The actual insulation is stupid cheap when you look at the ROI. However, installing in on an existing house is really tricky and you can cause a lot of moisture related damage if you do it in the wrong way.
The house I live in, like everyone's in my street, is from the late 1800s. Insulating those houses from the outside is very expensive, to the tune of 200k, where the house is worth around 500k in the inflated environment we're in. At the very same time, I haven't found a single insurance company willing to insure the house after insulation for insulation related moisture damage.
This makes insulating this house an absurdly bad financial decision, since once you have moisture damage in any part of this material, you can basically demolish most of the walls since it soaks like a sponge.
Mind you, I have not started going into the pitfalls of the legality of even touching a house this old in Germany, where you have to deal with "Denkmalschutz".
Insulating new construction and retrofitting insulation are two absolutely different things and barely have anything to do with each other from the cost-benefit ratio.
Absolutely, and the thing about old houses is that they've evolved to keep moisture and ventilation in check without access to modern tech. Modern tech, as you point out, usually makes things worse.
My house in Sweden is from the 1950's and I still had to be _very_ mindful when adding insulation. The best thing I've found is cellulose-based insulation, basically shredded newspapers lined with salts and pressed to sheets. The can buffer a lot of moisture without developing mold.
Same here in Spain. But in this climate I don't need insulation anyway.. My energy costs are 60 euro a month in winter and 30 in summer. No point even trying to insulate.
In addition to what was already said, there definitely is and has been many moisture issues on walls and exteriors in new construction. A condo building near me had to have its exterior completely torn off, and lots of sheathing, insulation, cladding, etc replaced due to a botched install of EIFS. Google for “EIFS failures.” This was a big problem in the 80s/90s or so but apparently it’s still happening, at least with subpar builders.
Because they use a dual layer construction with an air gap between the inner an outer walls. This gives space for condensation to occur and evaporate again without effecting the living space.
In old buildings with solid walls you can get condensation on the walls that will evaporate because the living space is heated and we'll ventilated. If you clad that wall in insulation you can prevent the evaporation which leads to damp problems.
Depending on the climate zone, up to half the insulation may be mandated to be on the outside (behind some kind of cladding (brick, stucco, siding) to protect it from UV and bulk water).
Per the above article, by having the outside be insulation, your interior structure is kept warmer, and so condensation is less likely to occur.
Do a search for "Joseph Lstiburek" and "Matt Risinger", who have lots of articles and videos on building science.
Old constructions are diffusion open, meaning moisture can move freely in and out of it. New constructions are diffusion closed, meaning you have a steam barrier on the inside which blocks the moist indoors air from traveling into the walls. This however requires an airtight house and mechanical ventilation.
