Which of the following contributes to high humidity levels indoors?

Improper insulation is a significant contributor to high humidity levels indoors because it can allow warm, humid air from outside to infiltrate into the living space. When insulation is not properly installed or is insufficient, thermal bridges can occur, leading to temperature variations within the walls and ceilings. This can cause condensation, particularly in areas where warm air meets cooler surfaces. As a result, moisture accumulates within the house, raising indoor humidity levels. Effective insulation serves to create a barrier that not only maintains temperature but also helps to control moisture transfer, thereby contributing to a more comfortable and healthier indoor environment.

In contrast, options such as increased ventilation, while sometimes leading to higher humidity, typically provide moisture control and air exchange, helping to combat humidity levels when managed properly. Air conditioning systems are designed to remove humidity from the air as they cool, which helps to lower indoor moisture levels. High outdoor temperatures may increase the potential for humidity indoors if not managed effectively, but this option does not directly contribute to the indoor humidity level itself in the same way that insulation issues do.