layers. They were distinctly warmer when I wore my hiking pants and shirt, plus windwear or rainwear over them in camp.

Performance In A Sleeping System—I routinely wore the Zyflex Thermals in my sleeping bag to keep my sleeping bag clean and to extend its warmth. In my 32 F (0 C) rated sleeping bag, the Zyflex Thermals were comfortable and warm when the nighttime temperature was in the 38-55 F range (3-13 C), but my legs were distinctly chilly at colder temperatures. On one night when the temperature dropped to 28 F (-2 C), my legs were cold in the Zyflex Thermal bottom, so I replaced them with a microfleece bottom (that I had brought along for comparison), and found that the microfleece bottom kept my legs warm for the rest of the night.

Warmth—The Zyflex fabric itself has little loft. Product information on the Zyflex website disputes the need for loft in order to provide warmth. I could debate their claims in this report, but the bottom line from my testing is that the Zyflex Thermals are not very warm for static activities, like wearing them as an insulating base layer in camp. On the other hand, I have found the Zyflex Thermals to be comfortable and useful for moderate exertion activities on the trail. At low to moderate exertion levels in cool or cold weather, the Zyflex Thermals performed similar to a medium weight polyester or wool layer. That is, they maintained a comfortable temperature while dispersing excess heat and moisture.

Odor Management—I did not detect any odor problems from extended wear while wearing them every evening and night on a six day trip.

Durability—The Zyflex fabric is quite durable. I wore the top as a hiking shirt on several occasions and there are no snags in it. Also, the fabric does not seem to pill.

Versatility—The Zyflex Thermals seem to perform best (in terms of comfort) for low to moderate exertion activities, like hiking, snowshoeing, or biking in cool or cold weather. They are not particularly warm for their weight in static activities as an insulating base layer.

Suitability For Lightweight Backpacking—Compared to microfleece or wool base layers, the Zyflex Thermals are heavy for the warmth they provide. Rather than use them for backpacking, I am more inclined to use them for day trips in cool and cold weather.

Acknowledgement
I would like to thank Zyflex and the BackpackGearTest Group for selecting me to participate in this test.

Will Rietveld