In general there is a square root relationship between thickness *d* of a static air layer and air velocity *v*. The exact function depends on the size and shape of the surface, but for the human body a useful approximation is:

Still air acts as an insulating layer with a conductivity _{} (a material constant, regardless of the shape of the material) of .026 W/mK, which has a heat transfer coefficient *h* (units of ^{}) (the conductive property of a slab of material) of:

(Kerslake 1972).

Radiant heat flow (_{}) between two surfaces is approximately proportional to their temperature difference:

where *T* is the average absolute temperature (in Kelvin) of the two surfaces, _{} is the absorption coefficient and is the Stefan-Boltzmann constant ( ). The amount of radiation exchange is inversely related to the number of intercepting layers (*n*):

Clothing insulation (_{}) is defined by the following equations:

where _{} is intrinsic insulation, _{} is (adjacent) air insulation, _{} is total insulation, _{} is average skin temperature, _{} is the average temperature of the outer surface of the clothing, _{} is air temperature, _{} is the dry heat flow (convective and radiant heat) per unit of skin area and _{} is the clothing area factor. This coefficient has been underestimated in older studies, but more recent studies converge to the expression _{}

Often *I* is expressed in the unit *clo*; one clo equals _{}.

McCullough et al. (1985) deduced a regression equation from data on a mix of clothing ensembles, using thickness of the textile (_{}, in mm) and percentage covered body area (_{}) as determinants. Their formula for the insulation of single clothing items (_{}) is:

The evaporative resistance *R* (units of s/m) can be defined as:

(or sometimes _{}, in )

For fabric layers, the air equivalent *(*_{}*)* is the thickness of air that provides the same resistance to diffusion as the fabric does. The associated vapour _{} and latent heat *(*_{}*)* flows are:

^{}

^{}

where *D* is the diffusion coefficient (), *C* the vapour concentration (_{}) and _{} the heat of evaporation (2430 J/g).

(from Lotens 1993). _{} is related to *R* by:

where:

*D* is the diffusion coefficient for water vapour in air, .