[Strawbale] At what point (R value) do we reach 'enough' insulation?

[Coralie & Andre de Bouter]

a question : 

When we draw the relation between insulation in cm and the thermal resistance obtained we obtain a curve.

For example:
0 cm insulation > 0 thermal resistance
5 cm insulation > x thermal resistance 
10cm insulation > 3x thermal resistance 

curve starts to slow down...

50 cm insulation > 30x thermal resistance 
100 cm insulation > 40x thermal resistance 

So at some point it is as good as useless to increase the insulation because it hardly adds up to increased performance.

***The the million dollar question is: At what point (R value) do we reach 'enough' insulation?*** 
I guess this is relative to the climate, is there any objective way of calculating this? 

I ask this because other ecological/natural building systems do not offer R6 (R=m².C/W), but I would like to form an idea on at what point we can say it is well enough insulated. rather that bluntly stating that SB has the best R value. (a bit like saying a Ferrari can do 300km an hour, when 100 km an hour is enough)

I know that some other building systems fonction in a different way (storing the heat or cold in thick high mass walls for instance) but I'm only looking to the insulation approach here.

Greetings,
André
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