The layering system, derived from your own heat balance

A layering system is a heat management problem before it is a kit list. A resting adult produces roughly 100 watts of heat, and the same person walking uphill under a pack can exceed 400 to 600 watts, so the insulation that is correct on the climb is several times too little for the ten minutes spent stopped at the top. Build the system around that gap and every other decision follows.

Start from the heat you make, not from three named layers#

Base, mid and outer is a packing convenience. It names garments rather than describing the problem, which is a balance: the heat your metabolism produces has to leave at the rate it arrives, or your core temperature moves. Heat leaves by conduction into what you touch, convection into moving air, radiation to sky and ground, and evaporation from skin and lungs. Clothing is the only term you can change while you are outside, and it works almost entirely by holding still air in place.

So the useful question is not how many layers to bring. It is how much heat am I making this hour, and how much resistance does it need in order to leave at a rate that keeps me level. Someone producing 450 watts on a climb must dump heat hard. Twenty minutes later, sitting on a rock with a sandwich at perhaps 110 watts, the same person needs to keep nearly all of it. No single jacket handles both.

What one clo actually is#

One clo is 0.155 square metre kelvin per watt of thermal resistance, chosen so that 1 clo is about what a resting adult needs to stay comfortable at 21 degrees C in still air. Because it is a resistance per unit area, values add: a 0.2 clo base layer under a 0.6 clo fleece is roughly 0.8 clo, before the thin film of still air clinging to the outside of everything you wear.

The definition is worth checking once, because it is the answer to a sum. Mean skin temperature at comfort sits near 33 degrees C, so at 21 degrees C the gradient is 12 kelvin. Take 1 clo of clothing plus roughly 0.6 to 0.7 clo of surface air film, near 1.6 clo in total, which is 0.248 square metre kelvin per watt. Heat flux is gradient divided by resistance: 12 divided by 0.248 is about 48 watts per square metre. An average adult carries roughly 1.8 square metres of skin, so that is about 87 watts of dry heat loss, and the balance of the roughly 100 watts leaves as warmed breath and insensible sweat.

Published values in ISO 9920 give anchors: a long sleeved base layer around 0.2 clo, a light fleece 0.4 to 0.6, a wind shell 0.1 to 0.2, a light down sweater 0.8 to 1.2, a heavy parka 1.8 to 2.5. Treat them as approximate, since manikin tests measure ensembles rather than single garments.

How much insulation the numbers actually call for#

Reverse the sum and it predicts any condition. Required total insulation is the skin to air gradient divided by 0.155 times the dry heat flux:

clo total = (skin temperature - air temperature) / (0.155 x dry heat flux in watts per square metre)

Dry heat flux is the part of your output leaving as conduction, convection and radiation rather than as evaporation and breath, and that fraction falls from about 80 percent at rest to nearer 50 percent working hard as sweating takes over. The table runs this over 1.8 square metres of skin, uses 33 degrees C skin at rest and 32 degrees C working, and subtracts the surface air film (0.7 clo standing still, 0.3 clo when your own movement strips it) to leave the clothing insulation you supply.

Read the table down a column rather than across a row. At 0 degrees C, climbing asks for about 1.2 clo and standing in camp asks for 3.6. The forecast did not change. You did. Two thirds of the insulation you carry exists for the hours you are not moving, which is why a gear list specifying one insulating layer answers half the question.

The resting rows also explain why standing at 0 degrees C in a warm jacket still feels cold after twenty minutes: the model wants over 5 clo, more than almost anyone carries. The same arithmetic is why people sleep cold, and why a bag rating describes someone lying still rather than sitting up.

The moving layer and the camp layer#

Here is the design rule the table produces, and the one most lists ignore. Split insulation into two objects with different jobs.

The moving layer covers roughly 0.5 to 1.2 clo and is judged on how much water vapour it passes, not on how warm it feels in a shop. Sweat rates under load are commonly reported between 0.5 and 2.0 litres per hour in warm conditions, lower in the cold but never zero, and all of it has to get out through what you are wearing. If this layer is comfortable standing at the trailhead, it is wrong.

The camp layer adds 2 to 3.5 clo and is judged on loft, coverage and how fast you can get it on. It is a different tool, not a thicker version of the first, and it lives at the top of the pack. The details in insulated jackets matter here: hoods, high collars, sealing hems.

Insulation generates no heat, it only slows the loss of heat you have, so it goes on while you still have a surplus. Put numbers on the cost of waiting. Stop at 0 degrees C in 1.2 clo: total resistance with the air film is 1.9 clo, so heat flux is 32 divided by (0.155 x 1.9), about 109 watts per square metre, or 196 watts over 1.8 square metres, while you produce about 120 watts. That is a deficit near 76 watts. Human tissue has a specific heat around 3.5 kilojoules per kilogram kelvin, so a 70 kg body stores about 245 kilojoules per kelvin. Twenty minutes at 76 watts is 91 kilojoules, about 0.37 kelvin of mean body temperature given away for nothing.

Why sweating into your insulation is the expensive mistake#

Evaporating one kilogram of water takes about 2,257 kilojoules (the figure at 100 degrees C, a few percent higher at skin temperature). That is why sweating cools you so effectively, and why it is costly once it happens in the wrong place.

Work the arithmetic on half a litre of sweat absorbed into a fleece, which is one hard hour. Evaporating it later takes about 1,130 kilojoules, and a resting adult at 100 watts produces about 1,080 kilojoules in three hours. A soaked mid layer is therefore roughly three hours of your entire resting heat output, and it collects that energy from you at the moment you have least to spare. It degrades the insulation while doing so, because water conducts at about 0.6 watts per metre kelvin against still air at 0.026.

