Fleece, wool, down, synthetic down: mid layers keep you warm, but which is best for you?

*A warm puffy and hot cuppa on a chilly morning near Mt Jagungal (Image Credit: Kylie Bevan)*

In Part One, we considered what kinds of clothes are best for hiking, and why the most expensive items are not necessarily the best. By clever shopping and reading labels you can find suitable clothing in all kinds of shops, not just outdoor ones: bargains online, in second hand shops and in KMarts. When you understand the foundational principles of the layering system and different materials, you hold the key to choosing correct clothing.

In Part Two, we considered base layers: the properties of different natural and synthetic materials, knits and weaves, plus design and sustainability aspects.

This series of articles is designed to be read together, which each building on the last. We highly recommend that, if you haven’t yet done so, you read at least Part Two now, because the characteristics of materials — nylon, polyester, tencel, different animal fibres — and fabric weights (GSM or oz/yd, microns etc) carry over into mid layers. Understanding these properties helps you to match clothing to conditions.

This Part Three explains how to choose the right mid/insulation layer, covering a wide range and the pros and cons of each to match your climate, hiking style and budget.

This article covers:

1. The Layering System: A Recap

2. Principles of Mid Layer Insulation

3. Base Layer or Mid Layer?

4. Weight and Packability

5. Materials

Wools
Polyester
Down
Synthetic Down

6. Design Considerations

The Layering System: A Recap

On the Australian Alps Walking Trail with a polyester mid layer over a base layer (Image Credit: Kylie Bevan)

Multi-function jackets are usually heavy, insulated, and are supposedly “waterproof.” This one jacket, you think, will do everything! Keep you warm, dry, and it’s just ONE item!

But that very sales pitch – usually aimed at city commuters – is exactly what makes these jackets terrible for hiking. They are perfect when you get up in the morning but, as soon as you begin walking with a pack, they’re too hot. When it rains, water wets the padded insulation. Or, the jacket doesn’t leak but it doesn’t breathe either, so becomes leaden with sweat.

The layering system is considered current best practice for outdoor activities like hiking. It varies somewhat between activity type, but the principle is a modular system of clothing that you mix and match to suit conditions. Unlike a single jacket, the system comprises a minimum of three (but up to five) layers that keep you cool or warm and/or dry as need be to maintain your normal body heat: shedding it during exertion, and preserving it when temperatures drop. You mix and match these thermo-regulating layers to suit different climates or changing weather throughout the day.

The system is based on three layers:

Base
Insulation/Mid and
Shell.

*Mid layers around camp: two polyester fleeces, one down jacket, and one synthetic jacket.*

Closest to your skin is the base layer, either long-sleeved or short sleeved, plus optional long bottoms to wear under or instead of pants/shorts. It is usually – but not always – relatively form-fitting, and its primary aim is to move moisture away from your skin to keep you dry from sweat, and hence warmer in cold weather.

The next layer, the topic of this post, is your insulation or mid layer. The primary aim of this layer is to keep you warm by trapping air and blocking wind. It also breathes well, so that the moisture that has passed through the base layer continues without condensing or soaking in. Some long sleeved, thicker, warmer layers can be worn either against the skin as a base layer, or over a base layer to become a mid/insulation layer.

The outer layer is the shell. A shell is waterproof and almost always windproof. The purpose of the shell is to keep you dry from rain, snow or sleet, and usually also to prevent wind chill.

These three modular layers can be mixed and matched for most conditions while you are on the move. Wear just the base layer if it’s warm. Add the mid layer if it’s cool. Add the shell if it’s cold, or raining. If you’re hiking in rain but it’s not cold, wear the base layer and shell without the insulation. You can immediately see how these three layers are more flexible than a single thick insulated jacket worn with a base layer.

Many people carry a fourth layer, or use a combination for their insulation/base layers – ultralight down puffies and wind shirts are two popular additions or alternatives.

