Wool=Base layer or insulation layer

I would see 320 as a mid layer, under which you could use a 180 merino base layer or the BPL merino such as Merino long sleeve shirt

I personally prefer merino base layers to synthetic but YMMV

When I was considering base layers I called customer service at Icebreaker. They suggested no more than a 200 weight base layer, particularly for bottoms, unless going into very frigid conditions and not moving around much. I concur now, since once in motion the 200 is very warm. I have a 320 top layer that I almost never use since I use wool only for the base.

Base layer merino, 140-200 weight. Better toward the lower end of that scale (prob 150-170). 300-ish is a good midlayer. My Icebreaker pullover hoody weighs something like 18 or 19 oz IIRC; my Patagonia Wool 4 hoody weighs about 13 oz.

Top off the 150-ish weight w/the 300-ish (260-320) weight, add a down vest (WM Flight, maybe Flash, FF Hyperion, Nun. Skaha) and a shell, you've got all the layers you'll need for 3-season use. I prefer for the base to be a zip-neck. Even in 80 degrees I just unzip the neck, push up the sleeves, and I'm good to go. If your back gets soaked (as would happen w/synthetic, too) just keep the shirt on once you dump the pack. Your body heat will dry it in ~20 minutes.

For weather in the (*F) teens, I find that a ~150g/m2 zip neck combined with a windbreaker and a thin hat is a good system. If you're dayhiking, you might start out with a thin fleece vest or something… but the reality is that you'll be "up to operating temps" after a mile or two. Gotta keep moving to keep warm; replace the down vest with a down parka for camp and longer rest breaks.

Reason I use merino and not synthetic is that I find the wool to regulate my temperature significantly better. Best way I can explain it, I find the wool actually does the things synthetics claim they can. Wool will keep you warm, but if you start to overheat it'll keep you cooler.

Weight-wise: Patagonia's Wool 2 zip weighs 6.9 oz; their Capilene 2 weighs 6.1. I never take that layer off over the course of a trip, and I'm much, much more comfortable in the wool. I'm okay w/0.8oz for the wool.

You may find the 320g weight great as a mid layer or it may be too heavy except for in camp, depending on how ‘hot’ you run and the outside temperature. Either way, I wouldn’t use it directly against the skin while active, as it would hold more cool sweat moisture than a thinner garment. That doesn’t make it a great base layer option, which should be designed to easily pass that moisture away from the skin and to the surface to evaporate.

I now use wool as my only base layer material and fully agree with Brad’s statement that “wool actually does the things synthetics claim they can.” …and without the smell. Brad, John and Roger have made many other good points as well.

In addition, I think you would find that two lighter wool garments layered would perform better as a “base layer” (really a base and mid layer) than a single, heavier wool garment for cold weather. I always use the lightest layer possible against my skin, and if necessary, add a second wool mid layer.

I find the heaviest wool base layers don’t wick as well as thinner garments when active and don’t provide enough insulation for their weight compared to down when at rest. I also find that I’ve rarely needed more than two light wool layers and a windshirt while backpacking (it must be quite cold and prolonged). YMMV

This is were a light wool base layer, a light or medium wool mid layer (if necessary) and down insulation layer really provide a great system. Add a windshirt and you can live comfortably with a 100 *F swing in temperature (single digits to +100 *F), as I have done with the following type of system:

~150-200g zip long sleeve base (always worn)
~200-300g long sleeve mid (worn only when well under freezing)
Windshirt (worn as needed)
Down jacket (worn at rest)

Again, YMMV. If it were going to be predominantly cold or hot, I would adjust accordingly. But, the above example is extremely versatile for a vast range of temperatures.

Maybe try some day trips in your anticipated conditions to test what your body is comfortable with while active and at rest. Everyone will be a little different when it comes to cold weather comfort.

Roger made a really good point… I'd never wear a 300-ish weight as a baselayer. Way too hot once you're moving. The reason I carry that piece is for moving around camp… end of the day, still a bit of sun, setting up camp (or tearing down the next day) the down layer would just be too warm. I could ditch the midlayer and be perfectly warm with the base and the down, but I like the mid for the layering flexibility.

Overall, I doubt you'd ever wear the two layers while actually hiking. Maybe in 0*F temp range under the windbreaker? But as Roger pointed out, we all metabolize a bit differently, too…

My first modern merino wool piece was a 320 weight Icebreaker zip tee turtleneck. It's a total bust for hiking — way too hot to hike in, and not warm enough around camp. I have since purchased 150 weight base layers from Ibex and Icebreaker, and wear them all year long.

The 320 top is useful for sedentary or moderate activities in very cold weather — I wear it for winter birding trips, for example.