How much rockwool on the outside is too much?

[Garald]

I've got a quote for insulating a wall on the outside using rockwool. (I've got similar quotes for polystyrene, but I think there are good reasons to prefer rockwool, one of them being permeability to water vapour.)

What is being proposed is 16cm thickness, for an R of 4.55.

 

Is there such a thing as too much rockwool on the outside? If not, why not go up to 18cm, or 20cm?

 

(Sure, there is the matter of cost, but only 1/3 of them are the cost of the insulating material itself. Going from 16 to 18cm should correspond to an increase in price of less than 5%.)

 

I have a further motivation - the wall is already insulated on the inside (I know, I know, not the best decision for a northern wall - the pseudoarchitect overruled both the builder and me on that, back before I knew she was no architect). Now, Ubakus tells me that just a little bit of insulation on the outside will be enough to forestall the risk of condensation, but, until I get WUFI to run and/or understand condensation myself, it's best to work with safety margins. Around here, people go by a "the R on the outside should be at least twice the R on the inside" rule - that seems like an exaggeration, but who knows?