*Geoff’s trusty fleece is enough for him in subalpine cool. I’m wearing a rain shell over mine for extra warmth.*

Principles of Mid Layer Insulation

Air is an excellent insulator, slowing the loss of heat from your skin. This might seem counterintuitive when we layer up to stay warm, but it is the pockets of air within the material, rather than the material itself, that slow the transmission of heat: imagine how cold your sleeping bag would be without its loft! The air is trapped within the layer, be it thick homespun wool, down-filled baffles of a puffy jacket, or plush or grid-like raised patterns in polyester fleece. Even materials such as mesh that are more air than material trap warmth, provided the air is truly trapped rather than being wafted away by the slightest breeze.

Some animal fibres are famously light and warm: like the quills of bird feathers, polar bear and possum hairs are hollow, reducing weight and increasing their insulative properties. Many animals boast a double layer of protection: fine down to trap air closest to the skin, with an outer layer of coarse, more robust protective hair or feathers; some insulation clothing mimics this with a tough outer layer with a brushed, fluffy inner surface. And although sheeps’ fleeces are similarly consistent throughout, they’re so oily and dense that neither air nor rain penetrate.

So always remember that it is not the material per se that is warm, but the air it is able to trap. Materials that trap the same amount of air are usually similarly warm, so any difference in weight depends on the nature of the fibre, the number of fibres per square metre that are necessary to trap that given quantity of air, and how well that warm air is trapped. For mid layers, look for materials that have a high warmth to weight ratio.

Base layer or Mid Layer?

We are each wearing two base layers, with my merino long-sleeved crew warm enough to act as a mid layer over a merino tee in mild weather. Geoff has a lighter weight long-sleeved merino crew under his usual base layer tee - like many men, he always wears a long sleeved top under a short sleeved one for no reason I can discern.

As we saw in Part Two, Base Layers, these are worn against your skin and their main purpose is to keep you dry from sweat rather than warm, although ‘thermal’ base layers have the added advantage of keeping you warm whilst doing so. However, at a certain point, base layers become thick and warm enough to be an insulating mid layer when they are worn over another base layer. For example, in warm climates, a merino thermal of only 200gsm might be sufficient as an insulating mid layer. Heavier weight merino — 300+gsm — is more mid layer than base layer in Australia though, as we’ll see, exceptions exist. Conversely, a medium to light weight microfleece such as the Mont Powerdry long sleeved crew shirt also makes an excellent base layer under a mid layer in alpine conditions, so stay open to using the same top in different ways on different hikes rather than buying a new one.

Weight and Packability

*Left to right:*
*- Macpac Nitro 90gsm Alpha Direct hoodie 115g/4.1oz (add wind shirt 47g/1.7oz to total 162g/5.7oz)*
*- Ghost Whisperer hoodless down jacket packed into its own pocket 203g/7.2oz*
*- Helly Hansen hoodless fleece 240g/8.5oz*
*- Hand knit pure wool cable jumper 810g/28.6oz*
*These aren’t like for like, of course, but give ballpark figures of what you might expect.*

On day hikes when you don’t need to carry much other than lunch, first aid kit, PLB and rain shell, clothing weight doesn’t matter, and you’ve plenty of space even in a day pack. However, on a multiday hike when you’re also carrying shelter, kitchen, bedding and food, every gram counts. A woollen jumper may be as warm as a polartec fleece, but is many times heavier especially when damp, and it is just one item of clothing that you’ll likely carry as much of the time as you’re wearing it. And the difference in weight between a thin short-sleeved base layer polyester and merino tee is less significant than for much larger, thicker mid layer garments.

Not only that, but conventional woollen knits are bulky and minimally compressible, taking up a lot of space; clothing that is equally warm with less bulk or more compressibility is generally preferable. Having said that, many old school hikers in damp climates such as in Scandinavian countries or the UK still swear by wool for its ability to generate warmth even when wet (heat of adsorption), and modern factory knits are more compact than hand knits.

Mid Layer Materials

Wool

Although less popular today, wools have been used for centuries and there is still a place for them as insulation layers for multiday hiking, especially short ones. However, pure wool knit is more expensive than polyester fleece, and even more expensive when you start considering the higher warmth to weight ones such as alpaca and yak (cashmere is too delicate for backpacking). Again, because insulation layers are generally bulkier and heavier than their base layer counterparts, ones with better warmth:weight ratios make a bigger difference in lightening your pack. Buy from secondhand shops in posh suburbs or join mailing lists for sales or promotions: our Arms of Andes alpaca mid layers are warmer than our equivalent merino weight ones, but eye-wateringly expensive at full price; we bought them at discount.

