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We present a novel, physically-motivated sub-grid model for HII regionfeedback within the moving mesh code Arepo, accounting for both the radiationpressure-driven and thermal expansion of the ionised gas surrounding youngstellar clusters. We apply this framework to isolated disc galaxy simulationswith mass resolutions between $10^3~{\rm M}_\odot$ and $10^5~{\rm M}_\odot$ pergas cell. Each simulation accounts for the self-gravity of the gas, themomentum and thermal energy from supernovae, the injection of mass by stellarwinds, and the non-equilibrium chemistry of hydrogen, carbon and oxygen. Wereduce the resolution-dependence of our model by grouping those HII regionswith overlapping ionisation front radii. The Str\"{o}mgren radii of the groupedHII regions are at best marginally-resolved, so that the injection ofpurely-thermal energy within these radii has no effect on the interstellarmedium. By contrast, the injection of momentum increases the fraction of coldand molecular gas by more than 50 per cent at mass resolutions of $10^3~{\rmM}_\odot$, and decreases its turbulent velocity dispersion by $\sim 10~{\rmkms}^{-1}$. The mass-loading of galactic outflows is decreased by an order ofmagnitude. The characteristic lifetime of the least-massive molecular clouds($M/{\rm M}_\odot < 5.6 \times 10^4$) is reduced from $\sim 18$ Myr to $<10$Myr, indicating that HII region feedback is effective in destroying theseclouds. Conversely, the lifetimes of intermediate-mass clouds ($5.6 \times 10^4< M/{\rm M}_\odot < 5 \times 10^5$) are elongated by $\sim 7$ Myr, likely dueto a reduction in supernova clustering. The derived cloud lifetimes span therange from $10$-$40$ Myr, in agreement with observations. All results areindependent of whether the momentum is injected from a 'spherical' or a'blister-type HII region.

Momentum feedback from marginally-resolved HII regions in isolated disc galaxies