Why Merino and Polyester Dry at the Same Rate

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About three and a half hours in a still room, under an hour with the air moving, six if the bathroom is humid. And the surprising part of that answer is that none of those three numbers has anything to do with the shirt being merino.

During the stage of drying that takes the time, the fibre does not appear in the arithmetic at all. A wet surface evaporates at a rate set by the air: how fast the air can carry vapour away, multiplied by how much more vapour the surface is holding than the room is. Two garments of the same wet area hanging in the same room lose grams at the same rate, whatever they are made of. What the fibre decides is how much water was on the garment when you started, and the point at which the straight line stops being straight.

Dry is an equilibrium, not a zero

Start with the measurement that makes this whole question answerable, because it is the one nobody states. Moisture regain is the mass of water a fibre holds when it has come into balance with the air around it, written as a fraction of its own bone dry mass. At 20 degrees Celsius and 65 percent relative humidity the standard figures are wool at about 17 percent, viscose 13, cotton 8.5, nylon 4.5, elastane 1.3 and polyester 0.4.

Those are not small differences. On a garment of 130 grams dry mass, a wool version is holding 22.1 grams of water when it feels completely dry and a polyester version is holding 0.52 grams. A factor of 42.5, between two shirts that both pass the back of your hand.

A pair of FALKE TK2 Explore merino wool hiking socks is the only wool garment in this catalogue, and its record publishes a great deal: a merino and synthetic blend, a three layer construction built to wick moisture, flat seams at toe and heel, medium cushioning, gender specific knitting for the left and right foot, mid calf length, model 16445, and the care instruction that matters here, machine wash cold and do not tumble dry.

What it does not publish is the wool percentage, and its own drawbacks say so in as many words: "Exact fiber percentages are not listed." That single number is the whole of both claims people buy merino for. A 20 percent wool blend sits at about 3.7 percent regain and an 80 percent one at about 13.7, which is a factor of 3.7 in the water the sock is carrying at the moment you call it dry, and a similar factor in the thermal argument. Full record in our review of the FALKE TK2 Explore socks.

One garment here does publish a composition, and it is the only one. A pair of COCOMARTS wide leg summer shorts is listed at 68 percent rayon, 28 percent polyester and 4 percent spandex, with a medium thickness, a slight stretch, an elastic waistband with a drawstring, side pockets, sizes S through XXL, and a care note to wash gently at 30 degrees Celsius, lay flat to dry and iron below 110.

Weight the regain figures by that blend and you get 0.68 times 13, plus 0.28 times 0.4, plus 0.04 times 1.3, which is 9.00 percent. So on the same 130 gram garment those shorts hold 11.7 grams of water at equilibrium, which is 22.5 times a polyester equivalent and a little over half the wool figure. Rayon is the most water loving fibre in ordinary clothing and it is in two thirds of the only blend we can check.

The rate, which is the air's business and not the fabric's

Here is the part that disposes of the usual argument. A wet surface loses water at a rate equal to a mass transfer coefficient multiplied by the difference in water vapour density between the surface and the room. Neither of those two terms contains a fibre.

Work it out for an ordinary room at 22 degrees Celsius and 50 percent relative humidity. Air at 22 degrees can hold at most 19.37 grams of water per cubic metre, so the room is carrying 9.69. The wet surface is not at 22 degrees, because evaporating water takes its latent heat out of the surface and cools it. Balance the sensible heat arriving against the latent heat leaving and the surface settles at 15.0 degrees, where saturated air holds 12.80 grams per cubic metre.

So the driving difference is 12.80 minus 9.69, which is 3.11 grams per cubic metre. In still air that works out at 41.1 grams per square metre per hour. A shirt on a hanger presents about 0.80 square metres of surface, front and back, so it is losing 32.9 grams an hour, and it would be losing 32.9 grams an hour if it were made of paper.

That surface temperature is the wet bulb idea, which our page on why an evaporative cooler runs out of degrees in humid air works through in the other direction, where the cooling is the product and the water is the input. Same two numbers, opposite purpose.

Three multipliers, and only one of them is the shirt

Everything that changes your drying time is a multiplier on that 32.9 grams an hour, and they are worth ranking because the ranking is not what people expect.

The fibre, through wet pickup only. Hand wringing leaves wool holding roughly 120 percent of its dry mass in water and polyester roughly 60, because wool's scaled surface and internal structure hold on to far more. On a 130 gram garment that is 156 grams against 78. Take out the water held inside the fibre at saturation, about 33 percent of dry mass for wool and about 1 percent for polyester, and the free water sitting on the surface where evaporation can reach it is 113.1 grams against 76.7. At 32.9 grams an hour that is 3.44 hours against 2.33. A factor of 1.48, and all of it is the starting mass rather than the speed.