Although wool stays warm when wet — up to 30% of its weight in water — it therefore dries much more slowly than polyester fleece, and this may be relevant depending on the length of your hike and your climate. A mid layer should be protected from rainy weather by your shell wear, but can absorb a lot of moisture from sweat in humid environments; conversely, some sweatier hikers prefer wool mid layers to synthetic ones for this very reason.

We prefer to use merino as a baselayer, and synthetics as insulation layers for hiking; YMMV.

Wool Fleece

As well as conventional knits, wool is available in a brushed inner ribbed knit, trapping extra air in a similar way to a polyester fleece. This form increases its warmth to weight ratio but it is still relatively heavy, with a heavyweight Icebreaker Realfleece TM Elemental full zip jacket almost 0.5kg/10oz for the women’s size small, much less than my 815g/29oz hand knitted woollen jumper. When you consider that today’s ultralight one person tents weigh around 450g/16oz, these weights are still sigificant!

However, Realfleece is available in lighter —but still not extremely light — weights. Note that women’s medium weight jackets are often 80-100g lighter than men’s because the former are shorter in the torso and narrower across the shoulders. These are weights for different knit weight full zip models in women’s small; quarter zips are lighter:

Other wool and wool blend fleece include XTM’s Aerogrid, Mons Royale Offgrid, and Ortovox.

Wool Down

Merinoloft TM is Icebreaker’s woollen alternative to down. It is warm for weight and, unlike down, machine washable; anecdotal reports suggest it maintains loft well through multiple cycles. However, it is extremely expensive (AUD540 at time of writing) and, at 500g/17.7oz in the women’s small, much heavier than conventional down. In our opinion, it’s stylish and sustainable but not an ideal insulation layer for multiday backpacking. If you’re concerned about animal welfare and bird down, choose brands with responsibly sourced RDS down.

Polyester Fleece

The good ol’ polyester fleece quarter or full zip jacket has become the most popular insulation layer for multiday hiking in Oz. This is because it is often — but not always! — relatively cheap (see Part 1), widely available, durable and warm for its weight — warmer than wool knit. It absorbs very little moisture and dries fast. Polartec is the most famous brand but you can choose others of comparable weight, thickness and quality: cheap versions may simply pill faster.

Weight/GSM

Polyester fleece comes in different weights. A bit like Goldilock’s three chairs, some of the lightest microfleece can be worn as base layers but are insufficiently warm for insulation; heavy ones might be good for sitting in camp but too hot for multiday hiking in most climates, whereas others are just right. Of course, ‘just right’ depends on climate, hiking style/speed/terrain and personal metabolism!

Helly Hansen writes:

“The higher the gsm [grams per square metre], the warmer the fleece.

<200gsm: Lightweight, performance fleece. Ideal for more active days - such as trail running or more challenging climbs - and layering with other jumpers or jackets. You may see this fleece referred to as microfleece due to its thinner fibres.
200-300gsm: Good all-round fleece that provides you with warmth, but is not too thick. This is a favourite weight for many hikers.
>300gsm: For the coldest days. Our warmest fleece are over 500gsm and will keep you warm in the harshest of conditions and climates.”

However, although this might apply within a brand, when comparing between brands, fabric thickness is more relevant than weight alone. A thicker material of lighter weight is warmer than a thinner one of heavier weight, because the former traps more air. If choosing a fleece jacket in a shop, feel for the one that is thickest for its weight. Pick up a whole lot at around the weight you want, then pinch them gently to compare thickness.

