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How Warm Is a Heavy Hoodie, Really?

Warmth is a measured property, not a fabric weight. What the hotplate and manikin standards say about heavy cotton, loft, fit and wind.

The first properly cold morning lands somewhere in September and the heavy hoodie comes back off the shelf. Then a question turns up in the search bar, phrased roughly the same way every year. How warm is a hoodie, and does a heavier one keep you warmer.

There is a real answer, because warmth is a measured property with a unit attached. Textile labs have put numbers on it for decades, and those numbers are less flattering to heavy cotton than a swing tag suggests.

Warmth has a unit, and it is not grams

Heat leaves a warm body through whatever is in the way. The property that matters is thermal resistance: how hard the material makes that trip. The reference method for cloth is the sweating guarded-hotplate test, published as ISO 11092:2014 and often called the skin model, because the plate is held at skin temperature and the fabric sits on top of it.

Writing in the SATRA Bulletin in June 2019, David McKeown set out what the rig reports. Thermal resistance, written Rct, is the temperature difference between the two faces of the specimen divided by the heat flow per unit area, in square metres kelvin per watt. Wet the plate and you also get water vapour resistance, Ret, in square metres pascal per watt: how hard the fabric makes it for sweat to evaporate away. Two numbers, one rig. Neither is a fabric weight.

Whole garments are measured differently, on a heated manikin, and quoted in clo. One clo is 0.155 square metres kelvin per watt. Kuklane and Toma, at Lund University, report their garment values in both units side by side, so the conversion can be checked rather than taken on trust.

The numbers are smaller than they sound

A 2025 paper in Materials by Vasile and colleagues put eighteen knitted skin-contact workwear fabrics through ISO 11092. The measured thermal resistance ran from 0.0052 to 0.051 square metres kelvin per watt: the lowest was a polyester, the highest an aramid blend. Convert the top of that range and it comes out around 0.33 clo. The whole spread of eighteen knits, from the thinnest to the thickest, is a third of a clo.

A single layer of knitted cloth, even a substantial one, is a small contribution. Insulation comes from stacking layers and from the air held between them.

Thickness carries the insulation, weight does not

The same paper is direct about what predicts thermal resistance. Thickness showed a moderate positive correlation with Rct, at r = 0.63. Bulk density went the other way, a negative correlation at r = -0.56. Denser cloth measured cooler, not warmer.

The physical reason is dull and consistent: still air is the insulator, and fibre is mostly a scaffold to hold it in place. Pack the same mass of cotton into a tighter, flatter structure and the trapped air goes with it. That is why two hoodies at the same fabric weight can behave differently. A lofted, brushed inside face holds more air than a hard, flat one, and the scale does not know the difference. One caveat: those eighteen fabrics were next-to-skin workwear knits, not 400 gram loopback terry, so the coefficients themselves do not transfer. The direction of the effect is the part worth carrying across.

A swatch on a hotplate is not a garment on a body

Fabric numbers and garment numbers are different animals. In a 2020 paper in Industrial Health, Kuklane and Toma measured 27 individual items and 14 assembled sets from an ambulance service on the thermal manikin at Lund according to ISO 15831, then checked the ISO 9920 summation method, which estimates the insulation of a whole outfit from the values of its parts, against those measurements.

Two things stand out. It works: across sets spanning 0.63 to 3.33 clo, calculated and measured values differed by between minus 0.17 and plus 0.18 clo, averaging 0.02 clo. The second is the shape of the formula they validated. Basic clothing insulation comes out as 0.161 plus 0.835 times the sum of the individual garment values. The multiplier is less than one. Garments stacked on a body give back slightly less than they measure alone, because layers compress each other and squeeze out the air that was doing the work.

For scale, the jackets in that ambulance system measured between 0.46 and 0.56 clo each. No hoodie was in the set, and we will not pretend one of those figures stands in for one.

Fit matters too, and not in one direction. In a companion 2021 paper the same authors looked at the clothing area factor, the ratio of the outer surface area of a clothed body to the same body bare. Heat exchange with the surrounding air scales with that outer surface. They found the usual estimating equations hold up for close-fitting Western clothing to about 1.5 clo and underestimate the real factor for loose garments. An oversized hoodie does two opposing things at once. It holds more air inside, and it presents more surface to the outside.

Wind is the part the hotplate never sees

ISO 11092 runs in controlled, still conditions. A knit with a lot of trapped air and an open structure can measure well on a plate and then perform badly on a bike, because moving air replaces the warm air inside the cloth faster than the body can heat it again. We have written about that separately in does a heavy hoodie block wind.

Environment and Climate Change Canada only reports a wind chill value once the temperature is zero or below and the wind is at least 5 km/h, which rules out most of September in most of the country. Their own clarification is worth reading: wind chill does not cool an object below the actual air temperature. It describes the rate at which a warm body loses heat, which is the thing a hoodie is meant to slow down.

What this actually means in September

  • Judge loft by hand, not by the fabric weight on the label. Squeeze the cloth. The one that springs back thick is holding more air.
  • Two moderate layers usually beat one heavy one at the same total weight, because you gain an extra trapped air gap. The summation formula says you lose a little to compression, not all of it.
  • Close the openings. Cuffs, hem and neck are where the warm air inside gets pumped out every time you move. That is a fit question, not a fabric question.
  • Damp changes the answer completely. Cotton sits at about 8.5 per cent moisture regain against 0.4 per cent for polyester, on the figures Vasile and colleagues cite, which is most of the reason a wet cotton hoodie stops behaving like an insulator.
  • Do not ask a hoodie to be a parka. Measured jackets in that ambulance system came in around half a clo apiece. More on stacking layers in how to layer a heavy hoodie for a Canadian autumn.

What the evidence does not settle

Quite a lot. We could not find published manikin data for a heavyweight cotton hoodie in the literature. Nobody should read a clo figure into a product description that has nothing behind it. Salamandra has not had its garments tested to any of the standards named above, and describing what a test method measures is not the same as claiming a result. The manikin work is also static. Kuklane and Toma tested items on a standing manikin, and walking pumps air through a garment in a way a standing test cannot capture.

And a lot of the common advice is folklore. The idea that a specific fibre is warm or cold, on its own, does not survive contact with the measurements. Structure and thickness do most of the work, moisture undoes it, and wind decides how much of it you keep. Care instructions for the cloth we do use are on the care page, including the bit that matters most here: repeated hot drying flattens the loft, and flat cloth measures cooler.

Sources

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