The room's humidity. Run the same solve at 70 percent relative humidity, which is an ordinary hotel bathroom after a shower, and the surface settles at 18.0 degrees with a driving difference of 1.77 grams per cubic metre. That is 23.4 grams per square metre per hour, 18.7 grams an hour off the shirt, and 6.05 hours for the same 113 grams. The ratio 3.11 over 1.77 is 1.757 and the ratio 6.05 over 3.44 is 1.759. A factor of 1.76, and it is exactly the ratio of the two driving differences, with nothing else in it.

Air movement. Still air gives a heat transfer coefficient of around 4 watts per square metre per kelvin. A gentle draught, an extractor fan, an open window with anything moving through it, takes that to around 15. Note what does not change: the surface temperature. The balance that put it at 15.0 degrees has the coefficient on both sides and it cancels, so moving air does not make the surface colder, it only moves more air past it. The flux rises in proportion, to 154 grams per square metre per hour, 123 grams an hour off the shirt, and 0.92 hours for the whole job. A factor of 3.75, the largest of the three by a wide margin.

So the answer to the question as asked is that the shirt being merino is the smallest of the three things deciding your drying time, and the thing that would halve it twice over is an open bathroom door and a fan. Our roundup of sock drying hangers is about the geometry side of the same point, since what a hanger does is stop two wet faces touching each other, and two faces in contact are one face as far as this arithmetic is concerned.

Why the last stretch always takes longer than the sum says

The 3.44 hours above is the constant rate period, and it ends when the surface stops being continuously wet. After that the water still in the garment has to travel out from inside the fibre, and that is a diffusion problem rather than an evaporation one. It is slower, it gets slower as it proceeds, and it is the half of drying people actually complain about.

Which is where the regain figures come back. The wool garment's 113 grams of free water goes in 3.44 hours and then it has 42.9 grams held inside the fibre, of which 22.1 will still be there when it reaches equilibrium with the room. So the tail is about 20.8 grams of water leaving by diffusion. The polyester garment's tail is under a gram. That is the real asymmetry between the two fabrics and it sits entirely in the last part of the curve, not the first.

It is also the honest reason merino feels dry before it is dry. Twenty two grams of water inside a fibre's structure does not read as damp to skin, because it is not on the surface. Polyester has nowhere to put water except the surface, so it reads as damp right up until it is not. Two different fibres, two different definitions of the word, and no listing on either side of the aisle mentions it.

A pair of PULI slim fit water resistant hiking cargo pants sits on the other side of this and shows the gap in our own records clearly. The listing publishes a lightweight, water resistant, breathable and stretchy fabric, an elastic waistband, articulated knees, seven pockets including a hidden phone pocket, and machine washability with minimal wrinkling. Its drawbacks are honest about the limit: "Water-resistant, not fully waterproof" and "Exact fabric content and weight not stated."

That second one is the problem. Without a composition and a mass, the water those pants hold cannot be computed, and water resistant and breathable are the two claims most in tension with each other. Our page on why a breathable jacket fills with water from the inside does the transport calculation for a membrane under a vapour pressure difference and finds the shell working on 4.9 percent of its rated figure. This page is the simpler case, a free surface with nothing across it, and the two are not substitutes: one is about getting vapour through a barrier, the other about getting it off a face.

The sealed bag, where the arithmetic is unforgiving

An IKVAOL travel vacuum packing cube with a USB-C pump publishes an unusually complete set of figures, and they are the right ones: a 5 kPa pump against the 3.5 kPa of standard pumps, up to 70 percent compression on puffy jackets, near 50 on sweaters and about 30 on jeans, a polyester and TPU shell with triple reinforced seams, a 180 degree clamshell opening, an IP68 seal, and sizing guidance of one bag for a 35 litre backpack, two for a 20 inch case and four for a 26 to 28 inch one.

Two things are checkable and both come out well. The claim that 5 kPa is 40 percent stronger than 3.5 is exact, since 5 divided by 3.5 is 1.4286. And 5 kPa is 4.93 percent of atmospheric pressure, which tells you what kind of figure it is. It is a squeeze pressure, not a fraction of the air removed. The up to 70 percent volume reduction on a jacket cannot be air taken out of the bag's free space; it is 4.93 percent of an atmosphere pressing the jacket's loft closed. And that is exactly why the same bag manages only about 30 percent on jeans, as its own drawbacks say: denim has very little void left to squeeze.