*Early heavy days on the Tour Mont Blanc. Geoff’s overkill fleece without a hood weighs an astonishing 680g/24oz!*

In our opinion, for Australian conditions a medium weight standard fleece jacket weighing around 240-290g/8.8-10.2oz for a medium size women’s full zip (about 100g/3.5oz more for a men’s, less for both with quarter zips) is a reasonable all-round solution for most conditions in most seasons; YMMV, so you may prefer heavier if you’re cold-blooded, hiking slowly, or in alpine, cold/exposed, and/or technical terrain. Remember too that you can change base layers — a long-sleeved, heavier/lighter or mesh thermal instead of a tee, for example — to tweak temperature further. And you can add your shell layer for warmth as well as protection from the elements. Conversely, if you’re super hot-blooded or hiking in the tropics, a microfleece might suit you best.

For many years, Geoff and I carried heavy standard fleece jackets but these were overkill, taking up a lot of space and weighing down our packs when we weren’t wearing them. If we needed more warmth, it was far more effective to wear a rain shell on top, which we were carrying anyway, so we switched to midweight fleece. Since then, Geoff has worn the same Berghaus full zip fleece for hiking for eight years and it is still going strong functionally, though it has lost a lot of its shape! The zip of my red fleece of a different brand failed after six years so I’m now wearing a plain green Helly Hansen bought for less than half price on sale at Anaconda. And of course, as we’ve seen in Part One, countless KMart and Woollies cheap fleece offerings are perfectly acceptable. Check for thickness!

Sustainability and Environmental Impacts

When laundered, polyester fleece releases microfibres that are damaging to the environment, particularly marine and aquatic life. This release can be significantly minimised (described in Part Two, Base Layers) but, for those concerned about sustainability, choose alternative materials such as synthetic down, with the polyester fibres safely sandwiched between facing materials, or wools.

Different Polyester Fabrics

As well as different fabric weights, polyester fleece is available in a range of fabrics. Some are arranged into into mesh or open knits of varying degrees. The most famous of these is Polartec Alpha Direct, an active mid layer. If you sew, search out patterns online.

Other polyester fleece is formed into grid or ribs with raised patterns on the inside rather than a uniform pile on the outside, which is usually a face fabric of varying weight and windproofness; you’ll find countless proprietary ones. The gaps between the raised patterns trap air, making for a material that’s lighter for the same thickness. Although they have a higher warmth:weight ratio, these fleeces are also often thinner than conventional fleece and therefore rarely as warm: choose carefully. Tejin’s Octayarn is one of the most well known in ultralight circles.

Alpha Direct

*Geoff in his Macpac Nitro hoody, far cheaper for Aussies than US-made equivalents.*

Alpha Direct, a polyester active mid layer, is flavour of the month and the material used by Senchi, Macpac, Timmermade, Farpointe, Sambob, Lightheart Gear and others. Unlike the original Alpha material that needs a facing layer, Alpha Direct does not. It’s available in 60GSM, 90, 100GSM and 120-190GSM, with the midweight 90GSM-100GSM ideally suited to Australia as a mid layer; the lightest weight is sometimes used as a base layer. Many hikers will find the heaviest weights too warm in all but the coldest climates, or when moving slowly in technical terrain.

Active base layers are not like standard fleece, designed to keep you warm when stationary. Instead, unlike standard fleece that gets too hot when you start walking, they are temperature buffering: not too warm and not too cold, moderating our temperature so that you dont need to keep doffing and donning your pack and layers as you start to hike and warm up. Alpha Direct does away with the tightly woven base of fleece and leaves just the loops and brushed fluff. Most designs have minimal cuffs and hems and no zips to maximise the warmth:weight ratio. The material dries incredibly fast and is insanely warm and light — close to half the weight of conventional fleece — such that there is a mental disconnect between what you expect when you put it on, and the amount of heat it holds… in still conditions.

Because of course, the slightest breeze wafts away that heated air, so on its own it is a poor insulation layer. Instead, team it with a windproof shell, either your waterproof rain shell or a wind shirt, and it comes into its own.

