Finally, the lightest option that I've tested is Patagonia's Nano-Air, which itself comes in three different weight classes per square meter: 20g, 40g, and 60g of insulation. 20g is the lightest possible weight, and Patagonia measures the men's medium to weigh 7.7 ounces. I'm testing the women's extra-small and it's barely even noticeable. This is what you wear when you want just a little more insulation than a base layer under a rain shell.

I have been trail running, hiking, biking, climbing, and camping in the 20g Nano Air pullover for a year now and this is my favorite insulating mid layer. Patagonia has made a proprietary fabric for the shell called FullRange that has been treated for both water and abrasion resistance, so it's much more durable than the feathery, high-loft alpha direct fleece that I was using before. It's made from 100 percent recycled polyester synthetic down and cut extremely slim to fit underneath jackets, backpacks, and climbing harnesses. I wore this in multiple 30- to 40-degree trail races underneath my race vest and only noticed it because I did not die of cold at the start or finish lines. This is an extremely minimalist pullover—it only has a small chest pocket to use as a stuff sack—but you can also buy it in heavier weights or configurations, like a hoody.—Adrienne So

Arc'teryx's Delta Jacket is an ultralight fleece made of Octa Fleece, one of the newer, high-tech fleeces to hit the market in the past couple of years. The name comes from the octopus-like weaving which creates air gaps, which help trap warmth while also allowing moisture to escape. It's very popular with cottage industry ultralight brands, but Arc'teryx adopted it for the 2025 revamp of the company's popular Delta fleece jacket.

I hesitated to put this jacket in this guide because, while I like it as an ultralight mid layer for summer trips, there is an older, heavier version of the Delta jacket. Judging by the angry reviews on Arc'teryx's website, it was very popular. This jacket is something else entirely, but disgruntled fans of the older version aside, this is a great mid layer for summer trips, especially when paired with a sun shirt hoodie. At 8.5 oz for this, and 6 oz for a sun hoodie like the one from Kuiu below, you have a combo that allows multiple layering options and still weighs less than many more popular grid fleece options. My only gripe is that fashion was, um, not taken into consideration in the design of this jacket. One user review on the Arc'teryx site likens it to Mr. Roger's sweater, which is about right. But hey, it's light, it's functional—who cares about fashion in the backcountry?

Grid fleece is awesome. It's warm for the weight, and very breathable, making it great for highly aerobic activities. Despite all that, I prefer the comfort of merino, and often bring my merino mid layers backpacking even though there's a definite weight penalty (see below). Ridge Merino's Cloudripper jacket combines merino and grid fleece, offering the advantages of both, keeping the merino against your skin, but using grid fleece to improve ventilation and make the jacket much lighter. The men's large I've been testing weighs just 16.1 ounces on my scale. That's 4 ounces more than our top pick grid fleece, the Patagonia R1 Air hoodie, but I have found the Cloudripper to be considerably warmer than the Air.

The Cloudripper jackets (there's also a 1/2 zip version and a crew neck version) are made of two fabrics. The first is wool grid fleece (265 gsm) which is 69 percent merino, 15 percent recycled polyester, 13 percent nylon, and 3 percent spandex. The secondary fabric is a grid fleece(180 gsm) that's 51 percent recycled polyester, 41 percent Merino wool, and 8 percent lycra. The more traditional grid fleece with the higher polyester count is used mainly on the hood and lower back, while everything that would touch your skin (if you don't have on a base layer) is merino.

The Cloudripper is my go-to mid layer on the cool fall mornings, keeping me warm when I'm first starting out, and venting off the excess heat I'm generating once I get going. It also doesn't get stinky like my purely polyester grid fleece.

I've worn this lightweight merino pullover hoodie from Kuiu as a second base layer when it's really cold, a mid layer for summer trips, and as a sun shirt on high altitude hikes, which is pretty good for a 7-oz hoodie (men's large). It's a good jack-of-all-trades shirt, though it is definitely the lightest mid layer here. I would use it as a base layer in anything but summer (Kuiu classifies it as a base layer, but I often wear it with another layer underneath.)

The Kuiu Ultra 120 is made of Nuyarn, which is a blended weave of 70 percent merino and 30 percent nylon. Nuyarn strikes a good balance between the benefits of wool and the durability of synthetics. Pairing this with the Arc'teryx above makes a more flexible system than a single mid layer like the R1 Air, and weighs just 2 ounces more than that single jacket.

There are such a huge array of mid layers that each manufacturer has developed its own cult-like following. Probably the most famous of these small-batch home-grown brands is the Colorado-based Melanzana (which famously used to never sell clothes online, oh well), but my local cult brand is Portland, Oregon-based Senchi Designs. During the Covid-19 pandemic, musician, hiker, and nursing student Ryan Windus got his hands on Polartec Alpha Direct, which was invented in 2017. It's a startlingly warm, fuzzy, lightweight fabric that makes you look like a Muppet. Senchi's hoodies were an instant hit on r/Ultralight. My Merlin hoody weighs less than 6 ounces.

Senchi's are cool, oversized, and unisex, and double as fuzzy streetwear. Windus only makes a few items at any given time, and when he does post stock it sells out instantly. I suggest signing up for his email list to be notified. (He also occasionally does garage sales, if you follow him on Instagram or happen to be in Portland.) With all that said, after a year of wearing it constantly, I very rarely wear mine anymore. It's very fragile and has to be washed in its own special wash bag to prevent the fibers from falling off and getting microplastics all over the Earth. But Senchi is very IYKYK, which matters to some people, and it's so soft and fuzzy that it doubles as a pillow while traveling, which is nice.—Adrienne So