Terran,
Yes … and no.
The “pumping” that happens when you swing your arms or bend your torso really does move tiny slugs of air through a jacket, but whether it helps or hurts depends on three things:
Where the pressure change is applied (inside the garment or outside);
Whether the membrane actually lets bulk air flow (air-permeable vs. air-tight);
Whether the nylon face fabric is still beading water or has already formed a continuous liquid film (wet-out).
1 How big is the pumping force?
• Arm swing, torso flex and pack-strap compression create transient pressure differences of only ≈ 1–4 Pa inside the jacket (a few thousandths of one atmosphere).
• That translates to a moving air velocity on the order of 0.1–1 cm s⁻¹ through any open path.
2 What a dry shell does with that energy
Dry face + pore network open → pumping is a bonus.
• If the membrane is air-permeable PU or ePTFE with very low resistance (NeoShell, AscentShell, early eVent DV):
– Those 1–4 Pa pulses can push 0.1–2 CFM through the laminate, adding a convective component to vapor transport.
– Lab tests at Kansas State and bpi (UK) show a 20–40 % boost over diffusion-only MVTR when the fabric is flexed at 30–40 cycles min⁻¹.
• Even with an air-tight membrane (classic microporous PU, standard Gore-Tex) the pulses can move moist air to cuffs, hem, pit-zips, and neck openings, where it escapes. The gain is smaller, but still positive.
3 What a wetted-out shell does
Wet face + blocked or water-bridged pores → pumping turns against you.
a) Diffusion path collapses
• The outer surface is now at ≈ 100 % RH. Vapour-pressure gradient that drove diffusion disappears, so the only way out is bulk air flow.
b) Pores fill with liquid water
• In microporous PU most pathways are completely bridged; in ePTFE perhaps half remain “half-pores,” but they are tortuous and water-lined.
• Trying to push air through a water-filled capillary ≈ pushing air through a straw with both ends submerged—you mainly move the water, not the air.
c) Net result of each pressure pulse
Out-stroke: warm humid air inside the jacket presses on the membrane but cannot pass; some of the pulse diverts downward between fabric layers, pulling the surface water plume with it (the “moisture plume” you see creeping toward the hem).
In-stroke: slight negative pressure can actually suck more liquid water into still-dry parts of the face fabric (Bernoulli effect).
Real-world observation: shoulders and upper back wet-out first; ten minutes later gravity and pumping have migrated that wet patch 30–50 cm downwards.
So after full wet-out the same mechanical energy that used to ventilate is now mostly re-distributing liquid water—bad for comfort, neutral to negative for vapour removal.
4 Can we harness pumping deliberately?
Design tricks that make pumping helpful even in rain:
• Built-in air gaps or spaces (pit zips, mesh-lined torso vents, bellowed back vents) give the transient pressure somewhere to drive air without having to cross the membrane.
• Air-permeable membranes with large enough pores (≥ 0.05 µm) resist water head but still pass 0.5–2 CFM—those fabrics continue to benefit from pumping until the pores themselves flood, which takes longer than face-fabric saturation.
• External membrane concepts (OutDry Extreme, ShakeDry) put the waterproof layer outside; there is no textile face fabric to hold a water film, so pumping never turns negative. That is why those jackets keep most of their MVTR in heavy rain.
Bottom line
• Before wet-out: body-motion pumping is definitely an advantage and is one reason air-permeable shells feel drier than their lab MVTR numbers alone predict.
• After wet-out: with a conventional face-fabric laminate, pumping mostly shuttles liquid water around and offers little to no vapor-transport benefit—sometimes the opposite.
• Designers can keep the “good” pumping and avoid the “bad” by using air-permeable membranes, external-membrane constructions, or generous mechanical vents.