*Zpacks 47g wind shirt (Image Credit: zpacks)*

**Wind Shirts**
When paired with zpacks 47g/1.7oz wind shirt, AD is still only two thirds the weight of a medium standard fleece, and almost as warm depending on GSM of either. It’s warmer than the micro fleece jackets I own, and it’s lighter even than my medium women’s 203g/7.2oz Mountain Hardwear Ghost Whisperer down jacket, but this time considerably less warm.
Timmermade also makes wind shirts, including a more breathable one for very high output activities (eg fastpacking). Larger manufacturers also make slightly heavier wind shirts, such as Patagonia’s Houdini or, at the other end of the spectrum for the budget conscious, the equally famous Dooy windbreaker.
A wind shirt — acting as a dry weather shell — incorporated into your layering mix can be worn over any base layer, over any base layer plus any non-windproof fleece, or under any rain shell. However, if you already carry an ultralight breathable rain shell such as Enlightened Equipment’s Visp or zpack’s Vertice, a wind shirt is superflous — just use the rain shell instead.

Alpha Direct is not without its drawbacks. It snags easily — a jagged fingernail pulled a thread the first time I put my arms through the sleeves — and is useless in scrubby terrain or bushbashing unless worn under a more resistant layer. Velcro, burrs and Australia’s spiky dry leaves are a nightmare to distangle. Pick your trails!

Nor is AD environmentally friendly, because its loose, brushed knit releases more microfibres when laundering; always use a fine wash bag and hang to dry. Fortunately, the material is remarkably resistant to stink unless worn under a non-breathable shell, and many hikers launder their AD mid layer garments with the same frequency as their shell jackets, monthly or seasonally rather than weekly. Be guided by your nose!

*MacPac Nitro women’s hoodie: no chest pocket so even lighter than the men’s model! (Image Credit: Macpac)*

In Australia, your cheapest AD option at time of writing is Macpac’s Nitro hoodie. The women’s model lacks the chest pocket of the men’s model but for once this is an advantage and saves weight in a garment whose prime purpose is a high warmth:weight ratio. The famous (or infamous) unobtanium Senchi hoodie is an eye-watering AUD270, or USD178 plus international postage at time of writing.
Mac Pac’s Nitro hoodie, without a quarter zip but of the exact same 90gsm Alpha Direct material, is AUD179/USD118 full price, AUD108/USD71 member price, but I bought Geoff’s at a clearance price of AUD89/USD59, with the women’s still available in small to medium sizes at that price at time of writing. Check in regularly for sales or get on their mailing list: it’s still a lot to pay for fleece!
I usually disregard colour when buying clearance clothing — colour is often the reason it’s remaindered! — but this highly impractical pale blue would require more frequent laundering than ideal.
Some manufacturers join AD to a facing material for you. MacPac’s 90GSM AD Pisa has a ripstop nylon outer but weighs 380g in a men’s medium, negating much of AD’s advantages. Timmermade’s 226g/8oz Alpha Shell jacket is customisable for both shell denier and AD GSM, (tradeoffs in weight, warmth, breathability and robustness) with a commensurate price tag. However, in our opinion, keeping wind and alpha layers separate is far more versatile.

Also in our opinion, for Australian conditions, AD plus windshirt is ideally suited to

day hikes, fastpacks, and single overnighters with a trusted forecast
open country or manicured trail
on hikes where you expect to wear just a tee or base layer most of the time, but carry an insulation layer in case of cool evenings or exposed ridgelines
3 season conditions or reliably warm conditions when you need only a light rain shell.

My ultralight Visp shell plus AD hoody total an impressively tiny 278g/9.8oz, for a versatile layering system over a synthetic or merino base. We admittedly don’t yet have extensive experience with AD, but I would not trust this combination (or wind shirt plus AD) on long multiday hikes where we could experience extended rain and scrubby trail primarily because none of them is sufficiently robust either alone or in combination. And as a slower hiker I would almost certainly get cold; Geoff might not and YMMV too. We will be trying them on upcoming shorter hikes first.

Octa Fleece

*Hollow, eight-armed fibre in cross section (Image Credit: Tejin)*

Tejin’s Octa, used in Northface’s Futurefleece (different GSMs), Mountain Hardwear’s 80gsm Airmesh, zpacks Octa (100 gsm), Arcteryx’s Delta and Patagonia’s Airfleece (238g/8.4oz women’s small), is another interesting material, with hollow fibres that have eight arms in cross section, so they are both very light, about 50% lighter than regular fleece of similar thickness, and extremely effective at trapping air.

