  We have performed spin and charge transport measurements in dual gated high
mobility bilayer graphene encapsulated in hexagonal boron nitride. Our results
show spin relaxation lengths $\lambda_s$ up to 13~\textmu m at room temperature
with relaxation times $\tau_s$ of 2.5~ns. At 4~K, the diffusion coefficient
rises up to 0.52~m$^2$/s, a value 5 times higher than the best achieved for
graphene spin valves up to date. As a consequence, $\lambda_s$ rises up to
24~\textmu m with $\tau_s$ as high as 2.9~ns. We characterized 3 different
samples and observed that the spin relaxation times increase with the device
length. We explain our results using a model that accounts for the spin
relaxation induced by the non-encapsulated outer regions.
