Best exterior wall insulation / Discussions

...insulation.  I don't really like the potential for harmful offgassing, but because we will not remove our interior drywall, I don't really know how we can air seal / vapor seal behind the drywall - this seems neccesary for other types of insulation. Does anyone have experience with re-insulation from the exterior of th...

...walls or should I do it all at the same time?  So far it seems the best route for insulation is an air barrier applied to the exterior then insulation then an exterior like painted wood or something engineered.  Should we use a foam insulation or could that lead to vapour issues?  Thanks!!