The Tug of Water: Hydrophilic vs. Hydrophobic Base Layers
Sweat leaves the body in two states: vapor and liquid. Fabrics sold on the same “wicking” claims can behave very differently once sweat collects as liquid. A test published by Backpacking Light in June 2026, “By the Numbers: The Tug of Water”, puts numbers to how hydrophobic and hydrophilic fabrics move liquid sweat.
The Basic Physics
Breathability specs, given as grams of water vapor per square meter per 24 hours or as a RET value, address vapor transmission. Those numbers matter at moderate exertion. During hard exertion, the body can produce sweat faster than it can evaporate. Liquid water accumulates at the skin interface. At that point, a fabric’s liquid-water behavior is the dominant variable.
Two design philosophies exist for handling that liquid.
Strategy One: Hydrophilic Absorption
Hydrophilic fibers have surfaces that water wets readily. They pull liquid sweat off the skin by capillary action and hold it between fibers. Merino adds a second mechanism: a water-repellent cuticle over a hydrophilic core makes the fiber hygroscopic (woolmark.com). Wool absorbs up to 35% of its weight before it feels wet, according to Woolmark (woolmark.com).
Two effects follow. First, the fiber buffers moisture, moderating the sensation of wetness. Second, wool releases heat as it absorbs moisture, which Woolmark credits for keeping the wearer warmer in cold, damp conditions. Drying reverses the exchange and draws that heat back.
The trade-off is retained water: the capillary forces that pull sweat in also hold on to it. In sustained rain or a wet-out without shelter, a saturated merino base feels heavy and takes body heat to dry. Merino is also less abrasion-resistant and generally pricier.
Strategy Two: Hydrophobic Transfer
Hydrophobic fibers repel water at the molecular level. Polyester and polypropylene are the primary examples, although a surface treatment can push a fabric either way. The fibers absorb no water, but a fabric is mostly empty space, and liquid collects in the pores. After hard exertion in the cold, Backpacking Light measured 87 g of trapped sweat in a Brynje mesh shirt (65% of its dry weight) and 62 g in an Alpha Direct shirt (55%).
Moving liquid into and through those pores takes pressure. In Backpacking Light’s testing, hydrophobic fabrics passed no water until enough pressure forced it in; in use, it comes from outer layers, pack straps, movement or a snug fit. Breakthrough pressure falls as pore openings get larger and rises as the fiber repels water more strongly.
A hydrophobic base with large, well-connected pores moves liquid to the next layer readily under sustained high sweat load.
The limitation: if the pores are too small for the pressure available, sweat stays pooled on the skin side. If water enters faster than it leaves, the pores fill and the fabric turns heavy and clammy. Once liquid reaches the outer face, it still needs a pathway to evaporate; a shell, a mid-layer or high ambient humidity slows that.
What the Test Data Addresses
The Backpacking Light methodology (source) tracks where liquid water ends up (on the skin, in the base layer or in the layer above) across eight base-layer fabrics, not vapor transmission. That distinction matters. Standard breathability tests such as ASTM E96 or ISO 11092 measure vapor transmission under controlled steady-state conditions. They do not simulate the dynamic, high-volume liquid load of sustained aerobic output.
Full results are for Backpacking Light members; the free preview states the pattern. Hydrophilic fabrics absorb sweat at once but do not necessarily release it easily, and they pass on less as the receiving layer gets wetter. When the receiving layer’s capacity was reduced, the hydrophilic samples transferred less while the hydrophobic ones held steady and stayed ahead. In two other tests, hydrophilic samples transferred less than hydrophobic ones, with two hydrophobic exceptions. The author limits the findings to the eight fabrics tested, none of them merino, and to liquid transfer, not drying time, breathability or comfort.
Hybrid Constructions
Some fabrics combine both mechanisms. The common dual-face design, often called push-pull, puts a hydrophobic inner face against the skin to limit rewetting, and a more hydrophilic outer face that draws liquid outward to evaporate.
Polartec’s Power Dry, a bi-component knit, is a wicking design rather than a blend of the two. Polartec credits capillary action and a hydrophilic enhancement, with yarns that give the outer face more room for water uptake (polartec.com). Backpacking Light tested its Silkweight version, which the author describes as treated for strong capillary force (discussion).
Hybrid constructions add manufacturing complexity. That typically adds cost and, in some cases, weight relative to single-face alternatives.
Trade-offs, Measured
Weight. Dry weight depends more on knit and thickness than on fiber type; consult grams-per-square-meter specs product by product. Both types gain weight when soaked: merino absorbs moisture into the fiber, hydrophobic fabrics hold it between fibers.
Breathability. Both fiber types can reach high vapor-transmission rates. Construction architecture matters more than fiber chemistry at the extremes. Published breathability figures reflect standardized lab conditions, not field performance.
Drying time. In a Backpacking Light hot-plate study (Stephen Seeber, 2025), the energy needed to dry a base layer tracked how much water it held, which depended mostly on fabric thickness; fiber type made no significant difference. On multi-day trips, a thinner base layer dries with less body heat.
Odor resistance. Merino holds less body odor than polyester: in a New Zealand study Woolmark cites, trained assessors rated wool 66% lower in odor intensity. Woolmark credits sweat absorption, which keeps skin drier and discourages bacterial growth, and fiber proteins that bind odor molecules until washing (woolmark.com). Synthetics can carry silver-ion or other antimicrobial treatments, whose wash durability is not always independently verified.
Durability. Synthetics generally resist abrasion better than fine merino. Polypropylene can degrade under repeated high-heat washing. Merino is sensitive to snag and puncture. Neither fiber type is categorically more durable across all failure modes.
Price. Entry-level synthetic base layers sit below comparable quality merino. Technical synthetics from specialty brands can meet or exceed merino pricing. Value calculations depend on use frequency and expected lifespan.
Where the Choice Is Consequential
The hydrophobic-versus-hydrophilic question is most significant in two contexts.
Sustained aerobic output in cool-to-cold conditions. Trail running, ski touring, fast alpine. Sustained high sweat loads make liquid transfer rate the dominant variable. Hydrophobic base layers with large, well-connected pores, worn snug enough for layer pressure to push sweat through, are the more defensible choice here if the layering system lets moisture escape outward.
Moderate exertion with variable conditions. Lower-intensity hiking, travel, and urban-to-trail crossover use. Merino’s lower perceived wetness during moderate sweat, odor resistance, and temperature regulation often outweigh the liquid-transfer advantage Backpacking Light measured for well-structured hydrophobic fabrics.
For techwear-adjacent applications — garments expected to perform under load and present well at rest — fabric hand, construction quality, and odor management carry more weight than peak liquid-transfer rate. Merino and merino-synthetic blends remain defensible in that category.
The Verdict
No single fiber type wins across all scenarios. The performance edge shifts with exertion level, duration, and layering context.
Backpacking Light members can read the full data set here. Cross-reference fiber performance claims against manufacturer technical documentation and, where available, independent lab results from standardized test methods before drawing firm conclusions.