It’s all about the tightness of the weave. Wool sweaters are loose woven
Felted wool is actually water resistant
I have some icebreaker wool items that dry very quickly
Much faster than fleece.
Same for cotton. We’ve all heard the old saw cotton kills. Some fine denier and tight woven twills dry very quickly unlike coarse diameter Jersey as in t shirts
Wool also has lanolin oil in it, which repels water. I treat my wool with a product that restores the lanolin content.
One attraction to wool is its sustainability, low impact manufacturing. There’s just something intrinsically pleasing about wearing a quality handmade product that doesn’t rely on the petroleum industry and won’t clutter the landscape for a thousand years. So few outdoor products are that way anymore. Guess I’ll stuff my wool sweater in my canvas pack and move on with my wood paddle. 
Wool socks, fleece liner.
@Rookie said:
A backpacker had the same question: http://woodtrekker.blogspot.com/2012/11/fleece-vs-wool-insulation.html
Hey Rookie, the results for the wool vs fleece look so close as to be suspicious. The same site shows results for cotton which are basically identical to wool or fleece. The test and measurement system are not validated. Hmmm.
The link below shows a study from 75 years ago. Figure 16 shows a clear benefit of pure wool over pure cotton but the perhaps surprising overall conclusion (page 28) is that the biggest factor for insulation of wet fabrics is the surface of the fabric and how well it makes contact with your skin. So maybe the itchiness of wool is helping to keep you warm.
https://nvlpubs.nist.gov/nistpubs/jres/32/jresv32n5p229_A1b.pdf
I like my synthetics since they dry out so fast, pack small, don’t itch, and in my experience they will keep me much warmer than cotton if soaked. But now I feel more guilt since castoff is educating us about microfibers from washing synthetic clothes.
Water repellency, sustainability, and fashion are of course totally separate discussions so I suggest a good flogging for Loon Watcher for raising such a broad topic.
Went back to that site and read a bit more. Most interesting was the first comment written five years ago about fleece fiber pollution in the oceans. Patagonia explains how to reduce fiber pollution; https://www.patagonia.com/blog/2017/06/what-you-can-do-about-microfiber-pollution/
Will be hiking along the Lake Michigan shoreline in a bit, around Sturgeon Bay. Definitely wearing wool, winter gloves and hat, and my big old thick LL Bean down parka. Warmth is preferable over fashion.
Picked up Patagucci’s Nano-Air vest a couple years ago…best purchase made in a while. Breathes while maintaining a comfortable temperature for me, with their Capilene baselayers of various thicknesses, no matter what the conditions.
Wool doesn’t have to itch
Some wools are smooth like alpaca and merino
Some are sheep wool that is unprocessed and still has the little barbs in it.
That’s used for felting. Turns out thick and the barbs interlock
Just as in Kevlar there are types of wool
Went back to that site and read a bit more. Most interesting was the first comment written five years ago about fleece fiber pollution in the oceans. Patagonia explains how to reduce fiber pollution; https://www.patagonia.com/blog/2017/06/what-you-can-do-about-microfiber-pollution/
Will be hiking along the Lake Michigan shoreline in a bit, around Sturgeon Bay. Definitely wearing wool, winter gloves and hat, and my big old thick LL Bean down parka. Warmth is preferable over fashion.
It’s a cool site. Being an engineer I got excited by the data and graphs, then got curious about cotton. On the page with the graphs of cotton performance there is a long comment from someone challenging the validity of the test method. Probably another engineer.
I use lightweight cotton dress shirts in the summer, lots of synthetics that I use all the time, and some wool stuff that I use on occasion including an army surplus blanket that is often in my “ready” pack. It does seem like wool socks are hotter than anything short of heated socks. It’s all good.
Wool, fleece, down and some rain gear or Filsons.
Synthetic materials can really smell after awhile in the bush. Especially if the the days are warm or if you are hiking hard like working or hunting. I would rather smell like a wet sheep than a rank human any day.
Fleece is made of petrochemicals and eventually will become microplastics in the ocean while wool is made of, well, wool, and eventually will be eaten by moths.
Fleece seems marginally lighter though not much more packable. I think part of the push for fleece was higher profit margins. Icebreaker is pretty great, but too expensive for my taste. Costco had some wool tops for $20, and Taiga makes stuff in Canada that’s quite nice.
I got a merino wool sweater from Duluth trading that is better than other loosely woven, poor yarn ones costing twice as much.
Oh sure ! I’ll jump in on this. Wool. No contest, not even close. Now wait, there are many different wools. I personally detest ye ole, itchy wool sweater. Ah, but warmth? I wear a cashmere sweater to bed on all fall/winter/spring canoe outings. A merino wool shirt or even long johns, for an early spring 200 to 500 miles? Outside entirely for April and May? Wool, Merino wool, lambs wool, cashmere, and for socks, there ain’t no contest, winter, spring, summer, fall,Bison down socks, or Bison down/yak, socks, bison down/merino wool socks. I paddle a canoe, in rapids I am on my knees, often for hours. I do not care how good your spray deck is, (Mine is wonderful.) or skirt, or what you are wearing, your knees and feet are soaked. What do you have to have on board, 2 quarts, 3 quarts, your feet or knees are soaked. Period. The entire day. Wool will keep you warm even if dripping wet. Only a diver’s wet suit can make the same claim.
Wool… But why wear, or compare the stinky nasty, itchy stuff. Spend a couple of bucks, Merino wool, cashmere, bison down. Fleece,I have few kind words to say about fleece. It works great if your trip is a day trip, or even an overnighter, a dry overnighter. Gonna spend a week or a month out? Use wool.
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I spent part of 2 years working in SE Alaska with 150 inches of rain. We waded rivers all the time. If it wasn’t raining the brush was wet. I learned to really appreciate wool garments. I gave up on rain gear entirely and went of Filson tin cloth which could take the brush without ripping, and was not slippery if I fell on a steep slope. I started every morning even in July and August with a suit of Stanfield wool long underwear made in Canada.
When wielding my axe I only wear wool.
Wool is the only material I know of that can get wet and maintain a decent amount of its thermal properties. In many situations this is not trivial. It probably saved my life, or at least kept me out of the ER, once in my younger days.
But if it makes you itch then fleece wins. The only place I can have wool against my skin w/o the itch is a my feet. Even alpaca and merino doesn’t make it for a top without a layer between.
I think someone forgot about neoprene and the various fuzzy rubber types of clothing. 
Neoprene is fine around the water. On land it is impossibly hot and sweaty and clammy.
It’s been in the high 80s, low 90s here. Don’t even mention wool or fleece. 
In the mountains, frost and snow are common in any month. Years ago it was 107 in Reno for many days in a row. It was record setting. I called two friends and we hiked up to Bishop Pass in the Sierra. We had frost every night at 10,000 feet. Last year I was hiking near my house up by Carson Pass, on August 20 it was 23 degrees one morning.
Another thing about wool is that it is self-extinguishing when hit by sparks from fire. Synthetics tend to melt, and some (like polypro) will actually burn through and stick to your skin. During the years I worked construction in the winter I used to buy Army and Navy surplus wool pants and shirts and re-tailor them to fit me. Wore like iron and I was never cold, plus I never had to worry about welding sparks setting my clothes on fire. I wear wool socks year round – the only fiber that keeps its loft and cushioning when totally wet. I still have a lot of vintage wool stuff including my Woolrich Alaskan shirt-jac from the 1970’s, but the newer washable wools are an excellent upgrade. I have even paddled in cold water using dense 100% natural wool Dachstein knit gloves.
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