h	Material thickness (m)
k	Thermal conductivity (W m−1 K−1)
ka	Thermal conductivity of air (W m−1 K−1)
kf	Thermal conductivity of fiber (W m−1 K−1)
ks−f	Thermal conductivity of staple PET fiber (W m−1 K−1)
kh−f	Thermal conductivity of hollow PET fiber (W m−1 K−1)
kb−f	Thermal conductivity of bi-component PET fiber (W m−1 K−1)
kalam	Measured thermal conductivity on an Alambeta device (W m−1 K−1)
kCust	Measured thermal conductivity on a custom-build device (W m−1 K−1)
kmeas	Measured thermal conductivity (W m−1 K−1)
kpred	Predicted thermal conductivity (W m−1 K−1)
l	Fiber length (m)
m	Empirical coefficient
R	Thermal resistance (W m−1 K)
va	Volume fraction of air
vf	Volume fraction of fiber
x	Fraction of fibers parallel to heat flow direction ($x+y=1$)
y	Fraction of fibers perpendicular to heat flow direction
z	Factor of fibers orientation to the heat flow direction
δ	Fiber in-plane orientation angle (°)
θ	Fiber through-plane orientation angle (°)
ψ	Critical fiber orientation angle (°)
Δ	Relative difference