Does it mean that when we double the insulation thickness, we would have reduced the heat transfer by half? How thick should the insulation be? Before we look into the diminishing effect of insulation thickness, we need to understand the key concept behind heat transfer.

Heat transfer is given by this formula below

where
Q is the heat transfer in W
U is the heat transfer coefficient in W/(m²K)
A is the area in m²
T is the temperature difference in Kelvin

and we know that

R is the thermal resistance in (m²K)/W

therefore

and R-value is a function of thickness by the formula below

L is the thickness of the material in metres and
λ is the thermal conductivity in W/mK.

Using hypothetical figures below
Area = 100m²
Temperature difference = 15K
and above mentioned formula for heat transfer, we are able to calculate heat transfer at different R-value as tabulated in the table below

This example illustrated; when we add 0.4(m²K)/W of insulation, we will reduce the heat transfer by about 80% from the original, by doubling the R-value to 1.0(m²K)/W, the heat transferred is reduced to 90% (ie a 10% increase from previous). Further increasing the R-value to 1.5(m²K)/W, the heat transferred is 93.3% less than the original.

There is a very big drop off in heat transfer reduction as R-Value increases, to the extent, it is not viable to add thickness for the little saving you get. The table showed the diminishing effect of insulation thickness.

Insulation thickness with R-value above 4 (m²K)/W, gives very little additional return on your investment, more so for ductwork insulations at R3 at max. I don't see the R-value stated in the BCA/ NCC be increased in the near future.