Which Baselayer Is Best? — Merino Wool vs Synthetics

Author: Sam Brilleman

They go by many names… baselayers, next-to-skin layers, thermal underwear, long johns. The list goes on.

Regardless of what you call them, your next-to-skin clothing layers are vital for ensuring your backpacking trip is safe and comfortable. A poor choice of baselayer can lead to chaffing or rubbing when you sweat. Even worse, it can lead to hypothermia if you get wet whilst wearing a fabric that won’t insulate.

Fabrics like cotton — that are great for wearing to the local cafe or to work — are often not suitable for use in an outdoor or wilderness environment. On the other hand, materials designed for active use like merino wool or synthetics are ideal for such purposes. But sometimes it isn’t obvious which one is best.

In this article we describe and compare the most popular materials used for hiking baselayers and thermal underwear — merino wool and synthetics. We discuss the pros and cons of each of these materials and try to help guide you towards the perfect choice of baselayer for your next outdoor adventure.

Leave your cotton at home

First up note that cotton is a no-go area for hiking underwear! This is because cotton is an absolute front-runner when it comes to fabrics that can absorb the most water.

In fact cotton can absorb around 20 times its own weight in water. And once wet, it becomes heavy, takes forever to dry and no longer insulates. This is because the tiny air pockets in the cotton fibres — that would usually insulate — are now water logged and remain that way for a long time.

Cottons high-absorption properties make it a great choice for luxury bath towels, but it is a terrible material to have against your skin when doing active exercise (where you will be sweating!) or in conditions that may be cold, damp or wet (think mountains!).

So now that we know that we’ll be leaving our favourite cotton undies in the drawer back at home, what do we wear?

Hiking underwear are generally made from either merino wool or a synthetic fibre such as nylon or polyester.

The better alternative: Merino Wool?

Merino wool is a natural fibre, obtained courtesy of the merino sheep.

And merino wool is epic!

It is highly breathable, insulates even when wet, is quick drying, soft to the touch, and is naturally odour-resistant. This makes it one of the perfect materials for hiking clothing. There is a reason it is so popular in the outdoor clothing industry.

When people first hear about merino, there are generally two common concerns.

The first concern: It’s wool! Won’t it be itchy?

Yes, it is wool.

But no, it won’t be itchy.

The merino sheep is a very special sheep. It’s ultra-fine wool is so soft that it just isn’t itchy to the human touch. It is much softer than most synthetic materials, and perhaps almost as soft as that cotton t-shirt sitting in your wardrobe back at home.

The second concern: It’s wool! Won’t it be too warm for summer?

The short answer is no.

A benefit of merino is that it can be wound into such thin fibres that lightweight garments like underwear and t-shirts are highly breathable and just as perfect for summer and warm weather use at they are for winter.

Merino fabric weights

If you are planning to use merino underwear — or even merino singlets or t-shirts for that matter — in a warm climate, then look for merino rated at around 150 gsm (that’s grams per square metre) or less, and definitely not heavier than 200 gsm.

At Ultimate Gear Lists we always opt for 150 gsm merino underwear (like the Smartwool Merino 150 Boxer Briefs) and a 150 gsm singlet or t-shirt (like the Smartwool Merino 150 T-Shirt) regardless of the weather conditions.

Then if we get cold we can layer a heavier 250 gsm zip-up (like the to Smartwool Merino 250 Quarter-Zip Top) over the t-shirt and vent using the zip as needed.

The Downsides of Merino Wool

So it’s obvious that we think merino wool is great. And at Ultimate Gear Lists merino wool is hands down our favourite material for hiking baselayers.

But its not perfect.

The main downsides of merino are that it is quite expensive, and it isn’t quite as durable as most synthetic materials.

If you are only using your hiking gear on backpacking trips and not daily, then merino is easily durable enough to stand the test of time. What’s more is that many manufacturers are now blending merino with a nylon core to increase durability.

Nonetheless synthetics are a cheaper and more durable alternative to merino wool. If you are stocking up on underwear or you’re on a tight undie-budget then synthetic underwear is probably the way to go. This usually means nylon, polyester, or a combination of both…

The upsides of synthetics

Synthetic underwear do have good moisture-wicking properties that will help move sweat away from your body, to the outside of the garment where it can evaporate.

Most synthetic fibres don’t absorb water either — that is they are hydrophobic — so they don’t get particularly heavy when wet and they dry fast. Most synthetic underwear will dry a little bit fast than lightweight merino, but not by much.

Because synthetics dry slightly faster than merino, many people claim that they are better for situations in which you will be wet or sweating profusely for prolonged periods.

But we don’t believe this to be the case. Yes, synthetics dry faster than merino. But no, they aren’t more comfortable when damp.

In fact, merino fibres have the amazing property of being able to hold a significant amount of water while still feeling dry to the touch. This means you will often feel more comfortable in damp merino than you will in damp synthetic, even though the merino wool might retain some of the moisture for longer (i.e. take longer to dry).

Odour-resistance

One last point. Stink factors.

Merino is naturally odour-resistant.

We can again thank the merino sheep for that. The merino sheep’s glands produce a natural wax called lanolin. It lines the fibres and is naturally anti-bacterial, so it will stop the material’s fibres from becoming the home to odorous bacteria.

Unfortunately, synthetics can’t claim the same fame.

Some manufacturer’s try to treat synthetic fibres with an anti-microbial coating. However, it doesn’t usually live up to it’s promises, and will fade after repeated washing anyway.

So unfortunately in the smell challenge, those synthetic undies aren’t likely to take home any trophies.

Take me to the products

Are you looking to find the best merino wool or synthetic baselayers? Then check out some of these super-duper relevant review articles:

• Best Men’s Hiking Underwear

• Best Women’s Hiking Underwear

• Best Men’s Hiking Shirts

Or check out some of our other backpacking related tips:

• Ultimate Guide to Dehydrated Backpacking Meals

• Hiking vs Trekking vs Backpacking — WTF is the difference?

• Ultimate Backpacking Gear List

• Goose Down vs Duck Down — WTF is the Difference?

Or get inspired by our hiking Destination Guides.

Whatever you decide, happy hiking and take care out there in the wild!