The loading tooltip mentions that molerat attacks can only start in rooms touching dirt. However, attacks do not start in elevators. As such, can I make my vault molerat proof by completely surrounding it with elevators?
If you search through this Reddit thread you can see pretty extensive testing was done and seems to bear up that this does in fact work!
More testing on my own. When I get a "X fires" objective, I create a 'Sad Room' and put two of my best dwellers in there*. I recently got 'put out 47'. I thought I'd do some experimenting myself, so I built an elevator right next to it. I failed around 100 rushes on that room. I didn't count, but it was about half Radroaches half fires, and not one Molerat. I was half expecting the fact that the room below touches ground to be enough.