Thermal Underwear Manufacturing in China: Basis Weight, Brushed Knits and the Claims You Can Make

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Thermal is a marketing word, not a specification. What determines whether a base layer keeps a customer warm is the basis weight of the fabric, the knit structure, the fibre blend and the way the seams are finished, and each of those can be written into a specification and tested.

Thermal is not a specification – basis weight is

Ask for grams per square metre.

Basis weight describes how much fabric there is per unit of area, and it is the first number that separates a base layer built for warmth from one built for appearance.

A heavier fabric generally contains more air, and still air is what insulates, which is why an increase in basis weight usually produces a warmer garment for the same fibre. That relationship is not unlimited: past a point the fabric becomes heavy and stiff, and the additional weight adds bulk rather than warmth. The useful way to specify a thermal programme is to state the basis weight for each weight tier in the range, alongside the knit structure and the fibre blend, and to confirm each combination in a sample. Where a factory quotes a thermal garment without a basis weight, it is quoting the word rather than the product, and the garment that arrives may be indistinguishable from an ordinary knit. Asking what the base fabric weighs before brushing and what it weighs after finishing is the practical way to pin the number down, because brushing adds surface without changing the base.

Knit structures: brushed interlock, grid fleece, rib and waffle

Structure decides where the air sits.

Brushed interlock gives a smooth exterior with a raised interior, grid fleece creates a pattern of raised and open areas, and rib and waffle structures build thickness through the knit itself rather than through a raised surface.

The choice is not simply about warmth, because each structure also behaves differently against the skin. A brushed interlock feels soft and familiar and is the easiest sell in a men’s basics range. A grid fleece moves moisture more effectively because the open channels between the raised areas give it somewhere to go, which matters in active use. A ribbed knit stretches and recovers well and therefore suits a close-fitting garment, and a waffle structure traps air in its cells but is bulkier under trousers. Asking what happens to the fabric after brushing is as important as the base specification, because brushing can reduce the fabric’s strength and its resistance to pilling if it is over-worked. A sample that has been washed several times shows this, while a new sample does not.

Fibre blends for warmth, wicking and odour

The blend trades warmth against moisture and odour.

Cotton is comfortable and quiet but holds moisture, a synthetic component wicks and dries but can hold odour, and man-made cellulosics sit between the two with a soft hand and good moisture handling.

The blends that appear most often in this category combine a cellulosic or cotton base with an elastane component for recovery, which lets the garment stay close to the body without restricting movement. A blend such as a bamboo and cotton with an elastane component gives softness and a natural feel; a modal blend gives a smoother surface and good drape; a wool blend would give warmth and odour resistance but sits at a different price point and requires different washing instructions. Where the garment is designed for activity rather than for stationary warmth, the wicking behaviour becomes more important than the insulation, because a base layer that holds sweat will chill the wearer once the activity stops. The blend decision should therefore be tied to the use case: an everyday winter base layer and an active base layer are different products even when both are called thermal. Where the fibre carries a claim, the chain-of-custody documentation behind it should be checked, which is the subject of the Textile Exchange standards.

Warmth claims and the test methods that support them

Support the claim with a measurement.

A warmth claim can be supported by a fabric-level measurement of thermal resistance, and the difference between a supported claim and a marketing phrase is whether that measurement exists and can be produced.

Thermal resistance is measured on a guarded hotplate instrument under a defined set of conditions, and the result describes the insulation the fabric provides rather than the warmth the customer will feel, which also depends on the fit, the activity and what is worn over the garment. Where a brand wants to make a comparative claim, such as that one weight tier is warmer than another, the comparison is much easier to defend when it rests on a measurement of the two fabrics made under the same conditions. Where a brand wants to avoid the measurement entirely, the honest alternative is to describe the construction: the basis weight, the structure and the brushing, and let the customer draw the conclusion. Claims that imply a specific performance benefit, such as a stated temperature rating or a health benefit, move the product into a different evidential category and in some markets into a different regulatory one, which is a reason to keep the claim at the fabric level rather than the body level.

Flatlock seams and the chafing problem in base layers

Seam bulk is felt in a base layer.

A base layer sits directly against the skin and under other clothing, so a raised seam is felt as a ridge, and the flatlock construction exists to remove that ridge.

A flatlock seam joins two panels and lies flat against the fabric, so the seam allowance does not stand proud of the surface. Where the seam is too bulky, too tight or incorrectly tensioned, the result is a ridge that chafes under a strap, a waistband or a rucksack, and the complaint appears after a period of wear rather than at first fitting. The seams that matter most are the shoulder, the side seam and the armhole, because those are the points where another garment or a strap presses against the seam. The check on a sample is a wear test under whatever will be worn over the base layer, which is the only test that reproduces the pressure. Ask what the seam construction is, whether the thread has been chosen to stretch with the knit, and how the flatlock tension is set, because a flatlock that is too tight will also pucker the fabric along the seam.

