Three fabrics tend to come up in any conversation about sustainable underwear: TENCEL, bamboo and modal. They sound similar, are often shelved next to each other, and are marketed in overlapping language ("eco", "natural", "sustainable", "soft"). They are not the same fabric, and the differences matter.

Below is the working comparison, with the marketing layer stripped off.

The three fabrics, side by side

The simplest way to see what is going on is to look at all three together. The table below shows what each fabric is, how cleanly it is processed, and how it actually behaves when you wear it.

	TENCEL Lyocell	Bamboo viscose	Modal (standard)
Source plant	Eucalyptus / sustainable wood pulp	Bamboo (fast-growing grass)	Beech wood pulp
Production process	Closed-loop (recovers 99% of chemicals)	Open-loop with sodium hydroxide and carbon disulphide	Open-loop, similar to viscose but slightly cleaner
Chemistry residue in finished fabric	Very low	Moderate	Moderate
Feel against skin	Silky, drapes well	Soft initially, loses shape with washing	Soft, holds shape
Moisture regulation	Excellent	Decent	Average
Antibacterial properties	Genuinely natural	Mostly washed out in processing	Minimal
Durability over years of wear	High	Lower than expected	High
Compostable in pure form?	Yes	Yes (but rarely sold pure)	Yes (but rarely sold pure)
Typical blend in underwear	100% TENCEL or with cotton	95% bamboo + 5% spandex	50% modal, 45% cotton, 5% elastane
Brand certification	Trademark (Lenzing)	None standardised	None standardised (TENCEL Modal is the certified version)

The environmental honesty test

Sustainability claims around all three fabrics have been overstated in the last decade. Here is the honest version.

Lenzing TENCEL Lyocell sits at the top of the pile. The closed-loop process recovers around 99 per cent of the chemicals and water used. The wood pulp is FSC certified or sourced from sustainable plantations. The fabric is genuinely cleaner than most alternatives, both at production and through the lifecycle. It is also the most expensive to make, which is why it is not the default for high-volume underwear manufacturing.

Bamboo viscose comes in second. The plant grows fast and needs no irrigation, which is the source of most "bamboo is sustainable" claims. Its actually an invasive weed! The processing is the problem. Bamboo viscose production uses heavy chemistry, sodium hydroxide, carbon disulphide, and is rarely closed-loop. Which means the environmental benefit of the plant is partly cancelled out at the factory. The carbon disulphide step is also a significant occupational hazard for textile workers, and can destroy the habitat of communities in developing nations when it is allowed to reach the water table, which is rarely mentioned in the marketing.

Modal is closer to bamboo viscose than TENCEL unless it is specifically TENCEL Modal. Standard modal is a step up from regular viscose but does not match TENCEL Lyocell on processing.

The 'back to nature' question

This is where the comparison gets interesting. All three fabrics are technically biodegradable in their pure form. The catch is that almost no underwear is sold in pure bamboo, pure modal or pure TENCEL.

Walk into any underwear shop and check labels. "Bamboo" briefs are almost always 95 per cent bamboo and 5 per cent spandex. "Modal" t-shirts are almost always 50 per cent modal, 45 per cent cotton, 5 per cent elastane. The 5 per cent synthetic content is enough to render the entire garment non-compostable, regardless of how clean the main fabric is.

For underwear that actually composts, you need 100 per cent of one fibre, plus natural rubber elastic, plus cellulose thread. That combination is rare in any of these three fabrics, and rarer still on the high street. Our 100% TENCEL underwear hits this spec end-to-end. We have written more about why the small spandex percentages matter in our post on sustainable bras.

Which to choose

If you are choosing between the three for sustainable underwear, the order of preference is straightforward.

TENCEL Lyocell. The cleanest choice. Best for sleepwear, hot weather, sensitive skin and women who run warm. Particularly good in nursing and pregnancy contexts because of the moisture regulation. Look for the trademark.

Bamboo viscose. Avoid if processing matters to you. The "bamboo is sustainable" claim is mostly true at the plant stage and mostly false at the factory stage. If a brand is using bamboo lyocell rather than bamboo viscose, the calculation changes, bamboo lyocell is processed with the same closed-loop method as TENCEL. Bamboo lyocell is rare.

Modal. Fine if it is TENCEL Modal specifically. Standard modal is acceptable but not exceptional.

Where organic cotton fits in

None of the three above are inherently better than 100% organic cotton. Cotton has been the natural fibre default for centuries because it works. It breathes, it is hypoallergenic, it is durable, and the supply chain is mature. The reason TENCEL has gained ground is that it processes cleaner than non-organic cotton and feels silkier than basic cotton. Lenzing Tencel is an out-performer because of it's clean, closed-loop process.

For underwear specifically, the choice is usually between 100% organic cotton (familiar, durable, hypoallergenic) and 100% TENCEL (silkier, better moisture regulation, slightly more expensive). Both are solid choices. Both are infinitely better than synthetic blends. We use both: organic cotton in the main bra range, TENCEL in selected briefs and as a cooler-feeling alternative.

If you want a deeper read on why TENCEL works particularly well for underwear, our piece on what makes TENCEL different covers the manufacturing process in more detail.