Hello Fabian,
first of all, you should question yourself whether your intention to build a "KfW40" house is really your goal. A "KfW" title as an insulation standard without quoting the related Energy Saving Ordinance amendment brings nothing. It’s like a missing unit – as if your car only consumes 3... uh 3 gallons per 50km...
So make sure what KfW40 really is – currently it is pretty much the highest quality you can build.
FOR THAT you won’t get around a (filled) brick, if you want it monolithic.
By the way, a ventilation system is not a question of insulation standard, but of the method how your house is built. If you have a brick exterior wall in mind, nowadays such walls are basically only glued. Partly still dipped, mostly a thin layer of mortar is applied with a mortar sled. This is already relatively airtight – normal mortar didn’t have “gaps” either...
If you use KfW funds, you also have to meet requirements for airtightness. These have to be proven accordingly after the construction measure – monitored during the process and of course considered in execution.
This especially applies to all building openings like windows, doors etc. for which an airtight installation method is mandatory. In this context, one always talks about installation according to RAL or installation by a RAL certified company.
This in turn carries the fact that the air exchange rates prescribed by standards in the rooms can no longer be achieved by manual ventilation. The “draft ventilation” of poorly installed windows is missing and therefore you would probably have to cross-ventilate every 2 hours to get an acceptable air exchange rate.
In short: You cannot avoid a ventilation system if you want to build the building according to standards. (I also consider it unrealistic that someone ventilates every 2 hours, even at night.)
And by the way: You will love this thing.
Besides a bit of electricity and filter changes (which happen quite frequently...) you mainly get one thing:
Always fresh air! Everywhere. Every evening I lie down in bed, take a deep breath and think how cool this ventilation system is!!! No joke! Every evening!!!
I would never build without it again! The gain in comfort is enormous (never having to ventilate) and the gain in well-being immeasurable (always fresh air).
Regarding your question... If you really mean KfW40 according to the Energy Saving Ordinance 2009, you will go toward a 49er wall with a 0.07 stone... whether that still makes sense is up to each person to decide.
If you mean KfW40 according to the Energy Saving Ordinance 2004 (and most people even today mean that when they say KfW40 and hear the term repeatedly among acquaintances...), then that roughly corresponds to today’s KfW70/KfW85 – that works with a 0.10 in 36.5cm...
you see – it depends on your definition.
greetings
-ks