Fit: compression, next-to-skin and loose thermal blocks

Fit decides how the fabric performs.

A base layer has to stay in contact with the body to manage moisture, which means the fit is a functional decision rather than only a stylistic one.

The three common blocks are a compression fit that is tight and highly elastic, a next-to-skin fit that follows the body with some ease, and a loose thermal block that sits away from the skin like an ordinary undershirt. The compression block gives the best moisture transfer and the least bulk under other layers, and it demands accurate sizing because a garment that is too tight restricts movement and one that is too loose loses contact. The next-to-skin block is the compromise most brands settle on, and it depends on the elastane content and the recovery of the knit to maintain contact through movement. The loose block is the easiest to fit and the least effective as a base layer, which is why it suits a positioning that emphasises comfort over performance. Whichever block is chosen, the grading has to be checked across the range rather than at one size, because the elastane content that gives a close fit at one size can produce a restrictive garment at another.

Pilling, shrinkage and wash durability

Wash the sample and look at the surface.

A brushed knit is more prone to pilling than a smooth knit, so durability testing matters more on a thermal garment than on a plain one, and the visible failure is a surface that has gone fuzzy or lost its loft.

Pilling is assessed by washing and drying the sample through a number of cycles and then grading the surface against a reference, and the important result is not the exact grade but whether the garment still looks like a base layer after repeated laundering. Shrinkage should be measured after the same cycles, in both directions, because a garment that shortens in the body will ride up and one that narrows will lose its fit. The loft of a brushed surface can also be lost, which reduces the insulation without any change in the dimensions, so the visual comparison after washing matters as much as the measurement. Where the fabric is a blend with a cellulosic component, the surface can also go fuzzy at the points of highest abrasion, which on a base layer are the underarm and the side seams.

MOQ and the seasonal window for a thermal programme

A seasonal product with a hard deadline.

A thermal programme has a narrow selling window, which means the production has to be booked well before the season rather than during it, and the minimum is set largely by the fabric.

Because a thermal fabric is often a quality the factory produces in specific weights, the minimum for the order is usually produced by the fabric rather than by the sewing. Where the fabric is a running quality in a standard colour, the constraint is softer; where it is a new weight or a new blend, the fabric has to be produced and its minimum applies. The seasonal window compresses the whole schedule, because there is no value in a thermal garment that arrives after the cold weather, and the production has to be booked early enough to allow for the fabric, the sampling and the shipping. The practical consequence is that a thermal programme should be decided several months before the selling season rather than as a late addition to a basics range. Where a brand is testing the category, a single weight and a single colour in a tight size range produces more information than a multi-weight range that arrives in the middle of the season.

Cost per unit by weight and construction

The fabric dominates the cost.

The basis weight, the fibre blend and the knit structure together determine most of the unit cost, because the fabric is the largest input and its price scales with the weight and the fibre.

A heavier fabric uses more material and therefore costs more per garment, which means a warmer garment is more expensive for reasons the customer can see and feel. A more complex structure such as a grid fleece adds knitting cost, and a brushed finish adds a finishing step. A higher-cost fibre such as a fine cellulosic or a wool blend raises the cost further. The consequence for a range is that the price ladder should follow the weight ladder, so that the customer understands why the heavier garment costs more. Attempting to hold the price flat across a range of weights usually means one of the weights has been built with a cheaper fibre or a lighter structure than its label implies, which is the point at which the product stops matching its description.

Positioning thermal in a men’s basics catalogue

A seasonal add-on to a year-round range.

Thermal underwear works best as a seasonal extension of an existing basics range rather than as a standalone category, because it shares the customer and the sizing language with the undershirt business.

The natural positioning is a weight ladder: a lighter base layer for the shoulder seasons and a heavier one for the coldest part of the year, with the same sizing and the same brand language across both. This lets the customer buy the weight that matches their climate rather than choosing between a garment that is too warm and one that is not warm enough. A second positioning is the active base layer, where the grid structure and the moisture behaviour carry the story and the customer is buying performance rather than warmth. A third is the gift and winter-set positioning, where the garment is sold alongside a pair of thermal bottoms or in a pack, and where the presentation carries much of the value. In all three cases the specification should be written before the positioning copy, because a garment that has been specified to one use and described as another will generate the reviews that undo the positioning. Where the brand already runs an undershirt line, the seasonal planning logic in the winter SKU planning guide applies directly to the thermal extension.

Sources: OEKO-TEX STANDARD 100; Textile Exchange standards; TENCEL™ branded lyocell fibre; 16 CFR Part 303 textile labelling; FTC textile labelling guidance.

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