Our house was built in the 1950s and severely under insulated. We live in the Philadelphia metro area so we have cold winters and hot summers. The under insulation is most notably a problem in our living/dining space which has plasterboard on an exterior brick/cinderblock wall only using furring strips and no insulation. So there isn’t enough room to blow any type of insulation in. I don’t particularly want to go through the hassle of removing the entire wall (it’s about 26ft) to add insulation. Is it possible/advised/recommended to use foam board over the existing wall and then drywall over it all? if I can, should I use faced or unfaced foam board?