Cotton or Cotton-Poly for a Heavy Hoodie?

Late August in Canada is the awkward stretch. Mornings in single digits, afternoons still hitting 26, and most people buying for the six weeks after that. So the same question comes back around: 100% cotton, or a cotton-poly blend?

The short answer is that the fibre split tells you less than people assume, and the yarn and the knit tell you more. But the blend does change four things: how much the piece moves in the wash, how the surface holds up, how the cloth handles sweat, and how it smells the day after. Here is what is actually measured, and where the common advice runs out of evidence.

What the polyester is doing in there

In a fleece, polyester staple is blended with cotton before spinning, so a single yarn holds both fibres. Cotton is a cellulose fibre that swells when it takes on water. Polyester is a melt-spun thermoplastic that barely takes any on. Put them in one yarn and the cloth sits between the two, weighted towards whichever fibre dominates.

Mills add polyester for three reasons: it holds the knit's dimensions through hot washes, it is cheaper per kilo than combed cotton, and it takes a different class of dye, which is why a poly-content garment often reads heathered rather than flat. None of that makes a blend better or worse. It makes it different, in ways you can partly predict.

Shrinkage, and why one number is never the answer

Shrinkage in a knit is mostly relaxation. Yarn is held under tension during knitting and finishing, and the first proper wash lets it go. Cotton swells and the loops pull up. Polyester does not swell much, so a blend usually moves less. That is the real reason blends dominate mass-market sweats.

The industry way of putting a number on this is AATCC TM135, Test Method for Dimensional Changes of Fabrics after Home Laundering, currently in its 2025 edition. AATCC specifies four washing temperatures, three agitation cycles and four drying procedures, and a result only means something paired with the combination used. Anyone quoting a single shrinkage figure without the wash and dry settings attached is quoting half a sentence. The dryer, not the wash, is usually the variable doing the damage.

Pilling, and a test that admits its own limits

Pills are broken fibre ends that migrate, tangle into a ball, and stay anchored. The standard laboratory approach is ASTM D3512/D3512M-16, Standard Test Method for Pilling Resistance and Other Related Surface Changes of Textile Fabrics: Random Tumble Pilling Tester, which tumbles specimens in a chamber and rates them from 5, no pilling, down to 1, very severe.

The interesting part is the standard's own honesty. ASTM states the method is not recommended for acceptance testing, and that it should be used with caution if it is, because between-laboratory precision is poor. It also notes that laboratory results may not track real wear. So when somebody says flatly that blends pill more, that is a tendency, not a measured law. Polyester is strong enough that a pill which forms tends not to break off, which is the mechanism behind the folklore. But a long-staple, tightly twisted cotton yarn out-performs a cheap blend and a cheap cotton alike. That is a spinning question before it is a fibre question, covered in ring-spun versus open-end cotton.

Sweat, damp and how the cloth feels

Fibres hold different amounts of water at the same air humidity, and that difference is standardised: ASTM D1909-13(2020)e1, Standard Tables of Commercial Moisture Regains and Commercial Allowances for Textile Fibers exists so trade can agree on those values when fibre is sold by weight. Cellulose fibres sit far higher on that table than polyester. A cotton fleece takes up humidity from the room and gives it back slowly, which is why a heavy hoodie can feel cool and slightly damp on a muggy day with nothing spilled on it. More on that in why a heavy hoodie feels damp in August.

For liquid rather than vapour, the method is AATCC TM195, Liquid Moisture Management Properties of Textile Fabrics. It places a test liquid on one face and tracks absorption and transfer between both faces using electrical resistance between concentric sensor rings. What it reports is the behaviour of a whole fabric structure, not of a fibre in isolation. Knit and finish move those numbers as much as the blend ratio does, so a poly percentage on a label promises nothing in particular.

