Modelling effective thermal properties is crucial for optimizing the thermal performance of
materials such as new green insulating fibrous media. In this study, a numerical model is proposed
to calculate the effective thermal conductivity of these materials. The fibers are considered to be
non-overlapping and randomly oriented in space. The numerical model is based on the finite element
method. Particular attention is paid to the accuracy of the results and the influence of the choice
of the representative elementary volume (REV) for calculation (cubic or rectangular parallelepiped
slice). The calculated effective thermal conductivity of fibrous media under different boundary
conditions is also investigated. A set of usual mixed boundary conditions is considered, alongside
the uniform temperature gradient conditions. The REV rectangular slice and uniform temperature
gradient boundary conditions provide a more accurate estimate of the effective thermal conductivity
and are therefore recommended for use in place of the usual cubic representative elementary volume
and the usual mixed boundary conditions. This robust model represents a principal novelty of the
work. The results are compared with experimental and analytical data previously obtained in the
literature for juncus maritimus fibrous media, for different fiber volume fractions, with small relative
deviations of 7%. Analytical laws are generally based on simplified assumptions such as infinitely
long fibers, and may neglect heat transfer between different phases. Both short and long fiber cases
are considered in numerical calculations.

effective thermal conductivity; thermal insulation; numerical model; homogenization; fibrous media