  We investigate the effect of microlensing on the standardisation of strongly
lensed Type Ia supernovae (GLSNe Ia). We present predictions for the amount of
scatter induced by microlensing across a range of plausible strong lens
macromodels. We find that lensed images in regions of low convergence, shear
and stellar density are standardisable, where the microlensing scatter is <
0.15 magnitudes, comparable to the intrinsic dispersion of for a typical SN Ia.
These standardisable configurations correspond to asymmetric lenses with an
image located far outside the Einstein radius of the lens. Symmetric and small
Einstein radius lenses (< 0.5 arcsec) are not standardisable. We apply our
model to the recently discovered GLSN Ia iPTF16geu and find that the large
discrepancy between the observed flux and the macromodel predictions from More
et al. (2017) cannot be explained by microlensing alone. Using the mock GLSNe
Ia catalogue of Goldstein et al. (2017), we predict that ~ 22% of GLSNe Ia
discovered by LSST will be standardisable, with a median Einstein radius of 0.9
arcseconds and a median time-delay of 41 days. By breaking the mass-sheet
degeneracy the full LSST GLSNe Ia sample will be able to detect systematics in
H0 at the 0.5% level.
