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.