Multilayer Thermal & Vapour Stack

How to read this sheet

Resistances add in series, which is what makes a stack more than a repeat of a layer calculation: the still air trapped between layers carries real resistance and on a lofted system it is close to half the total. Each gap is capped at the thickness where natural convection begins, because past that point the air circulates and further thickness carries no further resistance; a model without that cap rewards a fourth loose midlayer that the wearer experiences as worse than the third. The default of 10 mm is a working figure and the field is exposed, since the true onset depends on orientation and on the temperature difference across the gap. Air gap vapour resistance is derived from its thermal resistance on the definition that a still air layer has a Woodcock index of 1; the fabric Ret values are taken as entered, from a hotplate test, and this site gives away a calculator that reduces those measurements. The Woodcock index is capped at 1 because a clothing system cannot pass vapour more freely than the air it displaces, and a stack whose entered figures imply otherwise is flagged rather than printed. Required insulation is a steady state dry heat balance - the skin-to-air gradient over the heat that must leave by conduction once respiration and sweat have taken their share - so it assumes a wearer in thermal equilibrium and says nothing about the transient of putting the garment on cold or about local cold spots at seams and closures, which is where real garments fail first. Wind is deliberately absent: it degrades the outer boundary rather than the stack, and the free wind-degraded tog calculator on this site handles it.