*Tejin Octafleece inner (Image Credit: Tejin Octa).*

The brushed back mesh inner is also arranged into a grid pattern, with the gaps able to trap even more air and warmth without adding weight, and allowing water vapour to pass through more easily.

Octa is often teamed with a smooth, slightly more robust outer that’s less likely to snag than AD Fleece. Some, such as Mountain Hardwear’s Airmesh, are reversible, with a reported but unverified 28% difference in warmth depending on whether you have the mesh or fluffy side closest to skin.

Different manufacturers seem to use slightly different weights and add different outer faces, but you can expect these fleeces to be about 50% warmer weight for weight than standard fleece, and weighing in at about 200g/7.1oz for a men’s medium depending on GSM and fabric. The outer faces are more windproof than AD, but not completely windproof, so you’ll need to team most of them with a shell or wind shirt in breezy conditions. It’s difficult to compare like for like with such a range, so read widely before buying online, and carefully consider the temperatures you expect on your hikes. Look at the weight you prefer in a standard fleece, then reduce it conservatively when trying these ultralight materials.

Octafleece absorbs more water and is reportedly slower to dry than AD, so consider this if you’re hiking in Australia’s wet winters or places. Here is an interesting local comparison between AD and Octafleece, although the vlogger doesn’t use a wind shirt.

Both Octayarn and AD have been made into pants by zpacks, Farpointe, Sambob, and Timmermade. They wouldn’t last a minute in Australian scrub but they are an excellent ultralight option to wear over thermal leggings and under rain pants in camp; down pants are warmer.

Down

Technical Terminology

Goose and duck down come in different grades, or fill powers, usually ranging from 600 - 900, rarely 1000fp. Fill power measures the fluffiness of down, or how much space it takes up. One ounce of 600fp down takes up 600 cubic inches of space, whereas one ounce of 900fp takes up 900 cubic inches of space. This means that a garment of higher fp down can be lighter for the same warmth, or warmer for the same weight. Cheaper down garments are often not pure down, but a mixture with feathers, often expressed as 90/10 or 85/15, with the down component coming first. Feather is far less insulating and heavier than down: preference pure down jackets. Search for the Responsible Down Standard (RDS) accreditation to avoid down sourced from live-plucked birds.

Down for Australian Multiday hikers

Down remains the gold standard insulation layer for many hikers. We’ve all seen down-bundled cross country skiiers trudging through snowy, icy landscapes in northern hemisphere winters... but how well does that translate to Australian conditions?

*Down plus fleece for me, fleece enough for Geoff.*

In our opinion, although down is great for in camp as an insulation layer, in Australia and other warm temperate climates it doesn’t work as a mid layer whilst walking with a pack. Other than in the highest alpine regions for a short part of winter, Aussie winters are wet, with the coldest areas +/-7C either side of freezing rather than in northern temperate climates with winter temperatures consistently below -10C with all precipitation falling as dry snow. Wearing a down jacket for sweaty hiking in our higher humidity, damp conditions moistens fill, reducing its loft and hence insulating properties. If you wear a waterproof shell over your puffy, it not only squashes the loft but also allows vapour to condense more quickly. Polyester fleece or synthetic down (see below) are almost always preferable as a standalone mid layer.

However, because down is so light and compressible with lightweight puffies packing into their own pocket, they excel in camp or when stopping for breaks as a second insulation layer over your polyester: most Australian hikers wear them like this and we recommend that you do too. Even Geoff packs a puffy, though he uses it far less often than I do mine.

Ultralight puffies — indeed most puffies — utilise thin nylon shells that are too fragile for bush-bashing through Australian scrub, but this doesn’t matter if you’re wearing them in camp. This approach also means that you don’t need to carry a huge thick puffy: ultralight ones aren’t really warm enough on their own but are perfect when combined with a fleece.

These light down puffies — the aforementioned Mountain Hardwear Ghost Whisperer , Montane’s Alpine 850 Nano Hooded Down Jacket and, for Aussie customers, Mont’s Zero Ultralight Down Jacket with its 1000fp down — are all ideal as a second insulation layer for multiday hiking. Compare weights of jacket, down and fillpower, as well as exchange rates, postage and taxes to find your best buy.

