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

[Herbert Gruber]

Dear Andre
Indeed a curve, but the breaking point depends on the insulation-price ­
heating-cost-saving - relationship; you can only solve the problem
(theoretically), if you assume a house´s lifetime of x years, multiply with
the heating costs per year and set the amount in relation to the additional
costs of the insulated construction.
In case of cellulose-/ or fibreglass-insulation you will find out a
thickness of about 24 cm, strawbale is 5times cheaper, so it will be about
120 cm, but: you have to add the price for the additional construction,
better insulated windows, automatically air-exchange-system and so on.
Probably you will find out, that the ideal thickness of strawbales is 35 or
50 cm.

Best wishes
Herbert Gruber, ASBN


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