Here's why:

* Thermal conductivity (k) is a material property. It represents how well a material conducts heat. It's inherent to the material itself and doesn't change with the thickness of the material.

* Thermal resistance (R), on the other hand, is affected by thickness. Thermal resistance is a measure of how well a material *resists* heat flow. It is calculated as: R = thickness / k.

* Therefore, if you double the thickness, you double the thermal resistance. This means the wall will be better at insulating and preventing heat flow.

In summary:

* Doubling the thickness of a wall doesn't change its thermal conductivity (k).

* It does double the thermal resistance (R), making the wall a better insulator.