The unit is the same one our page on what kPa means on a vacuum sealer works through, and the two numbers are instructive side by side. A food sealer quoting 96 kPa is describing how far below atmosphere it pulls, so about 19.2 times the squeeze pressure of this pump. Same unit, opposite convention, and the comparison only works once you know which one you are reading.

Now the part that matters for a damp shirt. Air at 22 degrees holds at most 19.37 grams of water vapour per cubic metre. A five litre bag therefore contains, at absolute best, 0.0969 grams of water vapour capacity. The shirt is carrying 113 grams of free water.

The bag's entire capacity is one part in 1,166 of the water in the garment. Seal a damp shirt in and the air inside saturates almost immediately, the driving difference falls to zero, and evaporation stops. It does not slow down. It stops, and the shirt is exactly as wet when you open the bag as when you closed it.

Which brings us to a line in our own guide to packing dirty laundry when you travel: "A roll-top closure folds down and buckles to seal the bag shut. That barrier keeps moisture and odor from reaching the rest of your luggage during transit." Both halves are true and the advice is right about odour, which is the thing it is for. On the damp case it needs the number above attached to it, because a traveller reading that sentence will seal wet swimwear next to a dry shirt and find two damp items instead of one. The right sequence is to dry first and seal second, and if that is impossible, seal only what you are willing to arrive with still wet.

What our own records cannot tell you

Not one garment in this catalogue publishes a mass. The socks publish a model number, a length and four size ranges. The pants publish seven pockets. The shorts publish a composition to the percent and a thickness in words. The leggings we carry publish a stated 25 and 28 inch inseam and, in the words of their own drawbacks, "Fabric composition and weight are not stated."

Those two figures, a composition and a dry mass, are the entire input to the question in the title. With them, the water a garment holds is one multiplication and the drying time is one division. Without them, every number on this page has to be anchored on a representative 130 gram garment, which is why the figures here are stated as ratios wherever a ratio will do: at 100 grams every gram total falls by 23 percent and every ratio is unchanged.

One more line from our own records deserves a note, because it is good advice that carries an unstated bill. The shorts listing says to lay flat to dry. That is correct for protecting the shape of a rayon blend, and it is the slowest geometry available: laying a garment flat puts one of its two faces against a surface where no air moves, so it roughly halves the exposed area and roughly doubles the clock. On the most water loving garment in the catalogue that is the difference between an afternoon and a day. Keep the advice and add a fan.

The honest limit, and the three numbers to carry

What is exact here is all of the psychrometry. Saturation vapour pressure comes from the Magnus relation, vapour density from the ideal gas law at water's own gas constant, and the wet surface temperature from a balance between the sensible heat arriving and the latent heat leaving, with the Lewis relation at an exponent of two thirds connecting the two transfer coefficients. That is where 15.0 degrees and 18.0 degrees come from, and the 3.11 and 1.77 grams per cubic metre that follow are what every ratio on this page is built on. The 1.76 humidity factor and the 3.75 air movement factor are exact consequences, not estimates.

What is representative is the moisture regain table, and it should be read as what it is: standard textile reference data at 20 degrees and 65 percent relative humidity, not a figure any listing here publishes. Also representative are the 130 gram dry garment, the 120 and 60 percent wet pickup after hand wringing, the 0.80 square metres of hanging area, the 4 and 15 watts per square metre per kelvin, the 22 degree room, and the five litre bag. Change the garment mass and every gram and hour moves with it; change nothing else and every ratio stays where it is.

What this page deliberately does not compute is three things, and each one is a real gap rather than a shortcut. It does not give a drying time for any specific product we carry, because that needs a dry mass and a composition and no listing supplies both. It does not compute the duration of the falling rate tail, because that needs the fibre's sorption isotherm and a diffusion coefficient through a knitted structure, which are laboratory properties and not specifications; all this page claims about the tail is its size in grams and its direction. And it says nothing about whether merino smells less after three days, which is the other half of why people travel with it, because that is a microbial question and there is no measurable anywhere in these listings that bears on it.

Three numbers to carry. Dry is an equilibrium, so a wool garment that feels dry is holding about 17 percent of its own mass in water and a polyester one about 0.4, which is why the two fabrics mean different things by the word. The evaporation rate belongs to the air, so a shirt loses about 33 grams an hour hanging still in an ordinary room regardless of fibre. And the one lever that beats everything else is moving air, at a factor of 3.75, which is why the answer to a sink wash in a hotel is the extractor fan rather than a better shirt.