Question

what happened to thermal conductivity if it's thickness becomes double?

Answer 1

it will also becomes double...........

Answer 2

Thermal Conductivity:
" The rate of flow of heat across the opposite faces of a metre cube of a substance maintained at a temperature difference of one kelvin is called as Thermal Conductivity of substance[k]"
Q/t=K*A(T1-T2)/L
WHERE  Q/T IS RATE OF FLOW HEAT.
Rate of Flow of Heat=Q/t
Rate of Flow of Heat [q/t] depends on :
1. Cross sectional area  of solid
Q/t   α A   --------(1)
Larger the cross sectional area A  of solid, larger number of molecules and free electrons on each layer parallel to its cross sectional area and hence greater will be rate of flow of heat through solid.
Thus, If Thickness of wall is doubled,The thermal conductivity becomes double.