  Linear polarization has been measured in several GRB afterglows. After a few
days, polarization arises from the forward shock emission which depends on the
post-shock magnetic field. The latter can originate both from compression of
existing fields, here the ISM magnetic field, and from shock generated
instabilities. For short GRBs, previous modelling of the polarization arising
from the forward shock considered a random field fully or partially confined to
the shock plane. However, the ISM magnetic field likely consists of both random
and ordered components. Here we study the impact of a more realistic magnetic
field having both ordered and random components. We present our semi-analytical
model and compute polarization curves arising for different magnetic field
configurations. We find that the presence of an ordered component, even
significantly weaker than the random one, has distinct signatures that could be
detectable. In the presence of an ordered component not in the observer plane,
we show that: i) for an observer inside the jet, the polarization angle
$\theta_p$ either remains constant during all the afterglow phase or exhibits
variations smaller than the 90\textdegree swing expected from a random
component solely, ii) for an observer outside the jet, the polarization angle
evolves from $\theta_p^{\max}$, before the jet break to its opposite after the
jet break. We also find that the upper limit polarization for GRB170817
requires a random field not fully confined to the shock plane and is compatible
with an ordered component as large as half the random one.
