We consider the mirrors model in $d$ dimensions on an infinite slab and with unit density. This is a deterministic dynamics in a random environment. We argue that the crossing probability of the slab goes like $κ/(κ+N)$ where $N$ is the width of the slab. We are able to compute $κ$ perturbatively by using a multiscale approach. The only small parameter involved in the expansion is the inverse of the size of the system. This approach rests on an inductive process and a closure assumption adapted to the mirrors model. For $d=3$, we propose the recursive relation for the conductivity $κ_n$ at scale $n$ : $κ_{n+1}=κ_n(1+\frac{κ_n}{2^{n}}α)$, up to $o(1/2^n)$ terms and with $α\simeq 0.0374$. This sequence has a finite limit.