The smell question, and what the study actually showed

This is the one people search for. There is real work behind it. In the paper Microbial Odor Profile of Polyester and Cotton Clothes after a Fitness Session, published in Applied and Environmental Microbiology in 2014, Callewaert and colleagues had volunteers do an hour of spinning in cotton, polyester or blended shirts, sealed the shirts overnight, then had a trained panel assess them. The polyester shirts rated significantly less pleasant, at -2.04 against -0.61 for cotton, and were called more musty, more ammonia-like, more sweaty and more sour. Micrococci, a genus the authors link to malodour, turned up on almost all the synthetic shirts and were largely absent from cotton.

Be careful how far that travels. Those were thin fitness shirts worn through an hour of hard exercise, not 400 gsm loopback fleece over a t-shirt on a cool October evening. It is one study, with a modest number of participants, and it says nothing about heavyweight sweats. What it does support is a mechanism: cotton absorbs more, so the compounds carrying the smell are held rather than released, and the fibre surface seems to select for a different set of bacteria.

The evidence leans one way. It does not prove a blended hoodie will smell, and it does not make a cotton one self-cleaning.

What the label has to tell you, in Canada

The fibre split is not a courtesy here, it is a legal disclosure. Under the Textile Labelling and Advertising Regulations, section 11(1)(a) requires a label showing the textile fibre content of the article, section 11(1)(b) requires the name and postal address of the dealer, and section 11(3) requires that fibre content appear in both English and French, with a narrow exception where only one official language is used in trade.

The Competition Bureau's detailed guidance fills in how the disclosure is written. Generic names are listed in order of prevalence by mass, most to least. A fibre at 5% or more of total fibre mass must be named with its amount, and below 5% it can be shown either by generic name or as "other fibre". Abbreviations are not allowed in place of a generic name, apart from PBI and PLA. The Bureau also states that a tolerance of 5% is generally allowed on fibre content disclosures for blends, while no tolerance is allowed for single-fibre fabrics. So an 80/20 blend label sits inside a band, not on a point. More on reading the rest of the tag in what the label in a hoodie has to tell you.

How to choose, practically

Buying for feel and the long haul: go cotton-dominant. It breaks in, and the hand improves with washing rather than staying flat.
Buying for a piece that lives in a hot dryer: a blend holds its measurements better. If it will be washed by somebody who does not read labels, that is a real argument.
Worried about pilling: ask about yarn, not blend. Staple length and twist decide more than the ratio does.
Worried about smell: the published work leans towards cotton, on thin shirts. Treat it as a lean, not a guarantee.
Either way: wash cooler than you think and keep it out of a hot tumble. That habit outranks the fibre split for how the garment ages.

One last thing, plainly. Describing what AATCC TM135 measures is not the same as saying a garment has been through it. Those are methods, not badges. A test name quoted without a result and a set of conditions is scenery. Read the tag, ask what the yarn is, judge the cloth in your hands.

Sources

AATCC. TM135, Test Method for Dimensional Changes of Fabrics after Home Laundering, 2025. members.aatcc.org
AATCC. TM195, Liquid Moisture Management Properties of Textile Fabrics, 2011e2 (2017). members.aatcc.org
ASTM International. D3512/D3512M-16, Standard Test Method for Pilling Resistance and Other Related Surface Changes of Textile Fabrics: Random Tumble Pilling Tester, 2016. store.astm.org
ASTM International. D1909-13(2020)e1, Standard Tables of Commercial Moisture Regains and Commercial Allowances for Textile Fibers, 2020. store.astm.org
Callewaert C, De Maeseneire E, Kerckhof FM, Verliefde A, Van de Wiele T, Boon N. Microbial Odor Profile of Polyester and Cotton Clothes after a Fitness Session. Applied and Environmental Microbiology, vol. 80, no. 21, 2014. journals.asm.org
Government of Canada. Textile Labelling and Advertising Regulations, C.R.C., c. 1551. laws-lois.justice.gc.ca
Competition Bureau Canada. Detailed requirements for textile labelling requirements: Fibre, down and feather content. competition-bureau.canada.ca