(..) Yes you are right, and as a consequence, houses that are not constantly occupied do not require as much insulation (..). Now the question is what to do when duty cycle is say 50% but period is as short as 24hrs - people live in the house but go to work and so on.
I am glad you asked. Forget about Finland and EU, lets move to the Dreamland for a moment. If we relax this problem of multitude of constraints, it can be quite easy to
estimate the potential of OP's concept/question.
Imagine a house with super low RC, but with standard R. In terms of engineering design, imagine a model, a free standing single room house, insulated from INSIDE, with floors and walls layout and furniture made out of low Cp*mass materials and machinery for injecting/extracting heat. Not a concrete slab, in other words. Imagine RC=5s.
My conclusion is that in Dreamland a room only needs the amount of energy that goes through the walls
when there is someone in it, and only this part of the energy is used really. All the remaining energy for heating demand is not used but wasted. Under this definition, when you are not inside, there is no need for the temperature inside to be different than outside temperature, when no energy escapes through the insulation, losses=0.
Now, I do not know how about the rest of you but since usually people can typically be present only in one place at a time, this gives an estimation of what this setback concept is worth, the bordering case. Had buildings been built with low RC, these would have required only the fraction of the energy that comes from the
occupancy of rooms. So if P people live in Q room apartment, sleep 1/3 of the time, leave for shopping, work, etc, there is no way they could all be in more than P places at the same time.
Of course once you return back to Finland, add more constraints, insulated internal walls, RH, high powers required to heat up room to temperature in seconds, things get messy. But the lesson from it is that, depending on occupancy, there is a potential in
saving not wasting huge amounts of energy. Alternatively, keeping wasting same amounts of energy but at lower investment cost.