Yes, you can avoid mould in older buildings by carefully airing out rooms and keeping things dry and away from walls. But not if the previous three tenants had a mould issue and the landlord just painted over it.
An honourable mention to fitting Cavity Wall Insulation, heavily sold and encouraged by UK government energy saving schemes thorough the 1990's and 2000's.
Except by stuffing the wall cavity, you provide a nice moisture bridge to outside whilst simultaneously stopping air circulating in the cavity and whipping away moisture; thus an explosion of mould.
The policy was a disaster, as getting the stuff removed costs a small fortune.
As a retrofit without those things I guess I can see it being problematic.
One technique to mitigate this that does not require extraordinary care on the part of the installer is “flash and batt”: apply a thin layer of spray foam to the exterior wall and then fill the cavity with fiberglass, mineral wool or some other fluffy insulation. The spray foam adheres to the wall and leaves no space for humid air to reach, and the inner surface of the spray foam will stay warm enough to avoid condensation.
Frame first — you want the foam to stick to the exterior shell and to the framing members that are up against the exterior. Protect anything you don’t want foamed — the fresh foam sticks to everything and foam in your electrical boxes is no fun.
(The unreacted chemicals are not persistent — they’re too reactive to stick around.)
If you have this problem, then the only thing I found which helps is fungicidal wash, and keeping furniture away from the problem walls (especially north facing corners of the house). I suspect wallpaper would also help but I never tried it.
Some of these houses have been transformed into being unfit for human habitation due to the damp and mould. In some cases, the botched insulation has resulted in an increase in heating costs (along with the damp and mold). Some have suffered structural damage due to the botched insulation, costing tens of thousands to repair.
https://buildingscience.com/documents/building-science-insig...
A small amount of mould?
Oh you sweet summer child! You really need to see the damage it can do.
Like, central ventilation is not magical unobtainable technology. Simple heat recovery even vastly improves heating costs in a way insulation never can.
What of the buildings that don’t comply in time? Or can’t find trades to do it in time? Or we notice to our eternal shock that projects to ensure code tracking are priced at a serious premium?
Or, how many improvements to code would we decide weren’t desirable because of the costs of retrofitting, so now we lose even the low slope of improvement versus today.
Not to mention, a lot of places around the world care about the look and character of historical locations. If a structure wasn't designed for central HVAC, for example, then there's often nowhere to hide the condenser units, air handle units or the ductwork. Same with insulation -- if that exterior wall wasn't intended to have it you've got a couple options and they both hurt.
Last of all, I'll mention labor. The type of skilled labor that can do any specific trade at all is relatively rare in the aftermath of the "college-or-bust" era, but the kind of labor you'd want for renovation work (fast, efficient, can tackle multiple different aspects at once without calling in different trades, and gets it right the first time to minimize disruption/call backs) is even more rare. To carry out some kind of massive renovation project at a national level even with infinite money you're talking about a generational timeframe just due to labor constraints.
There is a lot of old buildings out there.
Not everyone lives in a disposable McMansion in a gated community.
These buildings will have ventilation eventually but the code needs about 20 years to take full effect.
Mind you that people have been living in those buildings for hundreds of years without issues but tennants these days don't seem to have the basic skill of airing their place.