Your question is premised upon gas envelopes being very fragile, but there is no reason this has to be so.
You could make the gas envelopes out of some strong fibers which could absorb most impacts (think kevlar). The overpressure of the impact would have to be absorbed, but some relief valves mean you lose some lifting gas rather than pop.
Smaller projectiles are of no concern because a polymer coating on the inside can be self-healing. So long as the projectile has not caused a hole so large the polymer cannot touch itself again, it could reseal the hole (as a little experiment on polymers, fill a plastic sandwich bag with water and stick it with a needle - when you pull out the needle, it will not leak).
Worse comes to worse, you lose one envelope - prudence dictates that your lifting body is comprised of several independent gas envelopes. This has been fairly common in real-world dirigibles.
I'm not sure how much weight it would add for an armor mesh (a thin mail armor to protect against shrapnel cutting through), but that is another possibility to bolster resistance. Keep in mind that this is not intended to be thick enough to stop everything, just some shrapnel from flack. One must accept the possibility of being hit - not much different than a surface fleet navy getting hit with shells from battleships.