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KUIU Ultra Merino 145 Zip-Off Bottom Long Underwear Review

Have you ever wanted to wear long underwear on a cold winter day, but decided not to because you know you’ll sweat too much? Imagine having long underwear with side zippers that you could vent if you get too warm, or even remove completely without having to take off your boots or outer pants.

KUIU Ultra Merino 145 Zip-Off Bottom Long Underwear

Ease of Care

Great Layering Piece

KUIU Ultra Merino 145 Zip-Off Bottoms can be vented or removed without taking off your boots, just like full zip rain pants, making them ideal for winter hiking and other snowsports.

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KUIU makes a variety of zip-off long underwear bottoms for this exact purpose. While KUIU focuses mostly on hunters, these zip-off bottoms are also ideal for hikers, skiers, or really anyone who is active outdoors in winter. They’re also very reasonably priced and available in wool, synthetic yarn, or fleece to suit your preferences.

Ultra Merino 145 Zip-Off Bottom

If you’ve ever owned a pair full-zip rainpants, like Marmot Precips, these KUIU long underwear zip-offs work the exact same way. They have side zippers along the outside of each leg and velcro in the waistband. You can unzip the legs partially to vent heat or unzip them all the way, open the velcro tabs at the waist, and pull them completely off without having to remove your pants or boots.

All of KUIU’s long underwear bottoms are boot length with a 26″ inseam, so they only go down mid-calf. I find that desirable because I mainly wear mine with high gaiters and winter hiking boots and my socks would get soaked with sweat if they were much longer.

KUIU makes the wool versions of their zip-off bottoms with a 90/10 wool and polyester blend called NuYarn that makes them more durable and reduces shrinkage. Traditional wool fabric is knit from single-ply yarns in which wool fibers are drawn into fine strands and twisted together for strength. NuYarn eliminates the twist in the yarn. Fibers are placed around a core and drawn out into a superfine fiber with zero twist. NuYarn uses a nylon core for added strength, hidden inside the fine merino-wrapped fiber. The resulting merino outperforms the world’s best ring-spun merino fabrics in almost every performance metric.

The KUIU Ultra Merino 145 Zip-Off Bottoms have a fabric weight of 145 g/m2 and weigh 7.3 oz in a size XL. They have flat seams and a gusseted crotch for agility, in addition to a “man-port”, so you can pee while you’re wearing them. You don’t feel the zippers on your skin when wearing them and they’re fairly thin, so they don’t bunch up under the softshell winter hiking pants I wear.

I only wear long underwear on very cold days, usually when temperatures are 10*F or colder. I run hot when I hike, so it’s nice to be able to remove the zip-off bottoms when the day warms up and I start to sweat. I’ve also found the 145’s very comfortable when worn as a sleeping base layer on overnights and I appreciate that they seamlessly fit right into my 3 and 4 season clothing systems.

The 145’s do take a little extra care when washing. It’s best to wash them cold with Woolite and not a harsh everyday detergent. KUIU does not recommend machine drying, so I just hang them and let them dry that way.

KUIU’s Ultra 145 Wool Zip Off Bottom can be removed without taking off your boots.
KUIU’s Ultra 145 Wool Zip-Off Bottom scan be removed without taking off your boots.


KUIU Ultra Merino 145 Zip-Off Bottoms can be vented or removed without taking off your boots, just like full zip rain pants, making them ideal for winter hiking and other snowsports. While they do require a little bit of extra care to wash, they’re a unique layering option for people who start cold and then warm up and want to delayer.  These KUIU zip-offs are such a simple idea, you have to wonder why no one thought of it before.

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Disclosure: The author purchased this product.

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  1. Love the video.

  2. Looks great. Starting to really look into companies like Kuiu, Mystery Ranch, and First Lite for hiking clothes and not just for hunting. The full zipper is a great idea because I am also quick to overheat so they would be very convenient. One more thing to add to the list.

    • I’m doing more “wilderness backpacking” these days off the crowded and beaten track and some of the gear and clothing from these companies makes a lot of sense for those more rugged conditions.

  3. Like what strippers wear, but with zippers instead of snaps?

  4. For years now I have been cutting into brand new long johns,and putting in a 6-8″light weight zip in the bottom of the legs so I can get them on and off over my you my know us kiwis always get around in shorts and long johns in the wet and cold.

  5. YKK side zips can be added to any legging (preferably sized up) to achieve a similar effect. I have used YKK’s 31” heavy duty zips on wool leggings.

  6. At some point it would be worthwhile to review the Pro Merio shirts and zip off long johns. The material is bonded merino to a synthetic shell. I’ve recently purchased a set and found the combination truly comfortable, warm, breathable and rugged. This new material raised the bar for me. I think you’ll like it if you try some.

  7. FirstLite makes the same type of base layer. They even offer one version that is full length

  8. Great post Philip. I’d like to better understand your choice to purchase the merino vs either of the Peloton options, especially since the fabric weight of both Peloton pants is lighter than the wool/poly fabric. Was this due to the superior odor control of the NuYarn? I can readily appreciate this advantage, especially on long sections between trail town resupplies and gear laundering. Thanks.

  9. El Diablo Amarillo

    Check out Canis and Stone Glacier for similar side zip gear. This is not a paid infomercial BTW. I started using Canis this hunting season. Im 100% backcountry big game hunting so its a 25-40# pack and 3-9 miles a day. If its a successful day Im coming back out 40-60# heavier. The ability to hump up and down steep slopes with no trails then possibly stop or sneak for a few hours is the norm so ability to vent, not sweat, insulate, then start all over multiple times a day is key. And often its a backcountry spike camp overnite so getting back to a truck is not a reality. Also started running Mountain Tools from Carmel CA VBL socks. I like the way they are sewn better than the RAB VBL socks
    Great article as always Mr. Werner! Thank You!

  10. Hey Philip,
    Glad to see you’ve taken me up on my suggestion of trying these! They are awesome – try the insulated Kenai version next, perfect for stargazing and winter camping,!

  11. Has anyone ever tried silk or angora longjohns?if so what was the performance like?

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