Hence the rule: regulate before you sweat, not after. Once moisture is in the fill, venting does not recover it quickly.

Wind and water, the two things that destroy still air#

Every clo value assumes a film of still air on the outside of your clothes, worth 0.6 to 0.7 clo on its own. Wind strips that film, then forces air through the fabric itself, flushing out the trapped air the insulation exists to hold.

Put the same numbers through the model. Standing at 0 degrees C in 1.2 clo, total resistance is about 1.9 clo in still air and about 1.4 clo once moderate wind has cut the surface film to 0.2. Heat flux rises from roughly 109 to 147 watts per square metre, a 35 percent increase, with no change of temperature and no change of clothing. A wind shell of 0.1 to 0.2 clo restores most of that, which is why a 90 gram windproof beats a much heavier fleece on an exposed ridge.

Water does the same job faster. Cotton holds many times its own weight in water against the skin, and wet fabric conducts far better than dry. That is a conduction argument, not a fashion one, and it is why a shell that stops shedding water matters: see rain jackets and breathability and reproofing rain gear.

Regulate before you sweat: a protocol for the next hill#

Run these in order. Each step is cheaper and faster than the one after it.

1. Start cool. Dress so the first five to ten minutes are slightly uncomfortable. Output rises three to four times over in that period, and what felt right at the car will be far too much.
2. Hat first. The head is roughly 7 to 10 percent of body surface area and sheds roughly that share of your heat, not the 40 percent folklore claims. It is still the first adjustment because it takes two seconds and needs no stopping.
3. Zips next. A front zip and pit zips vent by convection and cost nothing to use. Open the chest before removing anything.
4. Cuffs and hem. Sleeves pushed up and a released hem drawcord let warm damp air out at the wrists and waist.
5. Only then remove a layer. Stripping costs two or three minutes and cools you, so it is the last resort.
6. Reverse the order the moment you stop, camp layer on within a minute, before you feel cold.

Test it yourself: weigh what your layers are carrying#

Kitchen scales settle the argument about whether your system moves moisture or stores it. Weigh your base layer and mid layer dry, to the gram, before a hard walk. Wear them for a two hour effort with a climb in it, then weigh each garment again on return, before anything dries.

The difference is the water your clothing is holding. Multiply that mass by 2,257 kilojoules per kilogram to get the heat it will take back from you later: 200 grams is about 451 kilojoules, roughly 75 minutes of resting heat output. Repeat with a different mid layer and you have a real comparison between two garments, using equipment you already own.

Common mistakes#

Buying one jacket to cover everything. A single mid weight insulated jacket is too warm to climb in and not warm enough to stand around in: commonly owned, rarely the right answer.

Treating the shell as insulation. A hardshell adds 0.2 clo at most. It protects insulation from wind and rain, which matters, but it is not a warm layer.

Sizing the camp layer to fit over nothing. If it will not close over your moving layers you cannot use it when you need it. Check with the layers on.

Insulating the core and forgetting the ends. Hands, head and feet carry a high surface area for their volume and shut down first when the core is under pressure. Warm gloves and the moisture control in footwear and socks do more at 0 degrees C than another 100 grams on the torso.

Carrying every layer you own. Two well chosen insulation levels weigh less than four hedged ones, a base weight argument as much as a comfort one.

Frequently asked questions#

How many layers should I actually wear for hiking?#

As few as keep you slightly cool while moving, plus one substantial insulated piece you are not wearing. Between 10 and minus 10 degrees C that is usually a base layer, one light insulating layer, a wind or rain shell, and a warm jacket in the pack. Counting insulation beats counting layers: aim for 0.5 to 1.2 clo while walking.

What does clo mean on clothing?#

Clo is a unit of thermal insulation equal to 0.155 square metre kelvin per watt, set so that 1 clo is roughly what a resting adult needs at 21 degrees C in still air. Outdoor clothing is rarely labelled in clo, but ISO 9920 gives approximate values: about 0.2 for a base layer, 0.4 to 0.6 for a light fleece, 1.8 to 2.5 for a heavy parka.

Why do I get cold as soon as I stop walking?#

Two things happen at once. Heat production falls from perhaps 450 watts to about 120 within minutes, and sweat held in your clothing starts evaporating, taking about 2,257 kilojoules per kilogram out of you. Add insulation on stopping rather than waiting for the cold to register.

Do I need a wind shell if I already have a rain jacket?#

Often, yes. A windproof of 90 to 150 grams has much lower evaporative resistance than a waterproof membrane, so you can climb in it without soaking your layers. Wind removes a still air film worth around 0.6 clo, so blocking it is among the highest returns per gram in the pack.

Is merino better than synthetic for a base layer?#

They fail differently rather than one being better. Merino resists odour and feels less clammy at low output; synthetics dry faster and hold less water, which matters when sweat rates are high. Both beat cotton, which holds many times its weight in water, and water conducts heat about 23 times better than still air.

How cold does it have to be before I need a proper insulated jacket?#

Less cold than most people assume, because the trigger is standing still rather than air temperature. The model above puts a person standing in camp at 10 degrees C at around 2.3 clo, which is more than a fleece. If your day includes long stops, an insulated jacket earns its weight well above freezing.

Does layering work in hot weather too?#

The same balance applies, with the emphasis on evaporation rather than insulation. Above roughly 25 degrees C you shed most heat by sweating, so the clothing job is to pass vapour and shade skin from radiation. Loose light fabric beats bare skin in strong sun and beats anything tight and coated.