The 1D conduction heat transfer in the case of walls if we keep on adding layers of insulating material would increase or decrease?

Ans: The addition of more and more insulation on the wall increases the total resistance.

The insulating material added to the wall in series reduces heat transfer.

As shown in the example below the total resistance is the sum of wall 1 and wall 2 (insulating material)

Now suppose we increase the length of insulating wall 2 by distance L3. Then total resistance increases as follows.

The more the total resistance the lesser and lesser will be the heat transfer from wall 1.