It traps moisture which leads to mold and for wood to rot. You nearly never want to install insulation without a vapor barrier. And especially not with spray foam. People do it because it’s cheap to do. You only have to punch a small hole in the wall and spray into it.
Also if you want insulate an old house consider you should install an HVAC system so that you are properly venting. Old houses self ventilate because they’re so loose compared to modern homes with tight envelopes.
> You nearly never want to install insulation without a vapor barrier. And especially not with spray foam
Closed cell foam insulation is a vapor barrier, which is where the problem comes from. You must never have two vapor barriers as any moisture that is between them cannot get out and this will cause mold and rot.
Punching a small hole in a wall and putting foam is fine so long as the foam and the existing vapor barrier are touching each other and thus form one barrier. However if there is something between the vapor barrier and the foam you have two barriers and problems.
Also if you want insulate an old house consider you should install an HVAC system so that you are properly venting. Old houses self ventilate because they’re so loose compared to modern homes with tight envelopes.