Timmermade’s “super duper UL”(SDUL series) customisable pullovers are without zips or pockets. With 1000fp down, the 1.1 weighs an astonishing 147g/5.2 oz in small. This model is comfortable to 0C/32F, below that for people like Geoff; Timmermade also sells warmer and cooler SDUL pullovers. The AUD504/USD330 price is surprisingly good; it’s a minimalist model but, because it’s so customisable (you can add a hood), it’s an excellent option for those with non-standard body shapes or sizes. Although the materials are delicate, Australians will wear this layer in camp, not on trail. Timmermade is a small cottage company with more demand that they can meet, so you need luck in their monthly customer lottery, and may need to enter multiple times!

Cheap knockoffs, ubiquitous online, use less and lower fill power down: if it sounds too good to be true, beware. Choose a reputable seller.

Good design plays a huge role in the efficacy of a down jacket. Consider

down/feather ratio (you generally want pure down)
Sewn-through vs box baffle construction — the former are lighter, but not as warm
Hoods or without (most people prefer hoods but, if you carry another hooded insulation layer, a beanie, balacava and/or neck gaiter, they are superflous)
Elasticised hoods, cuffs and hems that cinch to exclude drafts
Sizing that allows for another insulation layer underneath
Synthetic insulation in high moisture locations such as collars and sleeve ends
water-repellant coating
Width of baffles - wider ones are bulkier but allow more loft
Treated down that resists losing its loft in damp environments
fillpower - buy the highest you can afford, preferably over 800 pure down for multiday backpacking when weight is important.

However, a cheaper jacket with lower fillpower down can be equally warm, just slightly heavier. Geoff’s MacPac uberlight jacket with 75g of 650fp 90/10 down is just AUD176 (member price) or AUD95 on clearance sale at time of writing. It isn’t as warm as the Mont Zero with 75g of 1000fp down but it is ideal for those who run hot like Geoff, and only a few tens of grams heavier.

Synthetic Down

Synthetic down is made of materials that mimic the way down insulates. They require face fabrics to contain them, and some of these face fabrics are extremely light. Apex Climashield and Primaloft Gold are comprised of continuous filament that holds together rather than separating into clumps like down, so do not need baffles; some forms have shorter filaments that require widely spaced stitch lines to hold the batts in place. The North Face’s Thermoball and Patagonia’s Thermofill use fibres formed into clumps and feathery plumes respectively. Because they aren’t continuous fibre, they need baffles, compartments or some stitching to prevent material migrating.

Thinsulate is a very fine polyester fibre, sandwiched batting that’s less compressible and for hikers more commonly seen in beanies and gloves (see below).

Synthetic down garments retain their warmth and loft when damp (not wet!) and are light and compressible — but not as light and compressible as down — with Steven Seeber’s tests at paywalled Backpacking Light suggesting that Primaloft Gold has superior resistance to degradation after repeated compression as might be experienced after repeatedly scrunching it into a small stuff sack. In practice, it’s better to stuff any synthetic jacket more loosely into a pack around other items than into a tiny sack. Synthetic insulation does not last as long as down.

The huge advantage of Apex and Primaloft is that they don’t need baffles — and the resulting cold spots — to hold material in place, making it easy to sew and a hiker MYOG (Make Your Own Gear) staple around the world. Try Make your Own Gear Australia and Ripstop by the Roll in the US. If you can sew, it’s a fantastic option for an effective midlayer, just look for patterns online.

Many US hikers use the Torrid Apex jacket or similar instead of a polyester fleece insulation layer, but neither is warm when wet, it’s a myth — damp, yes, wet no. For Australia, either work as a standalone insulation layer with a waterproof shell. The Torrid jacket is far, far warmer than an equivalent weight fleece, but in Australia we rarely need that amount of insulation whilst hiking. It’s an excellent option for very cold conditions and the cold-blooded.

