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How to Layer a Heavy Hoodie for a Canadian Autumn

What the thermal standards actually measure about layering: trapped air, why fit beats bulk, and what each added layer really returns.

Late September in most of Canada packs a twenty degree swing into one day. Four degrees and damp at seven in the morning, sixteen and bright by two, back to six on the walk home. No single garment covers that. Layers do, but not the way the advice usually goes, and a heavyweight hoodie behaves differently in a stack than a thin one does.

The thermal comfort standards have measured most of this. They are worth reading before rearranging a wardrobe, partly for what they confirm and partly for what they refuse to claim.

Cloth is not the insulator. Trapped air is

Fibre conducts heat better than still air does. What a 400 gsm fleece really does is hold a lot of air still, in the loops and the brushed nap and in the gap between the cloth and skin. Compress that air and the warmth goes with it. This is why a hoodie squashed flat under a tight shell stops working, and why a hoodie worn on its own under a jacket with room in it works well.

The reference method for the cloth side is ISO 11092:2014, titled Textiles. Physiological effects. Measurement of thermal and water-vapour resistance under steady-state conditions (sweating guarded-hotplate test). It clamps a sample onto a heated, wetted plate that ISO calls a skin model and reports two numbers: thermal resistance Rct in square metres kelvin per watt, and water-vapour resistance Ret in square metres pascal per watt. Rct is the dry warmth. Ret is how hard the fabric makes it for sweat vapour to get out.

ISO is blunt about the limits of its own test in the scope clause:

The test conditions used in this International Standard are not intended to represent specific comfort situations, and performance specifications in relation to physiological comfort are not stated.

So a low Rct number is not a promise about a Tuesday in Thunder Bay. It is a flat plate in a lab. Useful, and narrower than most marketing makes it sound.

Each layer you add returns about 84 percent of itself

Whole outfits get handled by ISO 9920:1995, Ergonomics of the thermal environment. Estimation of the thermal insulation and evaporative resistance of a clothing ensemble, which carries tables of insulation values for individual garments and for full ensembles, measured on a standing thermal manikin.

The interesting part is how the standard combines them. Kuklane and Toma, writing in the journal Industrial Health, tested that summation method on 27 ambulance service garments and 14 real combinations spanning 0.63 to 3.33 clo. The equation they were checking is the ensemble insulation equals 0.161 plus 0.835 times the sum of the individual garment values. Read the multiplier: each garment you add contributes roughly 84 percent of what it measures on its own.

It held up well. Their measured and calculated values differed by between minus 0.17 and plus 0.18 clo, an average difference of 0.02 clo, with an R squared of 0.98 and no statistically significant gap between the two (p = 0.61). Individual comparisons still ran from minus 18 to plus 12 percent, so it is a good estimate rather than a precise one.

The authors also flag a limit that matters for anyone dressed in streetwear rather than uniform. Their garments were modern Western workwear with fairly even body coverage, and they note that items influenced by modern fashion ideas, or from traditional clothing elsewhere in the world, may not match the equation as neatly. A boxy hoodie with a wide body and a deep hood is not the shape the tables were built on.

Fit decides how much air you actually hold

Loose is not automatically warmer. Młynarczyk, Orysiak and Jankowski published a 3D scanning study of workwear fit in Materials in 2025, scanning garments on a thermal manikin and measuring both the air gap and the insulation it produced.

Their result runs against the instinct to size up for warmth. The highest total thermal insulation, around 0.23 square metres degrees Celsius per watt, came from the garment that fitted the body closely without being tight. Insulation rose with the air gap and then stopped rising: no further gain above roughly 28 to 43 millimetres of gap depending on how it was calculated, or above 50 to 55 cubic decimetres of air volume. The curve fitted a second order polynomial with an R squared above 0.8, rising then flattening then falling away.

They also measured with the manikin moving, and dynamic insulation dropped further than static insulation did. Air that gets pumped out of a garment every time an arm swings is not insulating anything. That is the same mechanism behind why a loose hem and open cuffs cost you warmth in wind, which we went into in does a heavy hoodie block wind.

The wet half of the problem

Autumn in Canada is not just cold, it is wet, and cotton holds water. Vasile and colleagues measured a set of knitted fabrics for skin-contact workwear in Materials in 2025, using ISO 11092 for Rct and Ret. Across their set, Rct ran from 0.0052 to 0.051 square metres degrees Celsius per watt and Ret from 1.43 to 5.96 square metres pascal per watt.

Their drying data is the part worth carrying around. Cotton-containing blends took over 15 minutes to dry in their test, while the pure polyester fabrics came in at 3 to 9 minutes, which they attribute to the higher moisture regain of natural fibres. Those were thin next-to-skin knits, not a heavy loopback, so treat the ratio as the lesson rather than the minutes. Heavy cotton takes far longer than either.

The practical read is that the cotton should not be the layer catching your sweat, and it should not be the layer catching the rain. We covered what happens when it does in why a wet cotton hoodie gets cold so fast.

A working order for a September morning

  1. Next to skin, something thin that dries quickly. A merino or synthetic tee, or a light cotton one if the day is dry and the walk is short.
  2. The hoodie, sitting close enough that the shoulders and chest are not floating. The air gap that pays is measured in millimetres, not centimetres.
  3. A shell over the top with enough room that it does not compress the fleece. If the hoodie flattens when the jacket zips, the jacket is a size too small for layering.
  4. Vent before you sweat, not after. Unzip on the hill, not at the top of it. Damp heavy cotton is a long recovery.
  5. Nothing between the hoodie and the shell unless it is genuinely cold. Two mid-layers usually just fill the space where the useful air was.

What the evidence does not settle

Plenty. ISO 9920 states in its own scope that it does not deal with adsorption of water, buffering or tactile comfort, and does not take rain and snow into account. Every clo value in those tables assumes dry cloth. Canadian autumn frequently does not.

The manikin studies measure heat flow, not how a garment feels after four hours. The plateau numbers for air gap come from workwear on a manikin, and nobody has published the equivalent for oversized streetwear that we could find. Advice about a specific layer count for a specific temperature is folklore. Insulation requirements depend on activity, wind, wet and the individual, and no table replaces walking outside and reversing course if you got it wrong.

For our part: we have not had Salamandra cloth tested to ISO 11092 or ISO 9920, and we are not claiming a number. What we do is build a fleece that holds air and sits close through the shoulder, which is the part of this the cut can control. The care that keeps that loft intact is in the club care notes.

Sources

PreviouslyHow Often Should You Wash a Heavy Hoodie? NextWhy Hoodie Pocket Corners Tear, and How to Fix Them
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