Cup Test Measured

Mass Lost g

Test Area cm²

Test Duration h

Test Conditions Driving force

Vapour Pressure Difference Pa

Latent Heat of Vaporisation kJ/kg

Ret Limit for "Breathable" m²Pa/W

MVTR

— g/m²/24h

Mass of vapour passed per square metre per day

Flux & Resistance

MVTR per Hour: — g/m²/h
Evaporative Heat Flux: — W/m²
Evaporative Resistance (Ret): — m²Pa/W
Margin Inside the Ret Limit: — m²Pa/W
Test Area: — m²

Upright cup, inverted cup and sweating hotplate methods give materially different numbers for the same fabric, so MVTR figures are only comparable within one method and one set of conditions. A negative Ret margin means the fabric sits outside the stated breathability limit.

MVTR & Evaporative Resistance (Ret) Calculator — free, with the formula and a worked example, at Textile School.