Beanies

Polyester fleece — usually partially doubled layers of microfibre fleece — beanies are much lighter than woollen beanies. Choose one that fits firmly but is large enough to pull down over your brow and ears, or one that has ear flaps. Pom-poms can snatch off your beanie on scrubby trail and are annoying under many rain shell hoods, so avoid or remove them.

Some beanies comprise Thinsulate sandwiched between microfibre fleece, and these hats are particularly compact, warm and comfortable, especially for sleeping (Geoff has particular need of one!). Other knitted nylon or polyester beanies are lined with Thinsulate; again, avoid excess bulk so they still fit well under your rain shell.

Design Considerations

The following design considerations are exactly that: considerations. Most are not right or wrong, but address specific issues that may or may not be relevant to you. It’s worth thinking about the kind of hiking you do, and which tips are helpful and which irrelevant. Flexibility is important but, where possible, less is more because every bell and whistle adds weight. If you don’t need it, why carry it?

Hoods

A highly personal choice. Tops with hoods are heavier, but efficiently block drafts around the ears and neck when they fit well. As for hems and cuffs, they should be snug, or able to be cinched.

Because Geoff and I already carry sun hats, fleece beanies and lightweight merino balaclavas, and use rain shells with hoods (in cold weather we wear all three of the latter simultaneously!), yet another hood is superfluous to our needs and we prefer mid layers without them. We often need beanies at night too, when we may not be wearing our fleece. Other hikers omit a beanie and carry a hooded insulation layer instead. It really depends on the kind of weather you expect, and what else you carry.

Full, half or no zip?

Zips are helpful in jackets because they allow you to regulate body temperature: just unzipping your jacket to expose the base layer over your chest is cooling, without having to stop and remove your pack to take off the jacket. Geoff and I prefer full zip mid layers. However, zips are a relatively heavy component, so many hikers choose quarter or half zips as a compromise. Other hikers are happy with a “pullover” type jacket with no zip at all, especially with down and synthetic down where a zip is an even more significant proportion of the garment’s weight.

Pockets

Pockets are convenient, but only if they are correctly placed and not blocked by your hip belt. Do they have zip closures so you won’t lose precious items as you contort under or over deadfall? Do you use pants pockets or hip belt pouches instead? Will you use pockets to keep hands warm or do you use trekking poles? Gloves? Pockets add weight, especially if they have zip closures.

Kangaroo Pouch

Hems and cuffs

On fleece mid layers, these should fit firmly enough to block drafts. Drawstring or elasticised ones are common on down garments. Avoid any mid layer that is baggy on you at the openings without any ability to cinch them to fit. Alpha Direct garments are an exception, with minimal hems and cuffs because the whole garment is drafty anyway.

Collar height

A collar that zips snugly up to your chin is warmest, unless like us you carry a balaclava. Many jackets have a protective flap that prevents the zip from irritating your skin.

Fit

Insulation mid layers vary in their fit, from form-fitting to baggy. You don’t want them too baggy (unnecessary weight) but, if you expect to wear them over another insulation layer – eg a puffy jacket over a fleece – ensure that it is big enough: you don’t want it to be tight and compress the loft.

Offset and flat seams

Because many mid layers are bulky, so are seams unless they are flat. Avoid seams along the top of the shoulders – look for raglan or offset designs. Less well-padded humans may also prefer offset seams at the hips, where a tight pack belt can turn any irregularity over the bone into a painful, bruising lump.

Droptail and length

Ensure that your insulation is long enough to be properly captured under the hip belt of your pack. Drop tail designs that are longer at the back are great for taller women (or choose a men’s model as these are longer in the torso).

Toasty warm in Lake Vera Hut, Tasmania. Geoff is wearing a short-sleeved merino tee base layer and his eight-year-old polyester fleece mid layer. Cold fish me is wearing a long-sleeved merino base layer, a polyester fleece insulation mid layer, and a second down insulation mid layer, also 8 years old. Our shell rain jackets are hanging out of shot but, if it were even colder, I would be wearing that too!

There are as many mid layer combinations as there are hikers. What works for you? Why? Where, on what hikes and in which climates? Share in the comments below!

Mid/Insulation Layers: Clothes for Multiday Hiking Part 3