Introduction
A base layer is like long underwear, pajamas or a sweat suit. It fills a number of rolls. This essay talks about materials, weights and uses because you need to consider all three in choosing a base layer.
Uses
The most common use for a base layer is to sleep in. It adds warmth to your sleep system and keeps your sleeping bag cleaner. It is a lot easier to wash than a sleeping bag and is warmer per ounce than a sleeping bag liner.
Base layers are also used as long underwear —to wick sweat and to add warmth. That means you will find them being used in a wide range of environments and sometimes as a sole item of clothing (like a long sleeve t-shirt would be) or in connection with another item (many people wear their base layer bottoms under their shorts rather than having pants).
Materials
The most common base layer material is merino wool.
The most traditional material is silk.
More and more synthetic base layers are being sold and used.
Originally nylon and nylon/spandex were popular. They make for very inexpensive base layers. Costco sells the 32 degree ones in bulk.
Polypropylene became very popular for a while. It is very hydrophobic which means it wicks sweat away from the skin and stays dry. The problem is that it retains odor and starts to stink if not cleaned with mild acids (like vinegar) or bleach.
It has been replaced in many applications with polyester resulting in Capilene and Polartec Silkweight (a military base layer).
Returning to merino wool. It is often 100% or with 10% nylon and a touch of spandex and has grown to dominate the market as it is warm, does not retain odors and takes a variety of colors well.
It does hold moisture.
Weights
Base layers come in different weights from barely there (a “50”) to substantial (300+).
To quote New Zealand
Weight:
Warmth Factor
120- 150g. — Featherweight for warm to hot conditions
150-170g. — Ultralight to wear all year round
170-200g. — Lightweight- still ok to wear all year round but with more warmth
200-300g. — Mid weight- good for the cooler months
300g and up. — Heavy weight for the coldest months
My personal experience
I started with nylon base layers on Appalachian Trail section hikes and used them to sleep in. I used an Underarmor top baselayer to hike in. it did a great job of wicking sweat and helping me stay cooler and dryer.
I like the nylon enough I tend to wear the long sleeve t-shirts from 32 degrees for daily wear in cooler months and to sleep in.
My wife bought me some Capilene bottoms which I paired with a wool top for cooler camping weather.
On the Appalachian Trail I picked up some wool bottoms. Here I am hiking through the Notch using them and hiking shorts. I have painters gloves for the grip and my then favorite fleece over a wool t-shirt.
On the PCT I took Polartec Silkweight top and bottoms. They are great but as it got warmer I sent them home and started wearing a sun hoodie as my top layer and my pants as a bottom layer to sleep in while hiking in my shorts.
For an April start on the PCT next year I’ll probably take the Silkweights at least through the Sierras.
In a lot of places a baselayer duplicates what you might use a fleece and long pants for for warmth. Once I started sleeping in wet gear on the AT to dry it out with body heat while I slept I started not using a sleep layer as much.
Other notes
A lot of people don’t bring both shorts and pants. They just have a pair of shorts they wear with their base layer bottoms or their rain/wind pants.
For warm weather the number and weight of layers is much different than for colder weather.
Many people wear a sun hoody regardless of the weather. In summer it wicks away sweat. In cooler temperatures it provides some protection against wind. 24-7 use means it also takes the place of a base layer top while sleeping.
There isn’t a single answer on baselayers. Your preferences and experiences will change how you use a base layer and what you prefer and there is a wide variety of good choices people make differently from each other.