Is it true that four inches of insulation does not save twice as much energy as two inches of the same kind of insulation?
It is true. The total thermal conductivity of the insulation is inversely proportional to the thickness. If 1 inch of insulation allows a thermal conductivity of 0.60 Btu per hour per square foot per degree Fahrenheit, other thicknesses will allow the following:
Thickness of insulation
Btu per hour per square foot per degree Fahrenheit
It is obvious that the more insulation you add the less savings you effect for the effort. Somewhere along the line there comes a point where more insulation doesn't pay.