Yes, insulation, but that's not without its problems either. If you're adding insulation to an old house you have to consider that the ventilation system in that house is probably gravity-based. What that means is hot air moves upward, through an unheated attic in the type of house I'm thinking of, and replacement air is pulled in through the breathable structures of the walls. When you insulate such a house heavily, that air intake is compromised and you lose ventilation. So for an insulated house you may need to add mechanical ventilation, which underpressurizes the house. You now need to add air vents, but if you don't have enough of them, replacement air may get drawn e.g. from the understructure of the house, pulling soil microbes into the air with it. Also, if you insulate in such a way that the breathability is compromised, you risk moisture problems, which lead to mold. In particular, adding modern fibreglass insulation to an old house is very problematic, because that type of insulation should not be allowed to draw moisture as it will not dry out and will grow moldy, and wood structures in contact with it may rot.

Basically, moisture control and insulation is a tough problem. In modern houses the solution is to have an airtight layer of plastic sheeting between the insulation and drywall, but those houses should also have been designed from the ground up to have an active, mechanical ventilation system.

Based on experiences here in Finland, a lot of such buildings built in the 60's and 70's have developed severe problems, and frankly even new projects seem to have issues all the time. The more you insulate, the more you have to take care that moisture does not gather in the insulation because you no longer have airflow that would dry it out.