  We study the anomalous diffusion phase $x ~ t^{\mu}$ with $0<\mu<1$ which
exists both in the Sinai diffusion at small bias, and in the related directed
trap model presenting a large distribution of trapping time $p(\tau) \sim
1/\tau^{1+\mu}$. Our starting point is the Real Space Renormalization method in
which the whole thermal packet is considered to be in the same renormalized
valley at large time : this assumption is exact only in the limit $\mu \to 0$
and corresponds to the Golosov localization. For finite $\mu$, we thus
generalize the usual RSRG method to allow for the spreading of the thermal
packet over many renormalized valleys. Our construction allows to compute exact
series expansions in $\mu$ of all observables : at order $\mu^n$, it is
sufficient to consider a spreading of the thermal packet onto at most $(1+n)$
traps in each sample, and to average with the appropriate measure over the
samples. For the directed trap model, we show explicitly up to order $\mu^2$
how to recover the diffusion front, the thermal width, and the localization
parameter $Y_2$. We moreover compute the localization parameters $Y_k$ for
arbitrary
  $k$, the correlation function of two particles, and the generating function
of thermal cumulants. We then explain how these results apply to the Sinai
diffusion with bias, by deriving the quantitative mapping between the
large-scale renormalized descriptions of the two models.
