Most houses built prior to World War II (and even some built soon after) have what is known as "knob & tube" wiring. The name is pretty descriptive of the appearance. While such wiring is not necessarily bad (only your electrician knows for sure), it does require a bit of additional care when insulation is introduced into spaces where it is present. As I understand it (again, not being an electrician), the biggest issues are, 1) bad splices / connections which can cause overheating and failure and, 2) the potential for overloading due to the fact that the wiring itself was not designed for modern electrical loads (after all, in 1909 they couldn't have imagined the electrical devices we cannot live without), and therefore may be susceptible to failure from running over-current.

These problems can be exacerbated by insulation. In their original state, they are cooled by air circulation all around the wires, even when enclosed in a wall or other building cavity. When they are encapsulated in insulation, that air circulation is greatly reduced and excess heat cannot dissipate as readily and when wires overheat, they fail sooner or later.

But, that doesn't mean you can't insulate. In walls (unlike, say, an attic space) it can be assumed that little alteration has taken place over the years, since the wiring is inaccessible, so the likelihood of improper splices is much lower. As to the issue of exceeding the current capacity of the wires, that's not hard to fix and the remedy is a good thing for the home, whether you plan to insulate or not. A qualified electrician can test and identify the affected circuits and verify that they are fused (if the original box is still present) or breakered (if it's been updated) at the proper amperage. The electrician would then leave some sort of certificate to verify that those precautions have been completed and you are then ready for insulation. That process was developed some years ago between electrical code-enforcement officials and the electrical, remodel and weatherization contractors in the area and it is recognized as the correct way to deal with that issue.

Again, the wiring checkup just makes sense whether you want to insulate or not and, after it's been completed and any minor problems corrected, we can insulate your home just as we would one with more-modern wiring.